Metabolic hubs in reproduction: The regulatory network of lipid droplets in gamete and embryo physiology (Review)

Introduction

Lipid droplets (LDs), typically considered inert reservoirs of neutral lipids, are recognized as dynamic and metabolically active organelles central to cell energy balance, lipid metabolism, proteostasis and stress adaptation (1,2). Structurally, LDs consist of a hydrophobic core enriched in triacylglycerols (TAGs) and cholesteryl esters (CEs), encased by a phospholipid monolayer that is uniquely enriched in proteins such as perilipin (PLIN), lipase and endoplasmic reticulum (ER)-associated scaffolding proteins (3). The formation of LDs begins at specific subdomains of the ER, where neutral lipid phase separation is initiated and nucleated by key regulatory proteins including seipin (SEIP-1) (4,5).

While extensively studied in metabolically active tissue such as adipocytes and hepatocytes, LDs have garnered growing attention in non-adipose cells where their functions extend beyond energy storage (2,6,7). In the central nervous system, immune cells and particularly in reproductive cells, LDs regulate cell signaling, redox homeostasis, steroidogenesis and embryonic morphogenesis (8-10). Reproductive cells exhibit notable plasticity in energy demand and metabolic activity, particularly during oocyte maturation, fertilization and preimplantation embryo development, periods marked by high biosynthetic and proliferative requirements.

Oocytes accumulate substantial quantities of LDs during proliferation and maturation, forming lipid-rich stores that support the metabolic needs of the embryo during early cleavage divisions (11,12). In lipid-rich species such as pigs, cows and humans, this cytoplasmic lipid reserve provides energy via β-oxidation and supports membrane biogenesis and redox buffering (13). Dysregulation of LD content in oocytes, either excessive or deficient, is linked to impaired maturation, fertilization failure and reduced developmental competence (14).

By contrast, mature spermatozoa are devoid of prominent LDs, but their development is dependent on lipid mobilization and LD dynamics in precursor germ and Sertoli cells (15). Sertoli cells exhibit cyclic changes in LD number and composition, reflecting their roles in nutrient provision, phagocytosis of residual bodies and energy buffering during spermatogenesis (16,17). Testicular Leydig cells also use LDs as platforms for storing and mobilizing cholesterol esters in response to luteinizing hormone (LH) stimulation, enabling rapid testosterone synthesis (18,19).

Granulosa and luteal cells in the ovary similarly use LDs for estrogen and progesterone production (20). These cells respond to follicle-stimulating hormone (FSH) or LH by activating hormone-sensitive lipase (HSL) to hydrolyze CE within LDs, liberating free cholesterol for steroidogenesis within mitochondria (21,22). Proteomic studies have identified key steroidogenic enzymes, such as CYP11A1 and 3β-hydroxysteroid dehydrogenase, localizing to LDs in these cells, suggesting that LDs not only serve as storage platforms but also scaffold sites for enzymatic reactions (20,23,24).

The nematode Caenorhabditis elegans is a powerful genetic model for studying LD function in reproduction. Its transparency, short generation time and well-mapped reproductive system enable in vivo tracking of LD dynamics during gametogenesis and embryogenesis (25). In C. elegans, SEIP-1 regulates a subpopulation of LDs that contribute to lipid layer assembly in the embryonic eggshell, which is key for embryo viability (25). Mutants lacking SEIP-1 exhibit disrupted permeability barriers and embryonic lethality, phenotypes that can be partially rescued by modulating PLIN-1 or Ras-related protein Rab-18 function, revealing parallel compensatory pathways (25,26).

Despite these advances, major questions remain unresolved. The temporal coordination of LD biogenesis and degradation during fertilization or implantation requires further elucidation. Molecular cues that determine LD targeting by lipophagy in reproductive cells remain to be identified. LD imbalance contributes to reproductive pathologies such as infertility or polycystic ovary syndrome (PCOS) (27-30), however the mechanisms that warrant deeper exploration. Understanding these questions is key given the metabolic sensitivity of reproductive cells and their susceptibility to lipid imbalance.

Coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS) microscopy enable label-free, real-time imaging of LD dynamics in living oocytes and embryos (31,32). Lipidomics and metabolomics, even at single-embryo resolution, have uncovered lipid profile shifts associated with in vitro maturation or developmental arrest (33). Genome editing tools such as clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (CRISPR-Cas9) and RNA interference (RNAi) screens in C. elegans or mammalian systems have revealed novel regulators of LD size, turnover and localization (34,35). Additionally, single-cell transcriptomics has revealed cell type-specific expression of LD-associated genes [PLIN2, Diacylglycerol O-acyltransferase DGAT2, adipose triglyceride lipase (ATGL) across the testis, ovary and early embryo (36).

LD dysfunction is increasingly linked to reproductive disorder (28,37). Obesity and metabolic syndrome alter lipid composition and increase oxidative stress in oocytes and sperm, decreasing fertility and embryo quality (38,39). In PCOS, altered lipid metabolism in granulosa cells impairs steroid hormone production, oocyte competence and follicular development (30). Genetic disorders affecting LD regulators, such as mutations in SEIP-1, the protein encoded by Berardinelli-Seip congenital lipodystrophy 2, in congenital lipodystrophy, typically involve hypogonadism and infertility (40).

The present review aimed to summarize the roles of LDs in reproduction across species and cell types, including oocytes, sperm and early embryos, and how supporting somatic Sertoli, Leydig, granulosa and luteal cells use LDs for metabolic coordination and hormonal output, as well as findings from C. elegans that uncover conserved regulatory mechanisms. The present review aimed to highlight key proteins such as ATGL, HSL, PLIN, DGATs, SEIP-1 and lipophagy-associated factors and assess how cutting-edge technologies are advancing the study of LD biology in reproductive physiology and how LD dysfunction contributes to reproductive disease.

Biological landscape and core functions of LDs in reproduction

Lipid availability in reproductive systems is heterogeneous, varying across cell types, developmental stages, and species. Oocytes, particularly in lipid-rich species such as pigs, cows and humans, accumulate abundant LDs during growth, whereas mature spermatozoa contain few if any visible lipid stores. By contrast, reproductive somatic cells, including Sertoli, Leydig, granulosa, and luteal cells, display dynamic LD populations that fluctuate in response to developmental cues and hormonal stimulation (24,41-43).

Beyond spatial heterogeneity, LDs also exhibit notable temporal dynamics. Their abundance increases during oocyte maturation and steroidogenic activation, is remodeled following fertilization and progressively declines during early embryonic development as stored lipids are mobilized (37,41,44). These spatial and temporal patterns establish the foundational context in which LDs serve not only as energy reserves, but as platforms that support metabolic coordination and signaling processes (Fig. 1).

Figure 1 Roles of LDs as metabolic and regulatory hubs in reproductive cells. (A) LDs are energy reservoir for development. LDs store TAGs that are mobilized via lipolysis to provide fatty acids for mitochondrial β-oxidation. This generates ATP, which is key for oocyte maturation, fertilization and early cleavage, while maintaining a balanced redox state (low ROS), which is critical for embryo viability. (B) LDs are regulators of signaling and cell fate. LDs release bioactive lipids (such as PUFAs) that act as signaling ligands for nuclear receptors (such as PPARs) to drive gene transcription and differentiation. LD-derived lipids contribute to membrane remodeling, influencing fluidity and morphogenetic events during cell division. (C) LDs are guardians of proteostasis (cell quality control). LDs function as sequestration sites for misfolded or aggregated proteins, acting as transient detoxification zones. These protein-laden LDs are cleared via lipophagy (autophagic degradation) or lysosomal pathways, which is key for maintaining proteostasis during high-stress periods such as spermatogenesis. (D) LDs are platforms for steroid hormone biosynthesis. In steroidogenic cells (Leydig, granulosa and luteal cells), LDs store cholesteryl esters. Following hormonal stimulation, these esters are hydrolyzed to free cholesterol, which is transported to mitochondria (facilitated by proteins such as StAR and HSL at the LD-mitochondria interface) to serve as the substrate for the synthesis of steroid hormones such as progesterone and testosterone. HSL, hormone-sensitive lipase; LD, lipid droplet; PPAR, peroxisome proliferator-activated receptor; PUFA, polyunsaturated fatty acid; ROS, reactive oxygen species; StAR, steroidogenic acute regulatory protein; TAG, triacylglycerol.

Spatial distribution of lipids and LDs in reproductive systems

Lipid availability in reproductive systems exhibits marked heterogeneity, with distinct patterns across germ and somatic cells as well as species-specific adaptations. For instance, while oocytes in lipid-rich species like pigs, cows, and humans amass numerous cytoplasmic LDs that confer opacity and fuel early embryogenesis through β-oxidation and membrane synthesis, murine oocytes maintain lower lipid reserves, highlighting metabolic divergences (9-12). In somatic support cells, LD profiles are tailored to functional demands: Sertoli cells show cyclic LD accumulation tied to phagocytosis and spermatogenic cycles (15-17), whereas granulosa and luteal cells prioritize cholesteryl ester storage for sustained steroidogenesis under hormonal regulation (20-22).

Temporal dynamics of LDs across reproductive stages

In addition to spatial heterogeneity, LDs exhibit notable temporal regulation throughout reproductive processes. During oocyte growth, LD content increases as neutral lipids are synthesized or imported and stored in preparation for fertilization and early embryonic development (11,12). Following fertilization, maternally inherited LDs undergo redistribution and partial consumption during cleavage divisions, reflecting a shift from lipid storage to utilization (13,31).

Similar temporal patterns are evident in reproductive somatic cells: In Sertoli cells, LD abundance fluctuates across the spermatogenic cycle, increasing during periods of active germ cell turnover and phagocytosis of residual bodies (16,17,45,46). In granulosa cells, LD accumulation intensifies during follicular maturation and peaks following luteinization, coinciding with maximal steroidogenic activity (24,47,48). These dynamic changes indicate that LDs are not static lipid depots but responsive organelles whose formation and turnover are associated with developmental timing and hormonal cues.

Metabolic use of stored lipids in reproductive cells

Once accumulated, lipids stored within LDs serve as key metabolic substrates. Triacylglycerols can be hydrolyzed to release fatty acids (FAs) that fuel mitochondrial β-oxidation, providing ATP during energetically demanding processes such as oocyte maturation, early embryonic cleavage and spermatogenic support by Sertoli cells (11,13,49). In parallel, LD-derived lipids contribute to membrane biogenesis, ensuring sufficient phospholipid supply during rapid cell division and cellular remodeling (50).

The balance between lipid storage and mobilization is regulated. Excessive lipid accumulation leads to lipotoxicity and oxidative stress, whereas insufficient lipid reserves compromise energy availability and developmental competence (14,51). Reproductive cells therefore rely on coordinated control of lipid synthesis, lipolysis and oxidation to maintain metabolic homeostasis across fluctuating physiological demands (49,50).

LDs as regulatory and signaling hubs

Beyond their metabolic roles, LDs integrate lipid metabolism with signaling and regulatory pathways. The controlled release of bioactive lipid species from LDs influences nuclear receptor activation, including peroxisome proliferator-activated receptors (PPARs), thereby modulating transcriptional programs associated with cell differentiation and developmental progression (52,53). In steroidogenic cells, LDs store cholesteryl esters that are rapidly mobilized in response to gonadotropic stimulation, coupling lipid storage directly to hormone biosynthesis (20-22).

LDs also engage in notable physical and functional interactions with other organelles. Increasing evidence supports the existence of membrane contact sites between LDs and mitochondria or the ER, enabling efficient lipid transfer, metabolic channeling and coordination of redox homeostasis (54,55). Ultrastructural evidence supports the concept of LDs as metabolic hubs: Electron microscopy studies have revealed tight membrane contact sites between LDs and mitochondria or the ER, indicating that these organelles are physically connected rather than randomly juxtaposed (54-57). Such contacts are hypothesized to facilitate efficient lipid transfer, metabolic channeling and coordinated regulation of energy production and lipid metabolism, providing a structural basis for the functional interactions illustrated in schematic models (58,59). Through these interactions, LDs serve not merely as passive reservoirs but as dynamic hubs that synchronize energy metabolism, signaling and cell adaptation during reproduction (2,10).

Accumulating evidence indicates that the LD-mitochondria interface represents a physically tethered unit rather than a transient or stochastic association (58,60). Ultrastructural analyses have revealed well-defined membrane contact sites that anchor LDs to mitochondria, thereby establishing stable platforms for lipid transfer and metabolic coordination (54,55,61,62). These contact sites are mediated by specific tethering proteins that physically link the organelles. Among these, PLIN5 is a LD-associated protein that promotes sustained LD-mitochondria coupling and facilitates the channeling of FAs from LDs to mitochondria for β-oxidation. In parallel, mitoguardin 2, a mitochondrial outer membrane protein, forms physical bridges between mitochondria and LDs, coordinating lipid trafficking and mitochondrial energy metabolism (54,63,64). Together, these tethering mechanisms support a model in which LDs and mitochondria serve as integrated metabolic units, providing a structural and molecular basis for the metabolic hub concept in reproductive cells.

The diverse and sometimes contradictory functions of LDs can be reconciled by viewing them as organelles with multiphase activities that are dynamically regulated across developmental and physiological contexts. In a storage phase, LDs primarily accumulate neutral lipids, serving as reservoirs that buffer energy availability and protect cells from lipid overload. During metabolic phases, LDs undergo controlled lipolysis, releasing FAs that fuel mitochondrial β-oxidation and support membrane biosynthesis. In signaling phases, LD-derived lipid species serve as bioactive molecules that engage nuclear receptors, such as PPARs, thereby influencing transcriptional programs associated with cell fate decisions and developmental progression (2,60,65).

The transition between these phases is not fixed but context-dependent, shaped by developmental timing, hormonal cues and cellular energy demands. This multiphase framework provides a conceptual basis for understanding how the same LD population alternately serves as a protective storage depot, a metabolic fuel source or a signaling platform during reproduction (2,41,48,66).

LDs in oocyte maturation and competence

The oocyte is a metabolically unique cell, characterized by large size, prolonged growth phase and dependence on stored reserves to sustain early embryogenesis. Among these reserves, LDs serve a key role in determining oocyte quality and developmental competence (37,41,49). While their role as energy depots is well-established, evidence highlights a broader regulatory role for LDs in shaping the oocyte redox balance, signaling landscape and cytoplasmic remodeling capacity, which are indispensable for meiotic progression and post-fertilization events (67-69) (Fig. 2).

Figure 2 LD dynamics and regulation during oocyte maturation. (A) COC (the follicular microenvironment). The oocyte is surrounded by cumulus cells, forming the COC. Cumulus cells modulate the oocyte lipid profile by transferring FAs and sterols through gap junctions. This supply chain is influenced by external factors such as gonadotropins and dietary intake (exogenous FAs). Cumulus cells maintain their own LD reserves to support this metabolic coupling. (B) LDs show intracellular dynamics across temporal phases. During the GV stage, the oocyte actively synthesizes and stores lipids. DGAT2 catalyzes triacylglycerol synthesis and PLIN2 facilitates coating of the LD surface to promote stability and prevent premature lipolysis, resulting in the accumulation of dispersed LDs. Following meiotic resumption (MII transition), LDs undergo spatial reorganization and functional activation. At mitochondria-LD contact sites, lipolytic enzymes (ATGL/HSL) mobilize stored lipids. The released FAs are directed into mitochondria for β-oxidation, generating ATP while maintaining redox balance (low ROS) and supporting membrane synthesis essential for meiotic progression and fertilization competence. COC, cumulus-oocyte complex; DGAT2, diacylglycerol acyltransferase 2; ER, endoplasmic reticulum; GV, germinal vesicle; HSL, hormone-sensitive lipase; MII, metaphase II; PLIN2, perilipin 2; ROS, reactive oxygen species; FA, fatty acid; LD, lipid droplet; ATGL, adipose triglyceride lipase.

LD accumulation is temporally regulated during oocyte growth

LD biogenesis in oocytes is a tightly orchestrated process that coincides with folliculogenesis. Throughout the growing phase, oocytes accumulate neutral lipids via both de novo synthesis and uptake of exogenous FAs, which are esterified and stored in LDs (37,68). During oocyte growth, LDs primarily operate in a storage phase, ensuring sufficient lipid reserves for subsequent developmental transitions. In large antral follicles, LDs become prominent cytoplasmic features (68). Their abundance and distribution vary across species: Porcine and bovine oocytes are lipid-rich and visibly opaque, whereas murine and human oocytes have fewer LDs and a clearer cytoplasm (41,42,70,71). These differences reflect fundamental differences in lipid metabolism, sensitivity to in vitro conditions and developmental strategies (49,72). For example, lipid-rich oocytes in porcine and bovine species rely heavily on β-oxidation of stored lipids for energy during early embryogenesis, as shown by reduced developmental rates when β-oxidation inhibitors are applied in culture (11). In contrast, murine oocytes exhibit greater dependence on glycolysis, making them less sensitive to lipid perturbations but more vulnerable to glucose fluctuations in vitro (73). Human oocytes, while lipid-moderate, show intermediate sensitivity, with in vitro maturation success influenced by media supplements that mitigate oxidative stress from lipid peroxidation (74). These variations underscore species-specific reproductive adaptations, where lipid-rich strategies buffer against nutrient scarcity post-fertilization, whereas lipid-poor ones prioritize rapid external nutrient uptake (49).

Oocyte capacity to accumulate and mobilize LDs is associated with the ability to resume meiosis and support embryo development (72,75). Disruptions in LD formation, either through inhibition of DGAT1/2 or alterations in FA composition, impair nuclear maturation and decrease blastocyst yield, underscoring the role of LDs in establishing developmental competence (49,69,76).

LD dynamics and cytoplasmic remodeling

LDs undergo dynamic spatial reorganization during meiotic maturation. In many species, LDs are dispersed throughout the oocyte cytoplasm at the germinal vesicle (GV) stage, then undergo clustering or partial consumption during GV breakdown and metaphase II transition (31,37,49,77). These changes may reflect a metabolic switch: As the oocyte transitions from quiescence to a highly active biosynthetic state, lipid oxidation increases, supported by mitochondrial redistribution and enhanced FA flux (67,68).

Moreover, LD remodeling is typically coordinated with organelle positioning. In mammalian oocytes, LDs have been observed in proximity to mitochondria and ER-derived vesicles, suggesting metabolic crosstalk and potential transfer of lipid species (54,55,61,62). These interactions may be key for shaping mitochondrial function, as lipid overload or misdistribution is associated with increased ROS production and mitochondrial dysfunction, which are detrimental to fertilization and embryo cleavage (63,64,71).

Regulatory mechanisms: Enzymes and LD-coating proteins

The functional integrity of LDs in oocytes is governed by a tightly regulated network of enzymes and structural proteins (37,69). DGAT2, which catalyzes TAG synthesis, is enriched in growing oocytes, and its inhibition leads to decreased lipid storage and impaired oocyte maturation (69,76). Similarly, the LD surface protein PLIN2 is abundantly expressed in lipid-rich oocytes, and is hypothesized to stabilize LDs by preventing premature lipolysis (78-80). Knockdown or pharmacological interference with PLIN2 results in dysregulated lipid metabolism and altered oocyte developmental trajectories (69,81).

In addition to synthesis and stabilization, lipid mobilization is precisely timed. Lipolytic enzymes such as ATGL and HSL are activated in peri-ovulatory periods, allowing controlled release of FAs for β-oxidation and membrane synthesis (82-85). An imbalance in this process, either via excessive lipid accumulation or hyperactive lipolysis, compromises oocyte viability (86).

Paracrine influences and somatic-oocyte interaction

LD content and composition in the oocyte are not solely determined by intrinsic metabolic programs. Surrounding cumulus and granulosa cells contribute to the oocyte lipid profile through paracrine signaling and metabolite transfer (41,87,88). Cumulus-oocyte complexes exhibit extensive gap junction communication, allowing transfer of small lipophilic molecules such as FAs and sterols (89-92). Cumulus cells express lipoprotein receptors, FA transporters and lipogenic enzymes, and can modulate the lipid environment of the oocyte in response to gonadotropic stimulation or dietary lipid availability (41,91,93).

Alterations in cumulus cell metabolism, such as those seen in high-fat diet models or PCOS, lead to excessive lipid accumulation in oocytes and are associated with lower fertilization and blastocyst rates (71,77,93). These findings emphasize that LD metabolism in the oocyte must be understood in the context of the follicular microenvironment (41,87,88).

LD metabolism as a marker and modulator of oocyte quality

Because LD content reflects metabolic history and readiness for fertilization, it is increasingly studied as a biomarker of oocyte competence (31,94,95). Non-invasive imaging modalities, such as CARS microscopy, have made it possible to quantify LD content in live oocytes and demonstrate its association with subsequent embryo development (31,96-99). Moreover, interventions aimed at modifying LD metabolism through culture media supplementation with oleic acid or antioxidants have shown potential to rescue poor-quality oocytes by rebalancing lipid profiles (96-99).

Altogether, LDs in oocytes serve as more than static energy stores; they are dynamic organelles whose content, composition and spatial behavior are critical determinants of oocyte maturation and developmental success (37,69). Their regulation is multifactorial, involving intrinsic enzyme systems, extrinsic paracrine input and inter-organelle coordination. Understanding and manipulating LD biology in the oocyte may thus represent a promising avenue for improving assisted reproduction outcomes (96,100,101).

LDs in spermatogenesis and sperm function

Spermatogenesis is a complex, highly regulated process that requires precise coordination of energy metabolism, membrane remodeling and quality control. While mature spermatozoa lack prominent LDs, increasing evidence suggests that LDs play key upstream roles during the earlier stages of germ cell development and in the metabolic crosstalk between developing spermatogenic cells and their supporting Sertoli cells (15,43,102) (Fig. 3).

Figure 3 LD dynamics and metabolic crosstalk in the testis. (A) Sertoli cells support germ cells via a metabolic axis. Within the seminiferous tubule, Sertoli cells support the developing germ cells. As spermatids elongate, they shed excess cytoplasm as residual bodies, which are phagocytosed by Sertoli cells. These lipid-rich remnants are sequestered into LDs (phagocytosis and lipid recycling). Stored LDs in Sertoli cells are catabolized via lipophagy (lysosomal degradation) and mitochondrial β-oxidation to generate ATP, fueling the high energy demands of spermatogenesis. Early germ cells (spermatogonia/cytes) contain transient LDs as energy reserves, which decline as cells differentiate into mature spermatozoa. (B) Sperm undergo membrane remodeling. Although mature spermatozoa lack LDs, their plasma membranes are enriched with PUFAs derived from upstream lipid metabolism. Integration of PUFAs into the sperm membrane is a critical factor for maintaining membrane fluidity, motility and fertilization capacity. FSH, follicle-stimulating hormone; HSL, hormone-sensitive lipase; LD, lipid droplet; PUFA, polyunsaturated fatty acid. (C) Leydig cells regulate interstitial processes. In the interstitial space, Leydig cells use LDs as reservoirs for cholesteryl esters. Under the regulation of LH, HSL mobilizes cholesterol from these LDs to synthesize Testosterone, which is key for maintaining spermatogenesis.

LDs in early germ cells and spermatogenic progression

During the early phases of spermatogenesis, including in spermatogonia and early spermatocytes, LDs are readily observed and serve as temporary energy reserves and platforms for lipid remodeling (15,102,103). As germ cells differentiate toward the elongated spermatid stage, LD content typically declines, which is associated with cytoplasmic condensation and organelle removal (15,102). However, disturbances in lipid storage and LD homeostasis during these early stages impair germ cell differentiation and decrease sperm output (43,104,105).

Model organisms such as Drosophila and C. elegans are key in dissecting LD function during spermatogenesis (43). For example, the Drosophila homolog of ATGL, Brummer, localizes to LDs in testicular germ cells and its deletion results in massive LD accumulation and spermatogenic arrest (43). Similarly, in mammals, ATGL and HSL are functionally important in lipid mobilization during spermatogenic progression (43,106). Deficiency in these enzymes leads to abnormal lipid accumulation, disrupted spermatid elongation and reduced fertility (106).

While studies in C. elegans have uncovered genetically conserved roles of LD in embryonic integrity (25,107,108), mammalian systems exhibit additional layers of complexity, particularly in steroidogenesis and hormonal regulation. For instance, LDs in mammalian reproductive cells integrate with hormone-responsive lipases to mobilize cholesterol for steroid hormone synthesis and interact with organelles such as mitochondria for enhanced metabolic channeling, features modulated by endocrine signals absent in simpler models (Table I).

Table I Conserved and divergent roles of LDs in reproduction.

Table I

Conserved and divergent roles of LDs in reproduction.

Biological process	C. elegans	Mammals	Conservation	(Refs.)
LD biogenesis	Regulated by SEIP-1 at ER-LD junctions	Conserved role of SEIP-1 in LD formation	Conserved	(215,216)
LD spatial distribution	Prominent LDs in germline and early embryos	LD-rich oocytes and steroidogenic cells	Conserved	(217,218)
Role in embryogenesis	SEIP-1-dependent LDs support eggshell lipid barrier formation	LDs support early embryonic energy supply and membrane biosynthesis	Functionally conserved	(37,219)
Steroidogenesis	Absent	LDs store cholesteryl esters for steroid hormone synthesis	Divergent	(220,221)
LD-mitochondria interaction	Functional coupling inferred genetically	Physical tethering via PLIN5/MIGA2-mediated contact sites	Partially conserved	(222,223)
Lipid mobilization	Lipolysis supports embryonic viability	β-oxidation fuels oocyte maturation and embryo development	Conserved	(215,224,225)
Regulatory signaling	Genetic pathways linking LDs to development	Nuclear receptor signaling (PPARs) linked to LD-derived lipids	Divergent	(215,226)
Clinical relevance	Model for conserved mechanisms	Direct relevance to infertility and ART outcomes	Divergent	(35,219)

[i] LD, lipid droplet; SEIP-1, seipin; ER, endoplasmic reticulum; PLIN, perilipin; MIGA, mitofusin-γ-interacting ankyrin; ART, assisted reproductive technology.

Sertoli cell-LD axis in supporting spermatogenesis

Sertoli cells provide structural and metabolic support to developing germ cells (46,109-111). Sertoli cells contain abundant LDs, the composition and abundance of which fluctuate in response to the spermatogenic cycle and phagocytic activity (45,46,110). One notable source of LDs in Sertoli cells is the engulfment of residual bodies (cytoplasmic fragments shed by spermatids during final maturation) (17,46,110,112). Lipid-rich components of these residual bodies are internalized and sequestered into LDs within Sertoli cells, where they may be catabolized via β-oxidation or lipophagy to fuel Sertoli cell metabolism (15,17,46).

The dynamic balance between LD formation and degradation in Sertoli cells is key for maintaining testicular homeostasis (103,113,114). Exposure to toxicants such as lead or cadmium disrupts lysosomal and autophagic pathways in Sertoli cells, leading to impaired LD clearance, lipid overload and testicular dysfunction (113). Furthermore, hormonal regulation, particularly by FSH, modulates LD content in Sertoli cells by stimulating lipid uptake and lipogenic gene expression (109). This endocrine-metabolic axis ensures that Sertoli cells are metabolically equipped to support the rapid turnover of lipids during active spermatogenesis (109,111,114).

While rodent models have provided insights into the metabolic coupling between Sertoli and germ cells, key differences exist between human and rodent spermatogenesis (115,116). Human spermatogenesis is characterized by a longer developmental timeline, distinct seminiferous epithelial organization and differences in hormonal regulation and metabolic demands compared with commonly used rodent models (117,118). Moreover, the dynamics of lipid metabolism and LD turnover in human Sertoli cells are less well characterized, in part due to limited access to human testicular tissue (115,117,119).

These species-specific features suggest that, although core principles of Sertoli-germ cell metabolic support are potentially conserved, direct extrapolation from rodent studies to human reproductive physiology should be approached with caution. Integrating findings from human tissue analyses, organoid systems and single-cell profiling is key to define the relevance of LD-mediated mechanisms in human spermatogenesis (120-123).

Lipids and membrane remodeling in sperm maturation

Although mature spermatozoa lack classical LDs (typical cytoplasmic organelles characterized by a hydrophobic core of neutral lipids encased in a phospholipid monolayer), lipid metabolism is key for their structural and functional integrity (43,124). The plasma membrane of sperm is rich in polyunsaturated FAs (PUFAs), which provide membrane fluidity necessary for motility and capacitation (124-130). The composition of these lipids is tightly regulated during epididymal maturation and influenced by prior LD metabolism in germ and Sertoli cells (43,103,124,131).

Enzymes such as acyl-CoA synthetase long-chain family members, which participate in FA activation and incorporation into complex lipids, are key for proper sperm development (75). Genetic disruption of these enzymes leads to altered lipid composition and defective sperm morphology (75). These findings suggest that early LD metabolism indirectly shapes sperm functionality by controlling the availability and remodeling of lipid precursors (43, 75,124).

LD-associated defects and male infertility

Aberrant lipid metabolism in the testis is increasingly implicated in male reproductive disorder (43,104,105,132). In both mouse and rat models and human patients, disturbances in lipid homeostasis, manifested as altered LD dynamics, defective lipolysis or accumulation of cholesteryl esters, are associated with low sperm count, impaired motility and hormonal imbalance (43,104-106,132). Notably, mouse models with HSL knockout exhibit lipid-laden Leydig cells, decreased testosterone levels and oligospermia, underscoring the importance of intact LD mobilization for androgen synthesis and spermatogenic support (106,133,134).

Lipidomics and histological analyses of human testicular biopsies have revealed elevated LD accumulation in Sertoli and Leydig cells of infertile patients, often accompanied by disrupted mitochondrial morphology and increased oxidative stress markers (119,135-137). These observations further reinforce the idea that LD dysregulation contributes to male infertility not only through energy imbalance, but also by disrupting redox homeostasis, membrane remodeling and hormone production (43,104,106,132,138).

LDs in reproductive support cells

Reproductive support cells (Sertoli cells in the testis and granulosa cells in the ovary) serve key roles in nurturing germ cells and regulating the hormonal environment of the gonads (139). Both cell types are metabolically active, highly responsive to hormonal cues and rely on tightly regulated lipid metabolism to perform their functions (140). LDs within these cells serve as key metabolic and regulatory hubs, participating not only in lipid storage and mobilization but also in steroidogenesis, phagocytic recycling and paracrine signaling (6) (Fig. 4).

Figure 4 Comparative functions of LDs in reproductive support cells. (A) In the seminiferous epithelium, Sertoli cells support spermatogenesis through a recycling mechanism. Sertoli cells engulf residual bodies (cytoplasmic remnants) and apoptotic germ cells via phagocytosis. These internalized materials are processed in lysosomes and their lipid content is recycled into LDs. Stored lipids are subsequently mobilized via lipophagy (autophagic degradation) and mitochondrial β-oxidation to generate ATP, providing the energy required for Sertoli cell metabolism and germ cell support. This process is regulated by FSH, which stimulates lipid uptake and storage gene expression. (B) In the ovarian follicle, granulosa cells use LDs primarily as substrate reservoirs for hormone synthesis. Upon stimulation by gonadotropins (LH/FSH), HSL hydrolyzes stored cholesteryl esters into free cholesterol. Cholesterol is transported to mitochondria via the StAR protein at LD-mitochondria contact sites. Inside the mitochondria, CYP11A1 initiates the conversion of cholesterol into steroid hormones (progesterone/estrogen). Granulosa cells also metabolically support the oocyte by transferring pyruvate and lipid intermediates through gap junctions, a process associated with the metabolic status of their own LDs. CYP11A1, cytochrome P450 family 11 subfamily A member 1; FSH, follicle-stimulating hormone; HSL, hormone-sensitive lipase; LD, lipid droplet; LH, luteinizing hormone; StAR, steroidogenic acute regulatory protein; FA, fatty acid.

LDs in Sertoli cells: Nutrient buffering and phagocytic recycling

Sertoli cells are the central architectural and metabolic support for spermatogenesis, forming the blood-testis barrier, providing nutrients to developing germ cells and clearing apoptotic or senescent spermatocytes (139). These cells contain prominent LDs, particularly during periods of active spermatogenic turnover or high phagocytic activity (15).

A notable source of lipids for LD formation in Sertoli cells is the internalization of residual bodies and degenerating germ cells (17). These engulfed materials, rich in membrane and cytoplasmic lipids, are processed in lysosomes, with a portion of the liberated FAs and sterols re-esterified and stored in LDs. These droplets are mobilized for energy production via β-oxidation, allowing Sertoli cells to meet their metabolic demands while supporting adjacent spermatogenic cells (141).

The degradation of LDs in Sertoli cells involves not only cytosolic lipases (HSL) but also selective autophagy of LDs, known as lipophagy (142). This process is regulated by nutrient availability and endocrine factors. Under physiological stress or toxicant exposure (lead, cadmium), lysosomal dysfunction or autophagy impairment leads to LD accumulation, disrupted lipid homeostasis and impaired Sertoli cell function, contributing to testicular atrophy and infertility (143).

Furthermore, hormonal regulation serves a key role in LD dynamics. FSH promotes lipid uptake and storage in Sertoli cells by upregulating lipoprotein receptors and lipogenic gene expression. This hormonally mediated lipid buffering ensures an adequate energy reserve during active spermatogenesis and facilitates metabolic synchronization between Sertoli and germ cells (109,144-146).

LDs in granulosa and luteal cells: Platforms for steroidogenesis

Granulosa cells, which surround and support the developing oocyte, undergo extensive metabolic reprogramming as follicles mature (147). In the pre-ovulatory phase, these cells proliferate, increase steroidogenic activity and accumulate LDs rich in cholesteryl esters (49). Following ovulation, granulosa cells differentiate into luteal cells, which are among the most steroidogenically active cells in the body (147). In both phases, LDs serve as central platforms for steroid hormone biosynthesis (148).

LDs in granulosa and luteal cells store cholesteryl esters that serve as substrates for estrogen and progesterone synthesis, respectively (149). Upon gonadotropin (FSH or LH) stimulation, HSL is activated and hydrolyzes cholesteryl esters into free cholesterol (148). The cholesterol is transported to mitochondria via steroidogenic acute regulatory protein, where it is converted to pregnenolone by CYP11A1, initiating the steroidogenic cascade (149).

Several studies have demonstrated that LD content and associated enzyme expression levels vary depending on follicular stage and endocrine environment (150-153). For example, mature preovulatory granulosa cells contain larger and more numerous LDs than their early antral counterparts, consistent with enhanced steroidogenic readiness (23). Disruption of lipid mobilization pathways via HSL inhibition, PLIN dysregulation or excessive lipid accumulation impairs estrogen and progesterone synthesis, follicular rupture and corpus luteum formation (150).

Proteomic and lipidomic profiling of granulosa cell LDs has identified key components of the steroid biosynthetic machinery localized to or enriched around LDs, including enzymes of the P450 family and mitochondrial contact proteins (90). These findings suggest that LDs are not passive lipid depots, but serve as biochemical scaffolds that facilitate rapid and localized hormone production (148).

LD-oocyte communication: The somatic-germ cell interface

Beyond their intrinsic functions, support cell LDs also influence germ cell development through metabolic coupling and paracrine interactions (154). Cumulus granulosa cells, which form the innermost layer of follicular somatic cells directly surrounding the oocyte, are metabolically connected to the oocyte via transzonal projections and gap junctions (155). These cells actively metabolize glucose and FAs, generating pyruvate, amino acids and lipid intermediates that are transferred to the oocyte to support growth and maturation (28,154).

LDs within cumulus cells reflect this metabolic activity (156). Their number and size increase with gonadotropin stimulation and they show dynamic responses to oxidative stress, FA exposure and endocrine disruption (157). Excess accumulation of saturated FAs in cumulus cell LDs, such as under high-fat diet or PCOS, alter oocyte lipid composition, increase ER stress and reduce developmental competence (156).

Therefore, somatic cell LDs contribute indirectly to oocyte quality by modulating the follicular lipid environment, buffering toxic lipid species and fine-tuning substrate availability for oocyte maturation (157). Disruption of these functions leads to broader metabolic dysfunction and subfertility, highlighting the need to consider support cell lipid metabolism in the assessment of reproductive health (89).

LDs in early embryo development and pluripotency

Early embryonic development is an energy-intensive process that depends on maternally derived reserves, since transcriptional activity is minimal until zygotic genome activation (37). Among the maternally supplied nutrients, LDs serve as critical reservoirs of neutral lipids and bioactive lipids that sustain cleavage division, lineage specification and metabolic reprogramming (69). Beyond their role as passive energy stores, LDs are increasingly recognized as dynamic organelles that integrate metabolic and signaling pathways, influencing not only embryo viability but also the maintenance of pluripotency (37,69,75,158) (Fig. 5).

Figure 5 Spatiotemporal dynamics and functional roles of LDs during preimplantation development. (A) Developmental timeline (zygote to blastocyst). Following fertilization, the zygote inherits a pool of maternal LDs. During early cleavage divisions, LDs undergo dynamic clustering (typically perinuclear) and partial lipolysis. They interact with mitochondria to provide fatty acids for β-oxidation, fueling the energy-intensive process of rapid cell division. At the blastocyst stage, LDs exhibit asymmetric distribution. TE cells (outer layer) contain larger and more numerous LDs, supporting their high lipid metabolic needs for implantation and steroidogenesis. By contrast, the ICM (inner cluster) contains fewer LDs, consistent with a glycolytic, pluripotent metabolic state. (B) Functional mechanisms. LDs act as detoxification sinks by sequestering toxic saturated fatty acids and peroxidized lipids, thereby protecting the embryo from lipotoxicity and oxidative stress (ROS) when antioxidant defenses are developing. In pluripotent cells, LD-derived acetyl-CoA enters the nucleus to serve as a substrate for histone acetylation. This metabolic-epigenetic link influences chromatin structure and gene expression, thereby regulating the maintenance of pluripotency and stem cell fate decisions. ICM, inner cell mass; LD, lipid droplet; ROS, reactive oxygen species; TE, trophectoderm; CPT, carnitine palmitoyltransferase.

LD inheritance and remodeling post-fertilization

Upon fertilization, the zygote inherits LDs from the oocyte, which vary in size, number and lipid composition across species (72). Following fertilization, LDs transition toward a metabolic phase characterized by enhanced lipolysis and FA oxidation (FAO). In lipid-rich species such as pigs and cows, zygotes contain abundant LDs that are readily visualized by light microscopy, while in mice and humans LDs are smaller (159). These maternal LDs are progressively redistributed during the first cleavage divisions (42). Time-lapse imaging studies reveal that LDs undergo dynamic clustering, dispersion and partial lipolysis in synchrony with cell cycle progression (158,160-162). This remodeling reflects a shift from maternal lipid storage toward active use, enabling the embryo to meet energetic demands of rapid mitosis (31). Beyond serving as lipid reservoirs, LDs in early embryos also participate in the storage and regulation of non-lipid macromolecules. Johnson et al (126) demonstrated that LDs in early embryos serve as storage depots for maternal histones and selectively recruit RNA-binding proteins involved in post-transcriptional regulation. Through this mechanism, LDs contribute to the temporal control of mRNA translation during early embryogenesis, a developmental window characterized by limited zygotic transcription. These findings expand the functional scope of LDs beyond energy metabolism, positioning them as organizational platforms that coordinate lipid storage with proteostasis and translational control in the early embryo.

The extent of LD remodeling is sensitive to culture conditions and external nutrient availability (163). Embryos cultured in vitro often display altered LD number and distribution compared with in vivo counterparts, which has been linked to reduced developmental potential (164). These findings highlight LDs as sensitive indicators of embryo metabolic state (71).

LD metabolism and early energy supply

During cleavage, embryos rely primarily on pyruvate and lactate metabolism, but FAO becomes increasingly important as development proceeds to the blastocyst stage (49). LDs provide a readily accessible pool of FAs through controlled lipolysis (165). Inhibition of FAO enzymes such as carnitine palmitoyltransferase 1 (CPT1) leads to developmental arrest, underscoring the importance of LD-derived FAs in sustaining blastocyst formation (166).

Mitochondrial-LD interactions are central to this process (37). In mouse and bovine embryos, LDs are typically found close to mitochondria, facilitating efficient FA transfer and oxidation (13). This spatial proximity suggests that LDs and mitochondria form functional units that coordinate energy production and redox regulation during early development (167).

An additional context in which LD-mitochondria interactions may be relevant is ovarian aging. Advanced maternal age is associated with progressive mitochondrial dysfunction in oocytes, including decreased oxidative capacity, altered mitochondrial dynamics and increased oxidative stress (168,169). Emerging evidence suggests that aged oocytes often display abnormal LD accumulation and altered lipid distribution, raising the possibility that impaired mitochondrial FA utilization contributes to LD dysregulation during aging (69,170,171).

Whether aberrant LD accumulation in aged oocytes represents a compensatory response to mitochondrial insufficiency or a maladaptive process that compromises oocyte quality remains unresolved. Clarifying how mitochondrial dysfunction intersects with LD storage, mobilization and oxidative stress control during ovarian aging is key for understanding age-associated declines in oocyte competence and embryo developmental potential.

In addition to their roles in energy metabolism and signaling, LDs have recently been implicated in protecting embryos from lipid peroxidation and ferroptotic cell death (172-174). PUFAs, while essential for membrane synthesis and developmental signaling, are susceptible to oxidative damage. Sequestration of PUFAs within LDs limits their availability for uncontrolled lipid peroxidation, thereby decreasing oxidative stress and ferroptosis during early embryonic development (174-177).

Recent studies have highlighted LDs as key buffers that spatially compartmentalize PUFAs away from pro-oxidant environments, particularly under conditions of heightened mitochondrial activity and reactive oxygen species production (175,176,178). This protective function may be key during early embryogenesis, when antioxidant capacity is limited and metabolic demands rapidly increase. Together, these findings position LDs not only as metabolic hubs but also as key guardians of redox homeostasis and embryo survival.

Carnitine shuttle system as the rate-limiting step of LD-derived FAO

While FA β-oxidation is a notable pathway through which LD-derived lipids support reproductive processes, its flux is constrained by a rate-limiting step: The transport of long-chain FAs into mitochondria (179-181). This process is mediated by the carnitine shuttle system, which consists of CPT1 on the outer mitochondrial membrane and CPT2 on the inner membrane (2,179,182,183). CPT1 catalyzes the conversion of long-chain acyl-CoAs into acyl-carnitines, enabling their translocation across the mitochondrial membranes, whereas CPT2 reconverts acyl-carnitines into acyl-CoAs within the mitochondrial matrix for β-oxidation (180,184-186).

In oocytes and early embryos where mitochondrial oxidative capacity is tightly coupled with developmental competence, disruption of carnitine-dependent FA transport leads to impaired energy production and developmental arrest, underscoring the importance of this regulatory checkpoint in lipid utilization (49,166). Notably, supplementation with L-carnitine or acetyl-L-carnitine improves mitochondrial function, enhances FAO and increases blastocyst yield in multiple assisted reproduction settings (187,188). These findings suggest that the carnitine shuttle not only represents a biochemical bottleneck of LD-derived energy metabolism, but also constitutes a clinically actionable node linking LD biology to gamete quality and embryo developmental outcomes.

LDs as regulators of redox homeostasis and stress responses

Beyond energy supply, LDs also protect early embryos from lipotoxicity and oxidative stress (2). Excess accumulation of saturated FAs in culture or maternal metabolic disorders can trigger ER stress and apoptosis in embryos (69). By sequestering potentially toxic lipids into LDs, embryos buffer against lipotoxic insult (189). Moreover, LDs can serve as sinks for peroxidized lipids, thereby limiting propagation of oxidative damage (190). This detoxification role is key during preimplantation, when embryonic antioxidant defenses are developing (161).

LD dynamics and lineage specification

As embryos reach the blastocyst stage, LD distribution becomes asymmetric between inner cell mass (ICM) and trophectoderm (TE) (42). TE cells, which contribute to the placenta, contain more and larger LDs than ICM cells, suggesting lineage-specific metabolic requirements (42,158,191). This differential LD allocation may reflect the TE role in steroidogenesis, nutrient transport and implantation, which demand robust lipid metabolism (158).

In pluripotent ICM cells, LDs are fewer but metabolically active, supporting a glycolysis-dominant phenotype typical of stem cells (192). Manipulating LD metabolism in vitro influences stem cell fate: Pharmacological activation of lipolysis promotes differentiation, while stabilizing LDs maintains pluripotency (193). These findings position LDs as regulators of cell fate transitions in the embryo (194).

Implications for assisted reproduction and stem cell biology

Understanding LD biology in embryos has practical implications (72). In assisted reproductive technology, embryo selection is typically based on morphology, yet LD content and dynamics may provide more sensitive biomarkers of viability (31). Non-invasive imaging techniques such as CARS microscopy have been applied to quantify LDs in live embryos, revealing an association between lipid distribution and implantation success (160).

In stem cell biology, insights into LD regulation in early embryos inform strategies to maintain pluripotency in vitro and direct differentiation (49). For example, modulating lipid availability or LD turnover influences epigenetic programming, since acetyl-CoA and lipid-derived metabolites serve as cofactors for chromatin-modifying enzymes. Thus, LDs not only support metabolic needs but also contribute to epigenetic landscapes that govern developmental trajectories (195).

Key regulators of LD biology in reproductive cells

The biology of LDs in reproductive systems is orchestrated by a complex network of enzymes, structural proteins, transcriptional factors and signaling pathways (6). These regulators not only govern LD biogenesis and turnover but also link lipid metabolism to the unique demands of gametogenesis, steroidogenesis and embryogenesis (49). Their coordinated activity ensures that lipid reserves are properly balanced between storage and mobilization, safeguarding reproductive success (69) (Fig. 6).

Figure 6 Molecular landscape of LD regulation in reproductive cells. Integrated network of enzymes, structural proteins and signaling pathways orchestrate LD biology, ensuring the balance between lipid storage and utilization. LD formation originates at the ER. DGAT1/2 catalyze the synthesis of TAGs, while ACATs synthesize CEs. These neutral lipids are packaged into the nascent LD core, establishing the metabolic reserves required for gametogenesis and steroidogenesis. The LD surface is coated by PLIN family proteins. PLIN2 stabilizes the LD by preventing uncontrolled lipolysis (crucial for oocyte lipid retention), while PLIN3 supports cholesterol storage in steroidogenic cells. These proteins serve as gatekeepers, regulating access to the lipid core. Controlled lipid breakdown is mediated by lipases. ATGL initiates TAG hydrolysis to release fatty acids for mitochondrial β-oxidation. HSL, activated via phosphorylation, hydrolyzes TAGs and CEs, liberating cholesterol for steroid hormone synthesis. The entire system is governed by upstream regulators. Hormones such as FSH promote lipid uptake/storage, whereas LH triggers lipolysis by activating HSL. At the nuclear level, transcription factors SREBP and PPAR modulate the expression of lipogenic and oxidative genes, respectively, adapting cell metabolism to developmental demands. ACAT, acyl-CoA cholesterol acyltransferase; ATGL, adipose triglyceride lipase; DGAT, diacylglycerol acyltransferase; ER, endoplasmic reticulum; FSH, follicle-stimulating hormone; HSL, hormone-sensitive lipase; LH, luteinizing hormone; PLIN, perilipin; PPAR, peroxisome proliferator-activated receptor; SREBP, sterol regulatory element-binding protein; LD, lipid droplet; TAG, triacylglycerol; CPT, carnitine palmitoyltransferase; CE, cholesteryl Ester.

Enzymes of neutral lipid synthesis

LD biogenesis begins with the synthesis of neutral lipids within the ER, a process primarily mediated by enzymes such as DGAT1 and DGAT2 for triacylglycerols and acyl-CoA cholesterol acyltransferases (ACATs) for cholesteryl esters (140). In reproductive cells, these enzymes facilitate metabolic readiness: In oocytes, DGAT2 activity underlies the accumulation of lipid stores necessary for meiotic progression (196), while in granulosa and luteal cells, ACAT-mediated cholesterol esterification establishes a pool of precursors for steroid hormone synthesis (149). Disruption of these synthetic pathways reduces oocyte competence and compromises hormone production, highlighting their key role in reproductive physiology (150).

LD-coating and stabilizing proteins

Once formed, LDs are stabilized and functionally tuned by coat proteins, most prominently the PLIN family (197). PLIN2, highly abundant in lipid-rich oocytes and granulosa cells, prevents uncontrolled lipolysis and ensures lipids remain available for developmental cues (37,48,68). PLIN3, enriched in steroidogenic cells, contributes to cholesterol storage and supports rapid progesterone synthesis following LH stimulation (22,24,47). These surface proteins not only regulate LD size and stability but also serve as molecular scaffolds that recruit lipases or tether LDs to partner organelles, integrating storage with utilization (2).

Lipolytic enzymes

Mobilization of LD lipids is mediated by lipolytic enzymes, whose activity is coupled to reproductive events (148). ATGL initiates triacylglycerol breakdown, providing FAs for mitochondrial β-oxidation, which is key for oocyte maturation and embryonic cleavage (166). HSL plays a dual role, hydrolyzing both TAGs and cholesteryl esters. In luteal cells, LH-induced phosphorylation of HSL rapidly liberates cholesterol for progesterone synthesis, directly linking lipolysis to endocrine function (147). Failure of these pathways, as seen in mouse knockout models of ATGL or HSL deficiency, results in lipid accumulation, impaired gametogenesis and subfertility (106,133).

Transcriptional and hormonal regulators

The transcriptional and hormonal control of LD regulators provides another layer of coordination (143). Sterol regulatory element-binding proteins activate the expression of lipogenic enzymes, ensuring reproductive cells adapt to energy demand and steroidogenic flux (198). PPARs fine-tune lipid storage and oxidation, with PPARγ promoting lipid accumulation in granulosa cells and PPARα enhancing FA use in embryos (199). Endocrine signals, particularly FSH and LH, further dictate LD dynamics: FSH promotes lipid uptake and storage in Sertoli and granulosa cells, while LH triggers lipolysis and cholesterol mobilization, exemplifying how systemic hormonal cues converge on LD regulation (20,109,200).

LD dysfunction and reproductive disorders

The regulation of LD biology is key for reproductive competence and its disruption is increasingly recognized as a pathogenic factor in infertility and reproductive disease (6,37,41). Aberrant LD accumulation, altered lipid composition or defective mobilization disturb gametogenesis, hormone production and embryonic development, ultimately compromising reproductive outcomes (49) (Fig. 7).

Figure 7 LD dysfunction as a driver of reproductive pathology. (A) Obesity causes lipid overload in granulosa cells and oocytes. Under conditions of obesity or dietary excess, high levels of free fatty acids cause lipid overload in granulosa cells and oocytes. This leads to mitochondrial dysfunction and the generation of ROS. The resulting metabolic stress impairs steroidogenic signaling and disrupts follicle development, culminating in follicular arrest and phenotypes associated with PCOS. (B) Toxicants disrupt LD homeostasis in the testis. Exposure to environmental toxicants (BPA, phthalates) disrupts LD homeostasis in the testis. In Sertoli cells, toxicants block lysosomal/autophagic pathways, preventing the clearance of phagocytosed lipids. This leads to the accumulation of phagocytic LDs and impairs the nutritional support provided to germ cells. In Leydig cells, disruption of lipolysis (HSL inhibition) prevents cholesterol mobilization, resulting in decreased testosterone synthesis (hypogonadism) and germ cell apoptosis. (C) Lipotoxicity leads to developmental arrest in embryos. In the context of IVF or maternal metabolic disorder, embryos may exhibit uneven distribution of LDs. The accumulation of peroxidized lipids creates a state of lipotoxicity and oxidative stress. These cell insults compromise blastocyst quality, leading to developmental arrest, decreased viability and implantation failure. BPA, bisphenol A; HSL, hormone-sensitive lipase; IVF, in vitro fertilization; LD, lipid droplet; PCOS, polycystic ovary syndrome; ROS, reactive oxygen species.

Excessive lipid storage or impaired lipid utilization is associated with ovarian pathologies (150). Patients with PCOS, one of the most common causes of infertility, display abnormal LD accumulation and dysregulated lipid metabolism in granulosa cells (156). This metabolic imbalance alters steroidogenic signaling, impairs follicle development and reduces oocyte competence (89). Similarly, maternal obesity and diabetes are associated with excessive LD deposition in oocytes and embryos, resulting in lipotoxicity, mitochondrial dysfunction and decreased embryo viability (201). These findings suggest that LD mismanagement contributes to the metabolic-reproductive interface that underlies certain female infertility syndromes (38).

In the male gonad, LD dysfunction also exerts notable effects (143). Sertoli cells, which provide structural and metabolic support for spermatogenesis, rely on LD turnover to supply energy substrates to developing germ cells (139). Disruption of lipolysis or autophagic clearance in these cells leads to LD accumulation, impaired nutrient transfer and defective sperm maturation (141). Leydig cells depend on LDs for cholesterol ester storage and rapid mobilization following LD stimulation; defects in this system decrease testosterone synthesis, contributing to hypogonadism and subfertility (202). Moreover, environmental toxicants such as phthalates and bisphenol A perturb LD homeostasis in testicular cells, linking lipid dysregulation to environmentally induced reproductive disorder (203).

Beyond the gonads, LD abnormality influences early developmental competence (69). In vitro fertilization studies reveal that embryo with excessive or uneven LD distribution exhibit lower developmental potential, potentially due to oxidative stress and impaired energy regulation (31,41,69,204). Defects in LD-associated proteins, including PLIN and SEIP-1, cause embryonic lethality or reduced implantation success, underscoring the importance of regulated LD dynamics for reproductive success (205).

Together, these findings position LD dysfunction as both a biomarker and a mechanistic driver of reproductive pathology (2). By disrupting lipid homeostasis, LD abnormality compromises gamete quality, hormone production and embryo viability (37). Clarifying the molecular underpinnings of LD dysfunction in reproductive disorders holds promise not only for understanding infertility but also for identifying novel diagnostic markers and therapeutic targets (206).

Technological advances in studying LDs in reproduction

The study of LDs in reproductive biology has benefited from technological innovations that allow improved resolution, sensitivity and functional insight (6). Traditional staining approaches, such as Oil Red O and BODIPY dyes, provided the first visualization of LDs in oocytes and embryos (207), but advances in imaging and molecular profiling permit dynamic and quantitative analyses of LD biology within reproductive contexts (69).

High-resolution microscopy has been key to these advances (13). Confocal and two-photon microscopy enable three-dimensional imaging of LD distribution in intact oocytes and embryos (31), while super-resolution techniques such as stimulated emission depletion and structured illumination microscopy reveal nanoscale details of LD-organelle interactions. Live-cell imaging with fluorescently tagged LD-associated proteins makes it possible to track LD dynamics during meiotic maturation, fertilization and early embryogenesis, providing functional insight into how LD turnover couples with developmental transition (160).

Complementing imaging, mass spectrometry-based lipidomics has transformed the characterization of LD composition in reproductive tissue (208). Shotgun and targeted lipidomics approaches identify neutral lipid species, cholesterol esters and signaling lipids within LDs, enabling the detection of metabolic alterations in pathological states such as PCOS or maternal obesity. Single-cell and spatial lipidomics approaches have been applied to oocytes and embryos, resolving metabolic heterogeneity that underlies differences in developmental competence (209-212).

Genetic and molecular tools have also advanced functional studies (37,213,214). CRISPR-Cas9 editing in mice and C. elegans allows precise manipulation of LD-associated genes, demonstrating their roles in gametogenesis and embryonic development (205). Fluorescent reporters, such as PLIN or SEIP-1 fusion proteins, permit in vivo visualization of LD subsets and their dynamic responses to hormonal or metabolic cues (196). In parallel, optogenetic and inducible systems are powerful approaches to manipulate LD formation or lipolysis in real time, offering a causal understanding of LD function in reproductive processes (167).

Finally, integrative approaches that combine imaging, lipidomics and systems biology are beginning to redefine the landscape of LD research in reproduction (2). Spatial multi-omics platforms map LDs in the context of transcriptional and metabolic states (42), while computational models help predict how LD dysfunction develops into cell stress and developmental failure (192). Such integrative technologies not only provide mechanistic understanding but also hold promise for translational applications, such as developing LD-based biomarkers of oocyte quality or embryo viability in assisted reproductive technology (89).

In sum, methodological advances have propelled LD research from descriptive observations to mechanistic and translational insight (49). By coupling dynamic visualization with molecular profiling and functional manipulation, these technologies reshape understanding of how LDs support reproductive success and how their dysfunction contributes to infertility (150).

Conclusion

LDs are multifaceted organelles that are key for reproductive physiology. Beyond their traditional role as passive lipid stores, LDs serve as dynamic hubs integrating energy supply, membrane biosynthesis, steroidogenesis and stress adaptation. From oocyte maturation to early embryonic development, their regulated formation, turnover and organelle interactions ensure reproductive success. Dysregulation of LD biology, conversely, contributes to a number of disorders including PCOS, male infertility, and embryonic developmental defects, underscoring their centrality to reproductive health.

Mechanistically, it is not fully understood how LD subpopulations are formed or differentiated (based on size, lipid composition or functional specialization) in reproductive cells and how their interactions with mitochondria, ER and lysosomes are spatiotemporally regulated. The contribution of lipid signaling molecules released from LDs to epigenetic programming and lineage specification during early embryogenesis also remains largely unexplored. Moreover, the interplay between systemic metabolic disorders, such as obesity and diabetes, and LD dysfunction in gametes and embryos requires investigation to explain the intergenerational transmission of reproductive risk.

Future research may benefit from continued integration of cutting-edge technologies. Advances in live-cell super-resolution microscopy, spatial and single-cell lipidomics and CRISPR-based gene editing provide powerful opportunities to dissect LD biology. LD-based biomarkers hold promise for assessing oocyte and embryo quality in assisted reproductive technology, while therapeutic strategies targeting LD metabolism may emerge as novel approaches to treat infertility associated with metabolic or endocrine dysfunction. Importantly, comparative studies across species, from C. elegans and mice to humans, may demonstrate the evolutionary conservation and divergence of LD functions in reproduction.

In conclusion, LDs represent a dynamic interface between lipid metabolism and reproductive biology. By bridging basic mechanistic insights with clinical applications, LD research may deepen understanding of fundamental cell biology but also to transform reproductive medicine.

Availability of data and materials

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Authors' contributions

LP conducted the literature review and wrote the manuscript. ZW constructed figures. YJ wrote the manuscript and provided supervision. All authors have read and approved the final manuscript. Data authentication is not applicable.

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Patient consent for publication

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Competing interests

The authors declare that they have no competing interests.

Use of artificial intelligence tools

During the preparation of this work, the authors used artificial intelligence-assisted tools (including Grok) to improve the readability and language of the manuscript, to draft the cover letter and to propose a running title. No artificial intelligence tool was used to generate scientific content, analyse or interpret data, draw scientific conclusions or create figures. After using these tools, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.

Acknowledgements

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Funding

The present study was supported by Shandong Provincial Medical and Health Science and Technology Project (grant no. 202402081168), Ji Nan Health High-Caliber Talent Project (grant no. 202512), Science and Technology Development Plan Project of Jinan Municipal Health Commission (grant no. 2024302002), the National Natural Science Foundation of China (grant no. 82401009), Shandong Provincial Natural Science Foundation (grant no. ZR2025MS1456), Science and Technology Development Program of Jinan Municipal Health Commission (grant no. 2024202001) and the Research Start-up Fee for Introducing Talents to Jinan Central Hospital (grant nos. YJRC2023001 and YJRC2023005).

References