According to their histological appearance, lesions that present in the epithelium during the process of carcinogenesis may be classified according to their reactive epithelial changes (such as hyperkeratosis, hyperplasia and acanthosis) or preneoplastic changes (including mild, moderate and severe dysplasia; Fig. 1B) (16) prior to the establishment of an invasive carcinoma (12,14,17). Oral cancer originates as an epithelial dysplasia and is characterized by the altered proliferation of dysplastic squamous cells on the surface of the epithelial layer, which subsequently degrades the subepithelial basement membrane (BM). Degradation of the BM results in local destruction and distant invasion via metastasis. Local invasion to the underlying tissue occurs via the islets and cords of epithelial cells (18).

The ability to metastasize is directly associated with the differential grade of tumor cells, similar to that of the neoplastic tissue architecture and normal epithelium (14).

Currently, two systems are used to histologically classify tumor lesions; the International Histological Classification of Tumors (Fig. 1C–E) and the pattern of the TIF (19). The initial classification of lesions is based on the degree of tumor differentiation (well-, moderately- and undifferentiated) (20), which is essential to evaluate the tumor’s growth rate and ability to metastasize (14).

The TIF constitutes the area of the lesion with the greatest depth of invasion and progression into the surrounding tissues (21). In addition, the cells of the TIF have differing molecular characteristics when compared with the cells at the superficial areas of the tumor (10,22). The TIF is considered to be the most representative area of the tumor (23) and is identified by four characteristics; the degree of keratinization, nuclear polymorphism, lymphocytic infiltration and pattern of invasion (PI) (23,24). Of these, the PI is considered to be a good prognostic factor in OSCC (1). To evaluate the severity of the invasion, several morphological criteria exist, associated with certain PIs, according to the following three categories (Fig. 1F–I): i) Islet-infiltrating cells with wide fronts of invasion; ii) thin infiltrating cords; and iii) individual infiltrating cells (1).

In the clinical field, the majority of medical centers base their decisions upon the clinical and pathological information. The TNM stage (T, tumor size; N, regional lymph node compromise; and M, metastasis) (25) and the degree of tumor differentiation (20), combined with the patient’s health status, are the predominant factors that determine the therapeutic strategy. To advance the knowledge of OSCC, numerous pathological and molecular clinical markers have been identified for the prediction of prognosis (1).

Ki-67 and p53 are the most commonly used tumor markers for studying cell proliferation. The p53 protein is one of the transcription factors that is implicated in cell cycle control, apoptosis and preservation of genetic stability (28). In addition, the p53 gene is one of the most commonly mutated genes in OSCC with mutations detected in >50% of OSCC cases (29). The activation of p53 has been reported in a number of processes, such as DNA damage, hypoxia and oncogene activation. In addition, p53 protects against tumor formation by preventing the accumulation of cells with DNA damage, which subsequently induces a loss of function in the majority of malignant neoplasms (30). Although not completely understood, Ki-67 is considered to be an important protein in cell division, as it has been observed that the antigen is expressed primarily during the cell cycle stages of G1, S, G2 and M, with a marked emphasis on the M phase. However, Ki-67 expression is not observed during the G0 phase and has a low expression in the G1 and S phases (31). Furthermore, Ki-67 is considered to be one of the best predictors of survival (Fig. 1J and K) (16) and recurrence (5).

Recently, novel markers have been used to assess morphogenesis and cell differentiation. Previous studies have demonstrated that the aberrant expression of genes is associated with cancer embryogenesis, particularly the HOX genes that may induce embryological development, as well as contribute to the onset and progression of tumors (32,33). Furthermore, HOX gene overexpression has been associated with carcinogenesis, including head and neck neoplasms (34) and HOXB7, a member of the family of homeodomain transcription factors, is a critical regulator of development, controlling the proliferation and survival of progenitor cells. In OSCC, HOXB7 is overexpressed (Fig. 1L and M) (32), which has been confirmed to be associated with a poor prognosis in OSCC and other types of cancer (32,35).

Infiltration is a key prerequisite for cancer metastasis, making it a significant factor in the prognosis of patients with OSCC (36). For the activation of the process, degradation of the BM must occur between the epithelium and lamina propria, which is located around the nest of cancer cells and blood vessels. The BM has been identified as a crucial structure in the regulation of tumor invasion. Its molecular assembly is a barrier for the invasion of the connective tissue, in particular of the epithelial cells, unless a molecular rupture occurs (37).

ColIV is the most important protein component of the BM and its integrity is altered by the degradation of the BM via matrix metalloproteinases (MMP) 2 and 9 that are present in OSCC (Fig. 1N) (38) and the surrounding tissues (36). Furthermore, MMP 2 and 9 facilitate the development of lymph node metastases (38,39). Therefore, monitoring the changes in the expression of ColIV may have prognostic value in OSCC patients (36,40).

Despite a marked recruitment of immune cells in the TME, immune cells do not represent the main population of tumor stromal cells; CAFs are the most abundant cells of the TME. CAFs are generally identified by the expression of α-smooth muscle actin, which is similar to the expression of myofibroblasts that occurrs at the site of wound healing and chronic inflammation, however, is absent in normal skin fibroblasts (44,45).

CAFs may be locally differentiated from normal fibroblasts or surrounding stromal stem cells that are derived from the mesenchymal cells of bone marrow, which is recruited by the tumor (46). The tumor stroma is rich in CAFs, which may be scattered or found in the tumor periphery. Certain evidence indicates that CAFs mechanically reshape the extracellular matrix, via the use of proteases, to facilitate the invasion of cancer cells (4). Previous studies have also demonstrated the existence of a molecular dialogue between CAFs and tumor cells, the latter of which secrete interleukin 1α, which stimulates the secretion of chemokine (CC motif) ligand 7 from the CAFs, resulting in tumor progression (6). The increased presence of CAFs observed in OSCC has been associated with a diffuse invasion pattern, preparing the environment for tumor invasion and metastasis (47), and is associated with a poor prognosis (48).