The difference between cervical transformation zone and squamocolumnar junction..

A representative parous adolescent cervix showing the outer boundaries of several important areas of exocervical epithelium that correspond to the histological representations in Figure 2. External os (os), new squamocolumnar junction (nscj), area of cervical ectopy (ect) between nscj and external os, cervical transformation zone (ctz) between the original squamocolumnar junction (oscj) and nscj that includes an area of immature metaplasia (im), which is lightened by the application of 5% acetic acid

The terms "transformation zone" and "squamocolumnar junction" are frequently used interchangeably in the literature. However, these are two distinct entities. The squamocolumnar junction is the area in which the squamous epithelium of the ectocervix meets the columnar epithelium of the endocervix. The cervical transformation zone is a dynamic entity of metaplasia throughout a women’s life and is histologically the area where the glandular epithelium has been replaced by squamous epithelium and it is the site of carcinogenesis mediated by infection with oncogenic subtypes of HPV. Thus, the squamocolumnar junction is part of the transformation zone, but the transformation zone comprises a larger area than just the squamocolumnar junction.

NB: Some data suggest that the primary site of carcinogenic HPV-related CIN and cervical cancer is not the entire transformation zone, but is a small population of cuboidal cells at the squamocolumnar junction. This group of cells has a unique gene expression profile similar to that found in squamous and glandular high-grade CIN and carcinomas.