Cervical intraepithelial neoplasia
The cervical epithelium is derived from two embryologically distinct sources. The portio vaginalis of the cervix is covered by nonkeratinised stratified squamous epithelium, similar to the vaginal epithelium. Mucus-secreting columnar cells of the same embryological derivation as the uterine endometrium cover the endocervical canal. At birth there is an abrupt junction between the original squamous epithelium and the columnar epithelium of the endocervix, known as the original squamo-columnar junction (OSCJ).
In the majority of cases, this junction is on the ectocervix, but may be on the vagina, particularly in diethylstilbestrol (DES) exposed women. At about one year of age, the cervix begins to elongate resulting in the migration of the squamocolumnar junction towards the external os. At the time of menarche or during pregnancy, both the uterus and cervix enlarge. Enlargement of the cervix is accompanied by alterations in its shape, resulting in greater eversion of the endocervical columnar epithelium towards the vagina.
Over time, the columnar epithelium exposed on the portio vaginalis of the cervix is remodelled and replaced by metaplastic squamous epithelium. As this occurs, the original squamocolumnar junction (OSCJ) moves towards the endocervical os or endocervical canal, creating a new SCJ. The area between the original and the new SCJs is called the transformation zone (TZ) and is characterised histologically by metaplastic epithelium.
The concept of the TZ is central to the understanding of th pathogenesis of squamous cell cancer of the cervix and its precursors, because virtually all cervical squamous neoplasia originates at the new SCJ and coincides with the distribution of the TZ. In addition, in reproductive life, the TZ is located on the exposed portion of the cervix and is amenable to cytological and histological sampling and colposcopic examination. While the TZ is difficult to visualise with the naked eye, its localisation is greatly enhanced by the application of 5% acetic acid and the use of the colposcope.