“Metaplasia” is the term used by the pathologists and “transformation” is the word used by colposcopists. The squamocolumnar junction is a dynamic area and keeps changing in the different periods of a women’s life. It moves outwards at early neonatal life, puberty and after childbirth trauma. As a result, the columnar epithelium gets exposed to the acidic pH of the vagina leading to proliferation of subcolumnar reserve cells. These cells multiply and form a layer that eventually produces a normally differentiating squamous epithelium. Morphologically, the reserve cells have a similar appearance to the basal cells of original squamous epithelium, with round nuclei and a little cytoplasm. The newly formed metaplastic epithelium has no stratification and is called immature metaplasia. It does not contain glycogen and appears faint white after acetic acid application; it also does not take up a brown stain after Lugol’s iodine. Squamous metaplasia usually begins at the original squamocolumnar junction at the distal limit of the ectopy, but it may also occur in the columnar epithelium. As the process continues, the immature metaplastic squamous cell differentiates into mature stratified metaplastic epithelium.

It is within this area between the new and the old squamocolumnar junction that all the dynamic physiological and pathological processes occur. During early stages of metaplasia, the epithelium may be vulnerable to the genetic change that results in a cell population that somehow acquires a neoplastic potential and produces the atypical transformation zone.

The process of metaplasia is also associated with compression of the capillary structures of the stroma in the villi leading to reduction in their height, ultimately forming a network under the epithelium. This is indistinguishable from the capillary network of normal squamous epithelium. In the early stage, there is loss of translucency of the top of the villus and later on there is fusion of villi due to rapid division of reserve cells This leads to obliteration of original humps of columnar villi which now appears as smooth, light pink epithelium. During the process of squamous metaplasia, the outlets of the crypts of columnar epithelium, not yet covered by the metaplastic epithelium, remain as persistent crypt openings. These gland openings usually mark the farthest extent of the transformation zone.

Nabothian cysts develop as a result of the occlusion of the cyst opening by the overlying metaplastic squamous epithelium leading to entrapment of the mucus produced by the columnar cells.

The original squamous epithelium is derived from the epithelium of urogenital sinus and begins at the vulvovaginal line. It lines the vagina and the ectocervix, whereas the endocervix is lined by the columnar epithelium. The location of squamocolumnar junction in relation to the external os depends on the age, hormonal status, oral contraceptive use, pregnancy and birth trauma. Prior to puberty, the original squamocolumnar junction is located at or very close to the external os. During the reproductive years, the female genital tract remains under the influence of oestrogen resulting in elongation of the endocervical canal thereby everting the columnar lining of the endocervical canal on to the ectocervix. This shifts the original squamocolumnar junction away from the external os. The everted columnar epithelium under the influence of vaginal acidity eventually gets destroyed and is replaced by the metaplastic squamous epithelium. Thus, a new squamocolumnar junction is formed between the newly formed metaplastic squamous epithelium and the columnar epithelium. As the women progresses towards menopause, the location of new squamocolumnar epithelium moves on the ectocervix towards the external os (Fig. 8.4). The area between the original and new squamocolumnar junction is known as the transformation zone.

It is within this area that all the dynamic physiological and pathological processes occur. During early stages of metaplasia, the epithelium may be vulnerable to the genetic change that results in a cell population that somehow acquires a neoplastic potential and produces the atypical transformation zone.

The colposcopic findings are described keeping in mind the following criteria: