At birth there is an abrupt junction between the squamous epithelium of the ectocervix (the original squamous epithelium) and the columnar epithelium of the endocervix. Through exposure to estrogen (at birth, during puberty, and throughout reproductive life), the glycogen in the exfoliated cells of the vagina is converted into lactic acid, accounting for the acidity of the vaginal secretions (pH < 4.5). This acidity, along with other factors, stimulates the replacement of the columnar epithelium with squamous epithelium. This process is known as metaplasia. Metaplasia results in the formation of a new SCJ. The area between the original SCJ and the new SCJ is known as the transformation zone. Metaplastic changes usually start from the periphery of the ectropion and spread towards the external os. Changes can also occur in discrete patches on the columnar epithelium. The new squamous cells originate from the totipotent “reserve” cells that remain dormant beneath the columnar cells.
The process of metaplasia starts with fusion of the tips of the villi, followed by covering of the columnar epithelium by thin transparent immature epithelium. The immature epithelium matures, with gradual proliferation and differentiation of the squamous cells. The mature squamous epithelium formed because of metaplasia is sometimes indistinguishable from the original squamous epithelium.
As the metaplasia advances, a “new” SCJ is formed, which gradually moves inwards towards the external os.
Metaplastic epithelium can be faintly acetowhite, and iodine uptake is often patchy on it.