Atlas of Colposcopy: Principles and Practice

The squamous epithelium and the columnar epithelium meet at a line called the squamocolumnar junction (SCJ). The SCJ is visible as the junction between the pink squamous epithelium and the red columnar epithelium after the cervix is cleaned with normal saline. The SCJ is much better visualized after application of acetic acid as a distinct white line. The junction between the brown squamous epithelium and the red columnar epithelium is also quite evident after application of Lugol’s iodine.

The colposcopist must try to trace the SCJ along its entire course. The SCJ is easily identified if it is at the external os or on the ectocervix. The cervix must be manipulated with an endocervical speculum to visualize the SCJ that extends inside the endocervix.

Failure to correctly identify the SCJ is the most common error in colposcopy. The importance of this part of colposcopy cannot be overemphasized. To identify the SCJ correctly, the columnar epithelium should be traced to its lowest extent rather than trying to locate the highest apparent extent of the squamous epithelium.

Microscopic features: The junction between the multilayered squamous epithelium and the monolayered columnar epithelium is visible as an abrupt step. The SCJ is prominently seen on colposcopy because of the difference in height between the two epithelia.