Some articles on endocervix:

Cervix - Histology
... The endocervix (more proximal, within the uterus) is composed of simple columnar epithelium ... The area adjacent to the border of the endocervix and ectocervix is known as the transformation zone ... When the endocervix is exposed to the harsh acidic environment of the vagina it undergoes metaplasia to squamous epithelium which is better suited to the vaginal environment ...