Pathology Thread

I. Gross Anatomy and Histology

Objectives

After going through this section you will be able to:

• list and identify the various structures that form the uterine cervix
• describe the gross anatomy and histology of the cervix
• describe the physiologic changes that occur in the cervix with age and hormonal changes

A. Anatomy

The cervix uteri is the inferior and narrow portion of the uterus with a supravaginal and vaginal portions, the latter being free and projecting into the vagina. It is barrel-shaped and in the adult female multigravida measures 2.5 to 3 cm. It has the following parts:

Ectocervix (exocervix, portio vaginalis), is the lower intravaginal portion of the cervix. It is delimited by the anterior and posterior vaginal fornices and separated from the supravaginal portion of the cervix by the attachment of the vaginal wall. The ectocervix has a convex, elliptical surface and is divided into anterior and posterior lips.

External os (ostium uteri, external orifice of uterus) is the opening of the cervix into the vagina. It occupies a central position in the cervix and is round in the nulligravida and "slit-like" in the parous woman.

Endocervix (cervical canal) is the cavity of the cervix and connects the external os with the internal os. It is fusiform in shape and has posterior and anterior oblique longitudinal ridges, the plicae palmatae. These are not exactly apposed but inter-lock like a zipper so that the canal is kept closed.

Internal os, the opening of the cervix into the uterine cavity.

Anatomy of the uterus

B. Histology

The cervix is made up of epithelium and underlying stroma. The stroma contains an admixture of smooth muscle and fibrous and elastic tissues. The ectocervix is lined by nonkeratinizing stratified squamous epithelium, which is composed of several layers divided into basal, parabasal, intermediate and superficial. The basal layer consists of a single row of cells and rests on a thin basement membrane. Active mitosis occurs in this layer. The parabasal and intermediate layers together constitute the prickle-cell layer. The superficial layer varies in thickness, depending on the degree of estrogen stimulation. The response of the epithelium to sex hormones is identical to that of the vagina. The endocervix is covered by mucin-secreting, simple columnar epithelium, which lines the surface and the underlying glands. These so-called glands are actually not true glands but deep, cleft-like infoldings of the surface epithelium with numerous blind, tunnel-like collaterals. The cells seen in a Pap smear mirror cells from the various layers of the epithelium of the ectocervical and endo-cervical epithelia.

The border between the stratified squamous epithelium of the ectocervix and the columnar epithelium of the endocervix is called the squamocolumnar junction (SCJ).

Squamocolumnar junction.

The location of the SCJ in relation to the anatomic external os varies with a variety of factors. At birth it is at or just above the external os with the original squamous epithelium outside and the endocervical columnar epithelium on the inside. With growth of the uterus, especially from puberty and throughout reproductive life, the SCJ comes to lie at variable distances below the anatomic external os. This is a continuous remodeling process that is also influenced by other factors, and occurs throughout the life of a female producing two different SCJs:

Original SCJ: is the site at which the neonatal squamous epithelium of the ectocervix meets the endocervical columnar epithelium at birth.

New, functional or physiologic SCJ: newly formed SCJ as a result of the dynamic remodeling that takes place during the life of the female (see endocervical ectropion below).

Question: What factors influence the position of SCJ?

Endocervical ectropion

Endocervical ectropion represents areas where the columnar endocervical epithelium everts onto the ectocervix. As the uterus grows and the cervix enlarges, the original SCJ everts from its initial position onto the ectocervix. During this process, the original endocervical columnar epithelium above the SCJ is also everted so that it is exposed and comes to lie intravaginaly. The fleshy, reddened endocervical tissue resembles an eroded tissue and so is often misnamed as "cervical erosion." The columnar epithelium undergoes squamous metaplasia (also called "transformation") due to direct exposure to vaginal secretions and also in response to hormonal changes.

During menopause, the process of ectropion reverses, and the new SCJ inverts to lie at or above the external os, often out of direct visual observation from the vagina.

Endocervical ectropion. Note everted columnar epithelium, which is reddened and looks like "eroded" tissue.

Transformation (transition) zone

As a result of endocervical ectropion and subsequent squamous metaplasia of the exposed endocervical columnar epithelium, a new SCJ is formed inward to the old one on the endocervix. The area, formed by the metaplastic squamous epithelium, between the original or old and the new SCJ is called the transformation or transition zone (TZ). The new SCJ, and hence the extent of the TZ, can be observed from the vagina. However, in up to 15% of women the TZ extends into the endocervical canal beyond the field of vision from the vagina.

Transformation zone. Columnar epithelium is in upper third and squamous epithelium is in lower two thirds. Note abrupt change of epithelial at SCJ.

Question: What is the clinical significance of the transformation zone?