I. Gross Anatomy and Histology
After going through this section you will be able to:
- list and identify the various structures that form the
- describe the gross anatomy and histology of the cervix
- describe the physiologic changes that occur in the cervix
with age and hormonal changes
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
Anatomy of the uterus
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).
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
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
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