Fig. 1.1
Gross anatomy of cervix
The portio vaginalis cervix is the part of cervix protruding into the vagina and surrounded by vaginal fornices. The supravaginal portion of the cervix is not seen on vaginal examination as it lies above the vaginal mucosa reflection. A central canal, known as the cervical canal, runs along its length and connects the cavity of the body of the uterus with the lumen of the vagina. The portio vaginalis opens in the vagina through an opening known as external os. The supravaginal portion of the cervix meets the uterine body at the level of internal os. It requires a certain amount of force to dilate the internal os in order to enter the uterine cavity as part of many surgical procedures. The mucosa lining the cervical canal is known as the endocervix. External os thus marks the junction between ectocervix and endocervix (Fig. 1.2).
Fig. 1.2
Axial (T2 weighted) image of pelvis showing uterus and cervix
The size and shape of cervix may vary according to the female’s age, parity, and hormonal status. In parous woman, it may be bulky with wide gaping or transversely slit-like external os. In contrast, the os is pinpoint in nulliparous woman. The external os on an average is of 3–4 mm in diameter in a nulliparous woman. It is smooth textured, rich pink in color. The external os may contain clear or cloudy mucus depending on the menstrual phase. The canal joining the internal os and the external os is endocervical canal or endocervix. The endocervix is 3.5–4 cm long, red or orange-red in color. The mucosa of the endocervix is thrown into longitudinal folds. This increases the surface area of mucus secreting columnar cells, and the arrangement is known as plica palmate. From plica palmate, further branching folds emerge out peripherally, creating a palm tree-like pattern which is referred to as arbor vitae. Intervening in between these folds are clefts whose depth ranges from 1–2 mm to 1.2 cm.
The upper part of the cervix is separated from the bladder by cellular connective tissue known as parametrium, which also extends over the sides of the cervix. Posteriorly, the supravaginal cervix is covered by peritoneum, which runs onto the back of the vaginal wall and then turns upward and onto the rectum forming the recto-uterine pouch.
The cervical stroma is made of dense fibromuscular tissue through which lymphatic, vascular, and nerve supplies of the cervix pass to form a complex plexus.
1.1.1 Embryological Development
Cervix is derived from the two paramesonephric ducts also called the Müllerian ducts, in the sixth week of embryogenesis. During development, the outer parts of the two ducts fuse, forming a single urogenital canal, which further forms the uterus, cervix, and vagina.
The original squamous epithelium of the cervix is derived from the urogenital sinus, and the original columnar epithelium is derived from the paramesonephric duct. The point at which the two meet is called the squamocolumnar junction.
1.1.2 Vascular Supply
The arterial supply of the cervix is derived from internal iliac arteries via the cervical and vaginal branches of the uterine arteries. The cervical branch of the uterine arteries descend lateral to the cervix at 3 o’clock and 9 o’ clock positions. The corresponding veins run parallel to the arteries and drain finally in the hypogastric venous plexus.
1.1.3 Lymphatics
Three channels facilitate lymphatic drainage from the cervix. The anterior and lateral part of the cervix drain into nodes along the uterine artery to the external iliac lymph nodes and finally into the para-aortic lymph nodes. The posterior and lateral cervix drains along the uterine arteries to the internal iliac lymph nodes and ultimately into the para-aortic lymph nodes. The posterior section of the cervix drains into the obturator and presacral lymph nodes.
1.1.4 Nerve Supply
The endocervix has rich neural innervation, while it is very poor in ectocervix. Therefore, procedures like cervical biopsy, electrocoagulation, and cryotherapy are easily and well tolerated by most women without the need of local anesthesia. The endocervix has abundant innervation by sympathetic or parasympathetic fibers emerging as S2–S3. These nerves travel along the uterosacral ligaments, which pass from the uterus to the anterior sacrum. Dilatation and curettage of the endocervix may sometimes lead to vasovagal reflex. To negate this reflex, atropine (anticholinergic) is often given by the anesthetist prior to the procedures involving dilatation of cervix.
1.2 Microanatomy
1.2.1 Ectocervix
Ectocervix or portio vaginalis cervix is covered with stratified squamous epithelium which is nonkeratinized. However, keratinization can occur after exposure to the environment as in cervical prolapse. This is opaque, pale pink and has 15–20 layers of cells. This epithelium can be native to the site (formed during embryonic period), known as original squamous epithelium, or it can be newly formed as a result of squamous metaplasia during adult period. In reproductive age group, the original squamous epithelium is pinkish, while newly formed metaplastic squamous epithelium is pinkish white on visual inspection.
It has four layers of cell types (from below upward) (Figs. 1.3 and 1.4):
Fig. 1.3
Stratified squamous epithelium
Fig. 1.4
Histological section from ectocervix showing nonkeratinizing stratified squamous epithelium (low power magnification)
- 1.
Basal cells: This is a single layer of cells lying directly on the basement membrane, which separates the epithelium from underlying stroma. As these cells are immature and have active mitoses, the nucleocytoplasmic ratio is higher. The epithelial-stromal junction is usually straight. But sometimes it can be undulating with short projections of stroma at regular intervals, known as papillae. The portion of epithelium in between these papillae is called rete pegs.
- 2.
Parabasal or prickle cell layer: This consists of several layers of cells, larger than basal cells, but with lesser nucleocytoplasmic ratio as cytoplasm is relatively more. It is named so because of the presence of intracellular bridges, and mitosis here is similar as in the basal layer.
- 3.
Intermediate layer: It consists of glycogenated cells larger than the parabasal layer, and the cells appear clear due to the presence of cytoplasmic vacuolations. Still above this layer is a layer which consists of non-vacuolated, flattened cells with basophilic properties.
- 4.
Superficial layer: It consists of progressively flatter and elongated cells without vacuoles but with small pyknotic nuclei and eosinophilic cytoplasm.
Overall, from basal to superficial layer, there is a gradual increase in the cell size with corresponding decrease in the nuclear size. The intermediate and superficial layer cells have abundant cytoplasmic glycogen, which stains mahogany brown or black after Lugol’s iodine and magenta after PAS stain application. This glycogenation of the intermediate and the superficial cell layer indicates normal maturation and development of the squamous epithelium.
Estrogen hormone is responsible for continuous remodeling of the squamous epithelium in the form of epithelial proliferation, maturation, and desquamation. In its absence, full maturation and glycogenation doesn’t take place. Thus, in postmenopausal females, the cells don’t mature beyond the parabasal stage and thus do not accumulate in multiple flat cell layers. As a result of which, the epithelium becomes thin and atrophic, which on visual inspection appears pale, often with subepithelial hemorrhagic spots since it is more prone for trauma.
1.2.2 Endocervix
The endocervical canal is lined by columnar epithelium. It is composed of a single layer of tall cells with dark staining nuclei close to the basement membrane (Figs. 1.5 and 1.6).
