The fallopian tubes, the uterus, and the adjacent part of the vagina are derived from two embryonal structures, the müllerian ducts, so named after Johannes Müller, a German anatomist of the early 19th century who first described them. The müllerian ducts fuse to become the uterus and the proximal vagina but remain separated to form the two oviducts (fallopian tubes). Imperfect fusion of the müllerian ducts results in formation of various degrees of duplication or subdivision of the uterus and the vagina, such as uterus septus and vagina septa. An excellent discussion of embryologic origin and congenital abnormalities of the female genital tract may be found in the book by Gray and Skandalakis (1972).

The vulva is the external portal of entry to the female genital tract. It is composed of two sets of folds or labia (from Latin, labium = lip; plural, labia), which frame both sides of the entrance to the vagina. The larger external folds, or labia majora (from Latin, majus = larger; plural, majora) are an extension
of the skin. The smaller inner folds, or labia minora (from Latin, minor = lesser; plural, minora), form a transition between the skin and the vagina. The outer surfaces of the labia minora retain some features of the skin, such as the presence of sebaceous glands, whereas the inner surfaces blend with the vagina. Located anteriorly between the labia minora is the female counterpart of the penis, the clitoris, provided with a retractile, prepuce-like structure. Located about 1 cm behind the clitoris is the opening of the urethra, the terminal portion of the urinary tract.

The lymphatic drainage of the vulva is to the inguinal lymph nodes, which are the primary site of metastases in malignant tumors of the vulva.

In virgins, the entrance to the vagina is protected by a thin, perforated membrane, the hymen. The torn hymen persists in the form of small vestigial elevations at the entrance to the vagina. Just behind the vestigial hymen, on both sides of the posterior and lateral aspect of the vagina, there are two mucus-secreting glands, the glands of Bartholin or Bartholin’s glands. During the childbearing age, the adult vagina is a canal, measuring approximately 10 cm in length, demarcated externally by the vulvar folds or labia, described above. The posterior end of the vagina is a blind pouch, the cul-de-sac. The anterior wall of the vagina, near the cul-de-sac, accommodates the uterine cervix. The area demarcated by the cervix and the blind end of the vaginal pouch is the posterior vaginal fornix. The fornix is quite deep and is the site wherein the secretions from the uterine glands, as well as exfoliated epithelial cells, accumulate. The wall of the vagina consists of three layers: the inner or mucosal layer of squamous epithelium, which shows transverse ridges or rugae. The mucosa is supported by a layer of smooth muscle. The thin outer serosal layer of the vagina is composed of connective tissue. The wall of the vagina is rich in lymphatic vessels. The lymphatic drainage of the anterior one-third of the vagina goes to the inguinal lymph nodes, whereas the posterior two-thirds drain into the pelvic lymph nodes.

Of importance are the anatomic relationships of the vagina, which are separated by thin connective tissue partitions or septa from the rectum posteriorly and the bladder anteriorly. Inflammatory processes and cancers of one of these organs may spread to the vagina and vice versa.

One of the rare but important congenital abnormalities of the vagina is vagina septa, in which the vagina, and possibly the uterus as well, is divided into two separate chambers. On occasion, this is of significance in tumor diagnosis, since cancer may be present in one part of the genital tract while the healthy part is being investigated with negative results.

The uterus is arbitrarily divided into two parts—the body, or corpus, and the neck, or cervix. The corpus and the cervix usually form an angle of 120°, with the corpus directed anteriorly. The body or corpus of the uterus is a roughly pyramidal organ, shaped like an inverted pear and flattened in the anteroposterior diameter. In the resting stage, it measures 4 to 7 cm in length and approximately the same at its widest point. The apex of the pyramid, which becomes the cervix, is directed downward, whereas the wide base, or fundus, is directed upward. The cervix is a tubular structure measuring approximately 4 cm in length and about 3 cm in diameter. Of its total length, about half is within the vagina and is called the portio vaginalis (also known as ecto- or exocervix); the rest is embedded within the vaginal wall and is continuous with the body of the uterus.

The bulk of the uterus is formed by smooth muscle, or the myometrium, which is capable of a manifold increase in size and weight during pregnancy. The muscle encloses the uterine cavity, described below, and is covered on its surface by a reflection of the peritoneum, known as the uterine serosa. The uterus is anchored in the pelvis by a series of bands of connective tissue, or ligaments, the most important being the
posterior round ligament, and by folds or reflections of the peritoneum. Lateral folds, extending along the sides of the uterus and filled with loose connective tissue rich in lymphatics, are known as the broad ligaments forming the left and the right parametrium (plural, parametria).

The cervix has a very close anatomic relationship to the urinary bladder, which is anterior, and to both ureters, which run along the lateral walls of the cervix to reach the bladder. This anatomic arrangement explains the frequent involvement of the lower urinary tract by cervical cancer.

The thick, muscular walls of the uterus contain a cavity that, within the cervix, is called the endocervical canal and is continuous with the endometrial cavity of the corpus. The opening of the cervical canal into the vagina is referred to as the external os (from Latin, os = mouth). The point of transition of the endocervical canal into the endometrial cavity is known as the internal os. The endocervical canal is normally very narrow, measuring at the most 2 or 3 mm in diameter. The endometrial cavity follows the outline of the body of the uterus and is roughly conical, with the apex of the cone corresponding to the internal os and the base directed upward to the upper part, or fundus, of the uterine body. On each side of the triangular endometrial cavity, the horns, or the cornua, of the fundus are in communication with the fallopian tubes, or the oviducts. The lumen of the endometrial cavity in the resting stage is quite small, measuring only a few millimeters in the anteroposterior diameter. The endometrial cavity during pregnancy enlarges to harbor the fetus.

The fallopian tubes (so named after Gabriello Fallopius, an Italian anatomist of the 16th century, who first described them), or the oviducts, measure between 8 and 12 cm in length and 3 and 5 mm in diameter. Their proximal ends are in direct continuation with the endometrial cavity, whereas their distal ends, with fingerlike folds, or fimbriae, open freely into the abdominal cavity, embracing the ovaries. The ova, released by the ovaries, find their way into the fallopian tubes, where they are fertilized by spermatozoa. The tubes are composed of three layers—the inner mucosal layer, followed by a layer of smooth muscle, and a serosal layer on the surface. A narrow canal, lined by the mucosa, is present throughout the entire length of the tube, thereby ensuring direct communication between the vagina and the abdominal cavity—a fact of some importance in the spread of infections and malignant tumors. The histology of the fallopian tubes is discussed in Chapter 15.

The ovaries are approximately ovoid structures, each measuring on the average 4 by 2 by 2 cm, located anatomically in the immediate vicinity of the abdominal or fimbriated end of the tubes, but not directly contiguous with the tubal lumens. In spite of this, the ova, formed in the ovary, find their way into the tubes and from there into the uterine cavity. The ovaries are loosely suspended, as are the tubes, by peritoneal folds. The histology of the ovaries is discussed in Chapter 15.

The term adnexa or uterine adnexa is used to describe, as a single entity, the structures peripheral to the uterus, which consist of the fallopian tubes, ovaries, parametria, and regional lymph nodes. The lymphatics of the uterus, the tubes, and the ovaries are the tributaries of the pelvic and the aortic lymph nodes.

Squamous epithelium of the female genital tract is of two different embryologic origins. The epithelium lining the inner aspect of the labia minora and contiguous with the adjacent vagina, presumably to the level of the cervix, originates from the urogenital sinus. The remainder of the vaginal epithelium and the squamous epithelium of the vaginal portio of the cervix are derived from the müllerian ducts by transformation (metaplasia) of the original cuboidal epithelium into squamous epithelium. This fact has considerable bearing on certain congenital, neoplastic, and drug-induced abnormalities in the vagina and the cervix. The original squamous epithelium, not derived from metaplasia, is sometimes referred to as native squamous epithelium.

The fundamental structure of the squamous epithelium is described in Chapter 5 (see Fig. 5-4). In the female genital tract, during sexual maturity, four layers or zones may be arbitrarily discerned and include the bottom, or basal, layer, which is the source of epithelial regeneration; the adjacent parabasal zone, imperceptibly blending with the intermediate zone, forming the bulk of the epithelial thickness; and the thin superficial zone (Fig. 8-2A). It is estimated that the process of squamous epithelial maturation
takes approximately 4 days. The process may be accelerated to 30 to 45 hours by the administration of estrogens. The mature squamous epithelium of the cervix and vagina is rich in glycogen, as documented by periodic acid-Schiff stain (Fig. 8-2B). Clinically, the presence of glycogen may be revealed by staining the squamous epithelium with Lugol’s iodine solution, which, by binding with glycogen, stains the epithelium mahogany brown. This is the basis of Schiller’s test, which serves to visualize nonstaining, pale areas of the epithelium suggestive of an abnormality that can be either benign or malignant.

The epithelial lining of the endocervical canal, and of the endocervical glands, is formed by a single layer of mucus-producing tall columnar cells with oval nuclei and clear cytoplasm, also known as picket cells (Fig. 8-6). The endocervical epithelium participates in the events of the menstrual cycle, described below, and this is reflected by the consistency of the endocervical mucus. During the preovulatory phase of the cycle, the mucus is thick and readily crystallized; it becomes liquid just before and during the ovulation, presumably to facilitate the entry of spermatozoa into the uterine cavity. Consequently, the appearance of the cytoplasm of the endocervical cells and the position of nuclei depends on the phase of the menstrual cycle. During the proliferative phase, the cytoplasm is opaque and the nuclei are centrally located (Fig. 8-6A). During the secretory phase, the transparent cytoplasm is bulging with accumulated mucus that pushes the flattened nuclei to the basal periphery of the cells (Fig. 8-6B). In such cells, the luminal surface is flat but may show tiny droplets or smudges, reflecting secretion of mucus. The nuclei of the normal endocervical cells are open (vesicular) and spherical, and measure approximately 8 μm in diameter. Ciliated cells are commonly present in the upper (proximal) segment of the endocervical canal, as confirmed in a careful study by Babkowski et al (1996). The nuclei of the ciliated cells are somewhat larger than those of nonciliated cells (see Figs. 8-19B and 8-20D). Located among the columnar cells at the base of the epithelium, adjacent to the basement membrane, there are small, triangular basal, or reserve, cells. These cells are very difficult to see in light microscopy of normal epithelium but have been clearly demonstrated by electron microscopy. Under abnormal circumstances, a hyperplasia of the reserve cells may be observed. The role of reserve cells as the cell of origin of squamous metaplasia of the endocervix is discussed in Chapter 10.

The endocervical glands are of the simple tubular branching type, and they may vary substantially in number and distribution. In some women, the normal glands may be situated deeply within the wall of the cervix, at a considerable
distance from the surface; this distribution of endocervical glands is of importance in the diagnosis of extremely well-differentiated endocervical adenocarcinoma (see Chap. 12). The presence of glands underneath the squamous epithelium of the portio, in the area of the external os (transformation zone), is normal. The epithelium lining the glands is identical to the surface epithelium.

The area of the junction between the squamous and the endocervical epithelium is of considerable importance in
the genesis of carcinoma of the uterine cervix (see Chap. 11). In a normal, quiescent cervix, the transition between the two epithelial types is often sharp and is known as the squamocolumnar junction, now usually designated as the transformation zone (Fig. 8-9). The term transformation zone is based on colposcopic observations of adolescent and young women, documenting that the glandular epithelium of the cervix in the area of the squamocolumnar junction is undergoing constant metaplastic transformation into squamous epithelium. The events of transformation are sometimes reflected in cervical smears, showing side by side endocervical glandular cells and young metaplastic squamous cells.

The anatomic location of the transformation zone varies considerably and is age-dependent (Fig. 8-10). In adolescents and young women, the junction is usually located at the level of the external os, but may extend to the adjacent vaginal aspect of the uterine cervix. In the latter case, the area occupied by the endocervical epithelium on the surface of the cervix may be visible to the naked eye as a sharply demarcated red area, sometimes inappropriately called an erosion, but better designated as eversion, ectropion, or ectopy. The redness reflects the presence of blood vessels under the thin endocervical epithelium. The ectropion is a benign, self-healing condition, which, however, may mimic important lesions of the cervix. The cytologic presentation and clinical significance of the ectropion are discussed in Chapter 9.

With advancing age, the junction tends to move up into the endocervical canal. At the time of the menopause, the junction is usually located within the endocervical canal and is hidden from view.

Because most of the initial precancerous changes in the uterine cervix occur within the transformation zone, this is an area of major importance in cervix cancer prevention (see Chap. 11). For this reason, much emphasis has been placed on sampling of the transformation zone by cervicovaginal smears (see comments on smear adequacy at the end of this chapter). It is evident that the transformation zone is more readily accessible in younger than in older women. For comments on cytology of the transformation zone, see below.

The transition between the endocervical epithelium and the endometrium usually occurs at the level of the internal os. The transition between the large picket cells of the endocervical mucosa and the smaller cells of the endometrium is usually quite sharp.

The endometrium is essentially composed of layers of surface epithelium composed of cuboidal cells, forming simple tubular glands, surrounded by stromal cells. During the childbearing age, the endometrium undergoes cyclic changes (menstrual cycle) to prepare it for the implantation of the fertilized ovum, hence for pregnancy. The appearance of the glands and the stroma changes with the phase of the cycle, as described below. If the implantation
does not occur, the endometrium is shed before the beginning of the next menstrual cycle. A detailed history of the cyclic changes and their hormonal background can be obtained elsewhere; for our purpose, only a brief summary is necessary.

In vaginal pool smears, the endocervical cells are relatively uncommon and rarely well preserved. In cervical smears obtained by means of instruments, particularly endocervical brushes, the endocervical cells are usually numerous and well preserved. When seen in profile, the endocervical cells are columnar and measure approximately 20 μm in length and from 8 to 12 μm in width (Fig. 8-19A). Shorter cells, of plump, more cuboidal configuration may also occur. The columnar endocervical cells may occur singly but, quite often, they are seen as sheets of parallel cells, arranged in a palisade (Fig. 8-19B). When the endocervical cells are flattened on the slide and are seen “on end,” they form tight clusters or plaques, wherein the cells form a tightly fitting mosaic resembling a honeycomb. In such plaques, the cell membranes form the partitions of the honeycomb and the centers are filled by clear cytoplasm surrounding the nuclei (Fig. 8-19A,C). The identification of such cells as endocervical is facilitated if columnar cells are present at the periphery of the cluster.

The cytoplasm of endocervical cells is either finely vacuolated or homogeneous and faintly basophilic or distended by clear, transparent mucus that is pushing the nuclei toward
the narrow end of the cell. Some such cells may become nearly spherical in shape because of cytoplasmic distention by mucus. On the surface of the mucus-containing cells, small droplets or smudges of mucus may be observed.

The nuclei are spherical or oval, vesicular in configuration, with delicate chromatin filaments, often showing chromocenters and very small nucleoli. The nuclei may vary in size. The dominant size of the nuclei is about 8 μm in diameter but larger nuclei, up to 15 or 16 μm in diameter, are not uncommon. The variability of the nuclear sizes may reflect stages in cell cycle or other, unknown factors. Multinucleated cells may also occur (Fig. 8-20A). The fragile cytoplasm of the endocervical cells may disintegrate, with resulting stripped, or naked, nuclei, usually of spherical or somewhat elliptical configuration (Fig. 8-20B). These nuclei may also vary in size and may be difficult to recognize, unless they are similar to, or identical with, the nuclei of adjacent better-preserved endocervical cells. Small intranuclear cytoplasmic inclusions in the form of clear areas within the nucleus may occur in endocervical cells (Fig. 8-20B).

At the time of ovulation, and sometimes during the secretory (postovulatory) phase of the menstrual cycle, the nuclei of endocervical cells form intensely stained, dark, nipplelike protrusions of various sizes, up to 3 μm in length, that are an extension of the nucleus into the adjacent cytoplasm (see Figs. 8-19C and 8-20C). The protrusions appear mainly on the lumenal aspect of the nucleus, facing the endocervical lumen. Sometimes the protrusions are split in two. All stages of formation of the protrusions may be observed, ranging from a thickening of the nuclear membrane to protrusions growing in size. In nuclei with fully developed protrusions, the remainder of the nucleus is usually less dense and transparent, suggesting that there has been a shift of the chromatin to the protrusion. The mechanism of formation and the nature of the protrusions are the subject of a considerable debate. Taylor (1984) thought that the protrusions occurred mainly in ciliated endocervical cells and that their formation was the result of high ciliary activity. McCollum (1988) observed the protrusions in women receiving the long-term contraceptive drug medroxyprogesterone, during periods of amenorrhea, when the estrogenic activity was low. McCollum thought that the protrusions represented an attempt at nuclear division arrested by progesterone and, therefore, consistent with events occurring at the onset of ovulation. Zaharopoulos et al (1998) studied the protrusions by a number of methods, including electron microscopy, cytochemistry, and in situ hybridization of X chromosome. These investigators observed the presence of nucleoli and single X chromosome within the protrusions and reported findings suggestive of formation of an abortive mitotic spindle attached to the protrusion, thus providing support to McCollum’s suggestion that the protrusion represents an attempt at mitotic division. Although further studies may shed some additional light on this very interesting phenomenon, it is quite certain that the protrusions do not represent an artifact, as has been suggested by Koizumi (1996). It is of note that similar protrusions may be occasionally observed in histologic sections of the endocervix during the secretory phase of the cycle and in epithelial cells of various origins, for example, in bronchial epithelial cells and in duct cells of the breast obtained by aspiration (see Chaps. 19 and 29). Zacharopoulos et al observed similar nuclear protrusions in occasional nonepithelial cells, suggesting that the phenomenon
is of a general nature and clearly worthy of further studies.