Female Reproductive Anatomy and Embryology

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Chapter 3 Female Reproductive Anatomy and Embryology

The scope of obstetrics and gynecology assumes a reasonable background in reproductive anatomy, embryology, physiology (see Chapter 4), and endocrinology (see Chapter 5 and Part 4). A physician cannot effectively practice obstetrics and gynecology without understanding the physiologic processes that transpire in a woman’s life as she passes through infancy, adolescence, reproductive maturity, and the climacteric. As the various clinical problems are addressed, it is important to consider those anatomic, developmental, and physiologic changes that normally take place at key points in a woman’s life cycle.

Most of this chapter deals with the disruptive deviations from normal female anatomy and physiology, whether congenital, functional, traumatic, inflammatory, neoplastic, or even iatrogenic. As the etiology and pathogenesis of clinical problems are considered, each should be studied in the context of normal anatomy, development, and physiology.

Development of the External Genitalia

Before the 7th week of development, the appearance of the external genital area is the same in males and females. Elongation of the genital tubercle into a phallus with a clearly defined terminal glans portion is noted in the 7th week, and gross inspection at this time may lead to faulty sexual identification. Ventrally and caudally, the urogenital membrane, made up of both endodermal and ectodermal cells, further differentiates into the genital folds laterally and the urogenital folds medially. The lateral genital folds develop into the labia majora, whereas the urogenital folds develop subsequently into the labia minora and prepuce of the clitoris.

The external genitalia of the fetus are readily distinguishable as female at about 12 weeks (Figure 3-1). In the male, the urethral ostium is located conspicuously on the elongated phallus by this time and is smaller, owing to urogenital fold fusion dorsally, which produces a prominent raphe from the anus to the urethral ostium. In the female, the hymen is usually perforated by the time delivery occurs.

FIGURE 3-1 Development of the external female genitalia. A: Indifferent stage (about 7 weeks). B: About 10 weeks. C: About 12 weeks.

Anatomy of the External Genitalia

The perineum represents the inferior boundary of the pelvis. It is bounded superiorly by the levator ani muscles and inferiorly by the skin between the thighs (Figure 3-2). Anteriorly, the perineum extends to the symphysis pubis and the inferior borders of the pubic bones. Posteriorly, it is limited by the ischial tuberosities, the sacrotuberous ligaments, and the coccyx. The superficial and deep transverse perineal muscles cross the pelvic outlet between the two ischial tuberosities and come together at the perineal body. They divide the space into the urogenital triangle anteriorly and the anal triangle posteriorly.

FIGURE 3-2 The perineum, showing superficial structures on the left and deeper structures on the right.

The urogenital diaphragm is a fibromuscular sheet that stretches across the pubic arch. It is pierced by the vagina, the urethra, the artery of the bulb, the internal pudendal vessels, and the dorsal nerve of the clitoris. Its inferior surface is covered by the crura of the clitoris, the vestibular bulbs, the greater vestibular (Bartholin’s) glands, and the superficial perineal muscles. Bartholin’s glands are situated just posterior to the vestibular bulbs, and their ducts empty into the introitus just below the labia minora. They are often the site of gonococcal infections and painful abscesses.

VULVA

The external genitalia are referred to collectively as the vulva. As shown in Figure 3-3, the vulva includes the mons veneris, labia majora, labia minora, clitoris, vulvovaginal (Bartholin’s) glands, fourchette, and perineum. The most prominent features of the vulva, the labia majora, are large, hair-covered folds of skin that contain sebaceous glands and subcutaneous fat and lie on either side of the introitus. The labia minora lie medially and contain no hair but have a rich supply of venous sinuses, sebaceous glands, and nerves. The labia minora may vary from scarcely noticeable structures to leaf-like flaps measuring up to 3 cm in length. Anteriorly, each splits into two folds. The posterior two folds attach to the inferior surface of the clitoris, at which point they unite to form the frenulum of the clitoris. The anterior folds are united in a hood-like configuration over the clitoris, forming the prepuce. Posteriorly, the labia minora may extend almost to the fourchette.

FIGURE 3-3 Female external genitalia.

The clitoris lies just in front of the urethra and consists of the glans, the body, and the crura. Only the glans clitoris is visible externally. The body, composed of a pair of corpora cavernosa, extends superiorly for a distance of several centimeters and divides into two crura, which are attached to the undersurface of either pubic ramus. Each crus is covered by the corresponding ischiocavernosus muscle. Each vestibular bulb (equivalent to the corpus spongiosum of the penis) extends posteriorly from the glans on either side of the lower vagina. Each bulb is attached to the inferior surface of the perineal membrane and covered by the bulbocavernosus muscle. These muscles aid in constricting the venous supply to the erectile vestibular bulbs and also act as the sphincter vaginae.

As the labia minora are spread, the vaginal introitus, guarded by the hymenal ring, is seen. Usually, the hymen is represented only by a circle of carunculae myrtiformes around the vaginal introitus. The hymen may take many forms, however, such as a cribriform plate with many small openings or a completely imperforate diaphragm.

The vestibule of the vagina is that portion of the introitus extending inferiorly from the hymenal ring between the labia minora. The fourchette represents the posterior portion of the vestibule just above the perineal body. Most of the vulva is innervated by the branches of the pudendal nerve. Anterior to the urethra, the vulva is innervated by the ilioinguinal and genitofemoral nerves. This area is not anesthetized adequately by a pudendal block, and repair of paraurethral tears should be supplemented by additional subcutaneous anesthesia.

Internal Genital Development

The upper vagina, cervix, uterus, and fallopian tubes are formed from the paramesonephric (müllerian) ducts. Although human embryos, whether male or female, possess both paired paramesonephric and mesonephric (wolffian) ducts, the absence of Y chromosomal influence leads to the development of the paramesonephric system with virtual total regression of the mesonephric system. With a Y chromosome present, a testis is formed and müllerian-inhibiting substance is produced, creating the reverse situation.

Mesonephric duct development occurs in each urogenital ridge between weeks 2 and 4 and is thought to influence the growth and development of the paramesonephric ducts. The mesonephric ducts terminate caudally by opening into the urogenital sinus. First evidence of each paramesonephric duct is seen at 6 weeks’ gestation as a groove in the coelomic epithelium of the paired urogenital ridges, lateral to the cranial pole of the mesonephric duct. Each paramesonephric duct opens into the coelomic cavity cranially at a point destined to become a tubal ostium. Coursing caudally at first, parallel to the developing mesonephric duct, the blind distal end of each paramesonephric duct eventually crosses dorsal to the mesonephric duct, and the two ducts approximate in the midline. The two paramesonephric ducts fuse terminally at the urogenital septum, forming the uterovaginal primordium. The distal point of fusion is known as the müllerian tubercle (Müller’s tubercle) and can be seen protruding into the urogenital sinus dorsally in embryos at 9 to 10 weeks’ gestation (Figure 3-4). Later dissolution of the septum between the fused paramesonephric ducts leads to the development of a single uterine fundus, cervix, and, according to some investigators, the upper vagina.

FIGURE 3-4 Early embryologic development of the genital tract (A to C) and vaginal plate (D). MD, mesonephric duct; MT, müllerian tubercle; PD, paramesonephric duct; UVP, uterovaginal primordium; US, urogenital sinus; VP, vaginal plate.

(Redrawn from Didusch JF, Koff AK: Contrib Embryol Carnegie Inst 24:61, 1933.)

Degeneration of the mesonephric ducts is progressive from 10 to 16 weeks in the female fetus, although vestigial remnants of the latter may be noted in the adult (Gartner’s duct cyst, paroöphoron, epoöphoron) (Figure 3-5). The myometrium and endometrial stroma are derived from adjacent mesenchyme; the glandular epithelium of the fallopian tubes, uterus, and cervix is derived from the paramesonephric duct.

FIGURE 3-5 Remnants of the mesonephric (wolffian) ducts that may persist in the anterolateral vagina or adjacent to the uterus within the broad ligament or mesosalpinx.

Solid vaginal plate formation and lengthening occur from the 12th through the 20th weeks, followed by caudad to cephalad canalization, which is usually completed in utero. Controversy surrounds the relative contribution of the urogenital sinus and paramesonephric ducts to the development of the vagina, and it is uncertain whether the whole of the vaginal plate is formed secondary to growth of the endoderm of the urogenital sinus or whether the upper vagina is formed from the paramesonephric ducts.

VAGINA

The vagina is a flattened tube extending posterosuperiorly from the hymenal ring at the introitus up to the fornices that surround the cervix (Figure 3-6). Its epithelium, which is stratified squamous in type, is normally devoid of mucous glands and hair follicles and is nonkeratinized. Gestational exposure to diethylstilbestrol (taken by the mother) may result in columnar glands interspersed with the squamous epithelium of the upper two thirds of the vagina (vaginal adenosis). Deep to the vaginal epithelium are the muscular coats of the vagina, which consist of an inner circular and an outer longitudinal smooth muscle layer. Remnants of the mesonephric ducts may sometimes be demonstrated along the vaginal wall in the subepithelial layers and may give rise to Gartner’s duct cysts. The adult vagina averages about 8 cm in length, although its size varies considerably with age, parity, and the status of ovarian function. An important anatomic feature is the immediate proximity of the posterior fornix of the vagina to the pouch of Douglas, which allows easy access to the peritoneal cavity from the vagina, by either culdocentesis or colpotomy.

FIGURE 3-6 Coronal section of the pelvis at the level of the uterine isthmus and ischial spines, showing the ligaments supporting the uterus.

UTERUS

The uterus consists of the cervix and the uterine corpus, which are joined by the isthmus. The uterine isthmus represents a transitional area wherein the endocervical epithelium gradually changes into the endometrial lining. In late pregnancy, this area elongates and is referred to as the lower uterine segment.

The cervix is generally 2 to 3 cm in length. In infants and children, the cervix is proportionately longer than the uterine corpus (Figure 3-7). The portion that protrudes into the vagina and is surrounded by the fornices is covered with a nonkeratinizing squamous epithelium. At about the external cervical os, the squamous epithelium covering the ectocervix changes to simple columnar epithelium, the site of transition being referred to as the squamocolumnar junction. The cervical canal is lined by irregular, arborized, simple columnar epithelium, which extends into the stroma as cervical “glands” or crypts.

FIGURE 3-7 Changing proportion of the uterine cervix and corpus from infancy to adulthood.

(Modified from Cunningham FG, MacDonald PC, Gant NF, et al [eds]: Williams Obstetrics, 20th ed. East Norwalk, Conn, Appleton & Lange, 1997.)

The uterine corpus is a thick, pear-shaped organ, somewhat flattened anteroposteriorly, that consists of largely interlacing smooth muscle fibers. The endometrial lining of the uterine corpus may vary from 2 to 10 mm in thickness (which may be measured by ultrasonic imaging), depending on the stage of the menstrual cycle. Most of the surface of the uterus is covered by the peritoneal mesothelium.

Four paired sets of ligaments are attached to the uterus (Figure 3-8). Each round ligament inserts on the anterior surface of the uterus just in front of the fallopian tube, passes to the pelvic side wall in a fold of the broad ligament, traverses the inguinal canal, and ends in the labium majus. The round ligaments are of little supportive value in preventing uterine prolapse but help to keep the uterus anteverted. The uterosacral ligaments are condensations of the endopelvic fascia that arise from the sacral fascia and insert into the posteroinferior portion of the uterus at about the level of the isthmus. These ligaments contain sympathetic and parasympathetic nerve fibers that supply the uterus. They provide important support for the uterus and are also significant in precluding the development of an enterocele. The cardinal ligaments (Mackenrodt’s) are the other important supporting structures of the uterus that prevent prolapse. They extend from the pelvic fascia on the lateral pelvic walls and insert into the lateral portion of the cervix and vagina, reaching superiorly to the level of the isthmus. The pubocervical ligaments pass anteriorly around the bladder to the posterior surface of the pubic symphysis.

FIGURE 3-8 View of the internal genital organs in the female pelvis. IVC, inferior vena cava.

In addition, there are four peritoneal folds. Anteriorly, the vesicouterine fold is reflected from the level of the uterine isthmus onto the bladder. Posteriorly, the rectouterine fold passes from the posterior wall of the uterus, to the upper fourth of the vagina, and thence onto the rectum. The pouch between the cervix and vagina anteriorly and rectum posteriorly forms a cul-de-sac, called the pouch of Douglas. Laterally, the two broad ligaments each pass from the side of the uterus to the lateral wall of the pelvis. Between the two leaves of each broad ligament are contained the fallopian tube, the round ligament, and the ovarian ligament, in addition to nerves, blood vessels, and lymphatics. The fold of broad ligament containing the fallopian tube is called the mesosalpinx. Between the end of the tube and ovary and the pelvic side wall, where the ureter passes over the common iliac vessels, is the infundibulopelvic ligament, which contains the vessels and nerves for the ovary. The ureter may be injured when this ligament is ligated during a salpingo-oophorectomy procedure.

FALLOPIAN TUBES

The oviducts are bilateral muscular tubes (about 10 cm in length) with lumina that connect the uterine cavity with the peritoneal cavity. They are enclosed in the medial four fifths of the superior aspect of the broad ligament. The tubes are lined by a ciliated, columnar epithelium that is thrown into branching folds. That segment of the tube within the wall of the uterus is referred to as the interstitial portion. The medial portion of each tube is superior to the round ligament, anterior to the ovarian ligament, and relatively fixed in position. This nonmobile portion of the tube has a fairly narrow lumen and is referred to as the isthmus. As the tube proceeds laterally, it is located anterior to the ovary; it then passes around the lateral portion of the ovary and down toward the cul-de-sac. The ampullary and fimbriated portions of the tube are suspended from the broad ligament by the mesosalpinx and are quite mobile. The mobility of the fimbriated end of the tube plays an important role in fertility. The ampullary portion of the tube is the most common site of ectopic pregnancies.

Normal Embryologic Development of the Ovary

The earliest anatomic event in gonadogenesis is noted at about 4 weeks’ gestational age (i.e., 4 weeks from conception), when a thickening of the peritoneal, or coelomic, epithelium on the ventromedial surface of the urogenital ridge occurs. A bulging genital ridge is subsequently produced by rapid proliferation of the coelomic epithelium in an area that is medial, but parallel, to the mesonephric ridge. Before 5 weeks, this indifferent gonad consists of germinal epithelium surrounding the internal blastema, a primordial mesenchymal cellular mass designated to become the ovarian medulla. After 5 weeks, projections from the germinal epithelium extend like spokes into the mesenchymal blastema to form primary sex cords. Soon thereafter at 7 weeks, a testis can be identified histologically if the embryo has a Y chromosome. In the absence of a Y chromosome, definitive ovarian characteristics do not appear until somewhere between 12 and 16 weeks.

As early as 3 weeks’ gestation, relatively large primordial germ cells appear intermixed with other cells in the endoderm of the yolk sac wall of the primitive hindgut. These germ cell precursors migrate along the hindgut dorsal mesentery (Figure 3-9) and are all contained in the mesenchyme of the undifferentiated urogenital ridge by 8 weeks’ gestation. Subsequent replication of these cells by mitotic division occurs, with maximal mitotic activity noted up to 20 weeks and cessation noted by term. These oogonia, the end result of this germ cell proliferation, are incorporated into the cortical sex cords of the genital ridge.

FIGURE 3-9 Migratory path of primordial germ cells from the yolk sac, along the hindgut mesentery, to the urogenital ridge at about 5 weeks.

Histologically, the first evidence of follicles is seen at about 20 weeks, with germ cells surrounded by flattened cells derived from the cortical sex cords. These flattened cells are recognizable as granulosa cells of coelomic epithelial origin and theca cells of mesenchymal origin. The oogonia enter the prophase of the first meiotic division and are then called primary oocytes (see Chapter 4). It has been estimated that more than 2 million primary oocytes, or their precursors, are present at 20 weeks’ gestation, but only about 300,000 to 500,000 primordial follicles are present by 7 years of age.

Regression of the primary sex cords in the medulla produces the rete ovarii, which are found histologically in the hilus of the ovary along with another testicular analogue called Leydig’s cells, which are thought to be derived from mesenchyme. Vestiges of the rete ovarii and of the degenerating mesonephros may also be noted at times in the mesovarium or mesosalpinx. Structural homologues in males and females are shown in Table 3-1.

TABLE 3-1 STRUCTURAL HOMOLOGUES IN MALES AND FEMALES

ANATOMY OF THE OVARIES

The ovaries are oval, flattened, compressible organs, about 3 × 2 × 2 cm in size. They are situated on the superior surface of the broad ligament and are suspended between the ovarian ligament medially and the suspensory ligament of the ovary or infundibulopelvic ligament laterally and superiorly. Each occupies a position in the ovarian fossa (of Waldeyer), which is a shallow depression on the lateral pelvic wall just posterior to the external iliac vessels and anterior to the ureter and hypogastric vessels. In endometriosis and salpingo-oophoritis, the ovaries may be densely adherent to the ureter. Generally, the serosal covering and the tunica albuginea of the ovary are quite thin, and developing follicles and corpora lutea are readily visible.

The blood supply to the ovaries is provided by the long ovarian arteries, which arise from the abdominal aorta immediately below the renal arteries. These vessels course downward and cross laterally over the ureter at the level of the pelvic brim, passing branches to the ureter and the fallopian tube. The ovary also receives substantial blood supply from the uterine artery through the uterine-ovarian arterial anastomosis. The venous drainage from the right ovary is directly into the inferior vena cava, whereas that from the left ovary is into the left renal vein (Figure 3-10).

FIGURE 3-10 Lymphatic drainage of the internal genital organs. IVC, inferior vena cava.

ANATOMY OF THE URETERS

The ureters extend 25 to 30 cm from the renal pelves to their insertion into the bladder at the trigone. Each descends immediately under the peritoneum, crossing the pelvic brim beneath the ovarian vessels just anterior to the bifurcation of the common iliac artery. In the true pelvis, the ureter initially courses inferiorly, just anterior to the hypogastric vessels, and stays closely attached to the peritoneum. It then passes forward along the side of the cervix and beneath the uterine artery toward the trigone of the bladder.

LYMPHATIC DRAINAGE

The lymphatic drainage of the vulva and lower vagina is principally to the inguinofemoral lymph nodes and then to the external iliac chains (see Figure 3-10). The lymphatic drainage of the cervix takes place through the parametria (cardinal ligaments) to the pelvic nodes (the hypogastric, obturator, and external iliac groups) and then to the common iliac and para-aortic chains. The lymphatic drainage from the endometrium is through the broad ligament and infundibulopelvic ligament to the pelvic and para-aortic chains. The lymphatics of the ovaries pass via the infundibulopelvic ligaments to the pelvic and para-aortic nodes (see Figure 3-10).

ANATOMY OF THE LOWER ABDOMINAL WALL

Because most intraabdominal gynecologic operations are performed through lower abdominal incisions, it is important to review the anatomy of the lower abdominal wall with special reference to the muscles and fasciae. After transecting the skin, subcutaneous fat, superficial fascia (Camper’s), and deep fascia (Scarpa’s), the anterior rectus sheath is encountered (Figure 3-11). The rectus sheath is a strong fibrous compartment formed by the aponeuroses of the three lateral abdominal wall muscles. The aponeuroses meet in the midline to form the linea alba and partially encase the two rectus abdominis muscles. The composition of the rectus sheath differs in its upper and lower portions. Above the midpoint between the umbilicus and the symphysis pubis, the rectus muscle is encased anteriorly by the aponeurosis of the external oblique and the anterior lamina of the internal oblique aponeurosis and posteriorly by the aponeurosis of the transversus abdominis and the posterior lamina of the internal oblique aponeurosis. In the lower fourth of the abdomen, the posterior aponeurotic layer of the sheath terminates in a free crescentic margin, the semilunar fold of Douglas.

FIGURE 3-11 Transverse section through the anterior abdominal wall just below the umbilicus (A) and just above the pubic symphysis (B). Note the absence of the posterior rectus sheath in B.

Each rectus abdominis muscle, encased in the rectus sheath on either side of the midline, extends from the superior aspect of the symphysis pubis to the anterior surface of the fifth, sixth, and seventh costal cartilages. A variable number of tendinous intersections (three to five) crosses each muscle at irregular intervals, and any transverse rectus surgical incision forms a new fibrous intersection during healing. The muscle is not attached to the posterior sheath and, following separation from the anterior sheath, can be retracted laterally, as in the Pfannenstiel incision. Each rectus muscle has a firm aponeurosis at its attachment to the symphysis pubis, and this tendinous aponeurosis can be transected if necessary to improve exposure, as in the Cherney incision, and resutured securely during closure of the abdominal wall.

The inferior epigastric arteries arise from the external iliac arteries and proceed superiorly just lateral to the rectus muscles between the transversalis fascia and the peritoneum. They enter the rectus sheaths at the level of the semilunar line and continue their course superiorly just posterior to the rectus muscles. In a transverse rectus muscle–cutting incision, the epigastric arteries can be retracted laterally or ligated to allow a wide peritoneal incision.

ABDOMINAL WALL INCISIONS

The most commonly used lower abdominal incision in gynecologic surgery is the Pfannenstiel incision (Figure 3-12). Although it does not always give sufficient exposure for extensive operations, it has cosmetic advantages in that it is generally only 2 cm above the symphysis pubis, and the scar is later covered by the pubic hair. Because the rectus abdominis muscles are not cut, eviscerations and wound hernias are extremely uncommon. For extensive pelvic procedures (e.g., radical hysterectomy and pelvic lymphadenectomy), a transverse muscle–cutting incision (Bardenheuer or Maylard) at a slightly higher level in the lower abdomen gives sufficient exposure. In addition, the skin incision falls within the lines of Langer, so a good cosmetic result can be expected. When it is anticipated that upper abdominal exploration will be necessary, such as in a patient with suspected ovarian cancer, a midline incision through the linea alba or a paramedian vertical incision is indicated.