ANATOMY AND PHYSIOLOGY
EXTERNAL GENERATIVE ORGANS
Ari Titin Mulyaningsih, 2010
(Williams Obstetrics, 22nd Edition)
The mons pubis, or mons veneris, is the fat-filled cushion that lies over the symphysis pubis. After puberty, the skin of the mons pubis is covered by curly hair that forms the escutcheon. In women, it is distributed in a triangular area, the base of which is formed by the upper margin of the symphysis. In men, the escutcheon is not so well circumscribed.
These structures vary somewhat in appearance, principally according to the amount of fat that is contained within the tissues. Embryologically, the labia majora are homologous with the male scrotum. The round ligaments terminate at the upper borders. After repeated childbearing, the labia majora are less prominent. They are 7 to 8 cm in length, 2 to 3 cm in width, and 1 to 1.5 cm in thickness, and are somewhat tapered at the lower extremities. In children and nulliparous women, the labia majora usually lie in close apposition, whereas in multiparous women, they may gape widely. They are continuous directly with the mons pubis above and merge into the perineum posteriorly at a site where they are joined medially to form the posterior commissure. Before puberty, the outer surface of the labia is similar to that of the adjacent skin, but after pubertythe labia are covered with hair. In nulliparous women, the inner surface is moist and resembles a mucous membrane, whereas in multiparous women, the inner surface becomes more skinlike. The labia majora are richly supplied with sebaceous glands. Beneath the skin, there is a layer of dense connective tissue that is rich in elastic fibers and adipose tissue but is nearly void of muscular elements. Unlike the squamous epithelium of the vagina and cervix, there are epithelial appendages in parts of the vulvar skin. A mass of fat beneath the skin provides the bulk of the volume of the labium, and this tissue is supplied with a rich plexus of veins.
The labia minora vary greatly in size and shape. In nulliparous women, they usually are not visible behind the nonseparated labia majora. In multiparas, it is common for the labia minora to project beyond the labia majora. Each labium minus is a thin fold of tissue that is moist and reddish, similar in appearance to a mucous membrane. The labia minora are covered by stratified squamous epithelium. Although there are no hair follicles in the labia minora, there are many sebaceous follicles and, occasionally, a few sweat glands. The interior of the labial folds is composed of connective tissue with many vessels and some smooth muscular fibers. They are supplied with a variety of nerve endings and are extremely sensitive. The tissues of the labia minora converge superiorly, where each is divided into two lamellae; the lower pair fuse to form the frenulum of the clitoris, and the upper pair merge to form the prepuce. Inferiorly, the labia minora extend to approach the midline as low ridges of tissue that fuse to form the fourchette.
The clitoris is the principal female erogenous organ. It is the homologue of the penis and is located near the superior extremity of the vulva. This erectile organ projects downward between the branched extremities of the labia minora. The clitoris is composed of a glans, a corpus, and two crura. The glans is made up of spindle-shaped cells, and in the body there are two corpora cavernosa, in the walls of which are smooth muscle fibers. The long, narrow crura arise from the inferior surface of the ischiopubic rami and fuse just below the middle of the pubic arch to form the corpus. The clitoris rarely exceeds 2 cm in length. Its free end is pointed downward and inward toward the vaginal opening. The glans is usually less than 0.5 cm in diameter and is covered by stratified squamous epithelium that is richly supplied with nerve. Gender differentiation. Genetic sex is established at the time of fertilization. At a time thereafter, the primordial gonad differentiates to testis if the SRY gene is expressed. The fetal testicular secretions effect male phenotypic gender differentiation. endings. The vessels of the erectile clitoris are connected with the vestibular bulbs. There is a delicate network of free nerve endings in the labia majora, labia minora, and clitoris (Krantz, 1958). Tactile discs are found in abundance in these areas. Genital corpuscles, which are mediators of erotic sensation, vary considerably in number. These structures are abundant in the labia minora and in the skin that overlies the glans clitoris.
The vestibule is an almond-shaped area that is enclosed by the labia minora laterally and extends from the clitoris to the fourchette. The vestibule is the functionally mature female structure of the urogenital sinus of the embryo. In the mature state, the vestibule usually is perforated by six openings: the urethra, the vagina, the two ducts of the Bartholin glands, and, at times, the two ducts of the paraurethral glands, also called the Skene ducts and glands. The posterior portion of the vestibule between the fourchette and the vaginal opening is called the fossa navicularis, and it is usually observed only in nulliparous women. The pair of Bartholin glands are about 0.5 to 1 cm in diameter, and each is situated beneath the vestibule on either side of the vaginal opening. They are the major vestibular glands, and the ducts are 1.5 to 2 cm long and open on the sides of the vestibule just outside the lateral margin of the vaginal orifice. At times of sexual arousal, they secrete mucoid material. These glands may harbor Neisseria gonorrhoeae or other bacteria, which in turn may cause infection and a Bartholin gland abscess.
The lower two thirds of the urethra lies immediately above the anterior vaginal wall. The urethral opening or meatus is in the midline of the vestibule, 1 to 1.5 cm below the pubic arch, and a short distance above the vaginal opening. Ordinarily, the paraurethral ducts, also known as the Skene ducts, open onto the vestibule on either side of the urethra. The ducts occasionally open on the posterior wall of the urethra just inside the meatus.
Embryologically, the vestibular bulbs correspond to the anlage of the corpus spongiosum of the penis. These are almond-shaped aggregations of veins, 3 to 4 cm long, 1 to 2 cm wide, and 0.5 to 1 cm thick, that lie beneath the mucous membrane on either side of the vestibule. They are in close apposition to the ischiopubic rami and are partially covered by the ischiocavernosus and constrictor vaginae muscles. The vestibular bulbs terminate interiorly at about the middle of the vaginal opening and extend upward toward the clitoris. During childbirth, they may be injured and may even rupture to form a vulvar hematoma.
Vaginal Opening and Hymen.
In most virginal women, the vaginal opening most often is hidden by the overlapping labia minora. There are marked differences in shape and consistency of the hymen, which is composed mainly of elastic and collagenous connective tissue. Both the outer and inner surfaces are covered by stratified squamous epithelium. The hymen has no glandular or muscular elements, and it is not richly supplied with nerve fibers. In the newborn, the hymen is very vascular and redundant. In pregnant women, its epithelium is thick, and the tissue is rich in glycogen. After menopause, the epithelium is thin, and focal cornification may develop. In adult women, the hymen is a membrane of various thickness that surrounds the vaginal opening more or less completely. Its aperture varies in diameter from pinpoint size to one that admits the tip of one or even two fingers. The appearance of the hymen cannot be used to determine whether a woman has begun sexual activity. A fimbriated type of hymen in virginal women may be indistinguishable from one that has been penetrated during intercourse. As a rule, however, it is torn at several sites during first coitus, usually in the posterior portion. Identical tears may occur by other penetration, for example, tampons used during menstruation. The edges of the torn tissue soon cicatrize, and the hymen becomes divided permanently into two or more portions that are separated by narrow sulci. Occasionally with hymenal rupture, there may be profuse bleeding. Changes produced in the hymen by childbirth are usually readily recognizable. Over time, the hymen consists of several cicatrized nodules of various sizes. Imperforate hymen is a rare lesion in which the vaginal orifice is occluded completely, causing retention of menstrual blood.
This musculomembranous structure extends from the vulva to the uterus and is interposed anteriorly and posteriorly between the urinary bladder and the rectum. The upper portion of the vagina arises from the mullerian ducts, and the lower portion is formed from the urogenital sinus. Anteriorly, the vagina is separated from the bladder and urethra by connective tissue, often referred to as the vesicovaginal septum. Posteriorly, between the lower portion of the vagina and the rectum, there are similar tissues that together form the rectovaginal septum. The upper fourth of the vagina is separated from the rectum by the rectouterine pouch, also called the cul-de-sac of Douglas. Normally, the anterior and posterior vaginal walls lie in contact, with only a slight space intervening between the lateral margins. Vaginal length varies considerably, but commonly, the anterior and posterior vaginal walls are, respectively, 6 to 8 cm and 7 to 10 cm in length. The upper end of the vaginal vault is subdivided into the anterior, posterior, and two lateral fornices by the uterine cervix. These are of considerable clinical importance because the internal pelvic organs usually can be palpated through their thin walls. Moreover, the posterior fornix provides surgical access to the peritoneal cavity. Prominent midline longitudinal ridges project into the vaginal lumen from the anterior and posterior walls. In nulliparous. The external genitalia with the skin and subcutaneous tissue removed from the right side. women, numerous transverse ridges, or rugae, extend outward from and almost at right angles to the longitudinal ridges. In postmenopausal multiparous women, the vaginal walls often are smooth. The vaginal mucosa is composed of noncornified stratified squamous epithelium. Beneath the epithelium is a thin fibromuscular coat, usually consisting of an inner circular layer and an outer longitudinal layer of smooth muscle. A thin layer of connective tissue beneath the mucosa and the muscularis is rich in blood vessels. It is controversial whether this connective tissue often referred to as perivaginal endopelvic fascia is a definite fascial plane in the strict anatomical sense. There are no vaginal glands. After giving birth, fragments of stratified epithelium occasionally are embedded in the vaginal connective tissue. They may form vaginal inclusion cysts, which are not true glands. In the absence of glands, the vagina is kept moist by a small amount of secretion from the cervix. During pregnancy, there is copious, acidic vaginal secretion, which normally consists of a curdlike product of exfoliated epithelium and bacteria. Lactobacillus species are also recovered in higher concentrations than in nonpregnant women (Larsen and Galask, 1980; McGregor and French, 2000).
The vagina has an abundant vascular supply. The upper third is supplied by the cervicovaginal branches of the uterine arteries, the middle third by the inferior vesical arteries, and the lower third by the middle rectal and internal pudendal arteries. The vaginal artery may branch directly from the internal iliac artery. An extensive venous plexus immediately surrounds the vagina and follows the course of the arteries. Lymphatics from the lower third of the vagina, along with those of the vulva, drain primarily into the inguinal lymph nodes. Those from the middle third drain into the internal iliac nodes, and those from the upper third drain into the iliac nodes.
The many structures that make up the perineum are illustrated in. Most of the support of the perineum is provided by the pelvic and urogenital diaphragms. The pelvic diaphragm consists of the levator ani muscles plus the coccygeus muscles posteriorly. The levator ani muscles form a broad muscular sling that originates from the posterior surface of the superior pubic rami, from the inner surface of the ischial spine, and between these two sites, from the obturator fascia. Some of these muscle fibers are inserted around the vagina and rectum to form efficient functional sphincters. In a recent study utilizing magnetic resonance imaging, Tunn (2003) and Hoyte (2004) and their colleagues used magneticresonance imaging and reported significant variation in the levator ani muscle, endopelvic fascia, and urethral support in nulliparous women. The urogenital diaphragm is external to the pelvic diaphragm and includes the triangular area between the ischial tuberosities and the symphysis. The urogenital diaphragm is made up of the deep transverse perineal muscles, the constrictor of the urethra, and the internal and external fascial coverings. The external anal sphincter is shown in, where its proximity to the posterior vaginal fourchette is apparent. The relationship of the internal and external sphincter is shown in. Damage to either sphincter increases the likelihood of rectal incontinence following vaginal delivery (Delancey and co-workers, 1997). Repair of the disrupted sphincter is shown in. The major blood supply to the perineum is via the internal pudendal artery and its branches. These include the inferior rectal artery and posterior labial artery. The innervation of the perineum is primarily via the pudendal nerve and its branches. The pudendal nerve originates from the S2, S3, and S4 level of the spinal cord.
The median raphe of the levator ani, between the anus and the vagina, is reinforced by the central tendon of the perineum. The bulbocavernosus, superficial transverse perineal, and external anal sphincter muscles also converge on the central tendon. Thus, these structures contribute to the perineal body, which provides much of the support for the perineum.