Chapter 2 Functional Anatomy of the Female Sex Organs Van Anh T. Ginger and Claire C. Yang Keywords Anatomy • Vagina • Clitoris • Uterus covered by pubic hair. The body of the clitoris extends into the mons for several centimeters, before bifurcating into the crura, which run bilaterally under the inferior pubic rami. Introduction Between the crura lie the clitoral bulbs, draped over the urethra , with the bulk of their tissue The emphasis of the study of the female sex organs lateral to the vaginal walls (Fig. 2.2 ). Only the has long been on understanding its reproductive glans clitoris is visible externally, approxi- role rather than sexual responsiveness. The majority mately 1 cm superior to the urethral meatus. of anatomic descriptions for these organs have been The labia majora are fatty, elongated, hair bear- from the context of reproduction. Yet, there is a ing folds of tissue forming the lateral boundar- growing awareness that while sharing some of the ies of the vulva. The medial aspects of the labia same anatomical structures and hormonal milieu, majora fuse with the labia minora , which are sexual function and fertility/reproductive function thin folds of skin outlining the introitus , or are distinct, with unique physiological responses. entry to the vagina. The anterior aspects of the The following is a brief presentation of the labia minora split and fuse with the ventral anatomy and histology of the female sex organs, aspect (or frenulum) of the glans clitoris and from the perspective of understanding the also unite over the glans as a hood, known as pathophysiology of sexual function. the clitoral prepuce. The posterior aspects of the labia minora fuse in the midline at the lower aspect of the introitus. The urethra is located immediately superior to the introitus. General Structural Relationships The internal genitalia are comprised of the vagina, the uterus (including cervix), uterine The external genitalia are referred to collec- tubes, and ovaries (Fig. 2.3 ). The vagina is imme- tively as the vulva and consists of the mons diately posterior to the urethra and bladder and pubis, clitoris and bulbs, labia majora, and labia extends from the perineum superiorly and poste- minora (Fig. 2.1 ). The mons pubis is an area of riorly toward the cervix, which protrudes into fatty tissue overlying the pubic symphysis, the vagina at approximately a 90° angle. The uterus typically is positioned posterior and supe- rior to the bladder. Posterior to the vagina is the rectum. Loops of intestine are found superior C.C. Yang ( ) and posterior to the uterus. The ovaries and uter- Department of Urology , University of Washington, Harborview Medical Center , Seattle , ine tubes extend laterally from the superior WA 98104-9868 , USA aspect of the uterus. J.P. Mulhall et al. (eds.), Cancer and Sexual Health, Current Clinical Urology, 13 DOI 10.1007/978-1-60761-916-1_2, © Springer Science+Business Media, LLC 2011 14 V.A.T. Ginger and C.C. Yang External Genital Anatomy Gross anatomical and histological study of the clitoris, clitoral bulbs, labia minora, and urethra reveals them to contain specialized vascular tissues that are sexually responsive [ 1 ] and consist of two histologically distinct types of vascular tissue. The fi rst type is the trabeculated erectile tissue seen in the clitoris and the bulbs. Gross examination of these structures demon- strate large, dilated vascular spaces that are fi lled with blood and are spongy in appearance (Fig. 2.4 ). On histological examination, the erec- tile tissues are trabecular, with smooth muscle underneath the epithelium of the vascular spaces. The erectile tissue of the clitoris and bulbs is very similar to that of the male corpora cavern- osa and corpus spongiosum. In contrast to the clitoral and bulbar erectile tissue, the labia minora and glans clitoris are composed of nonerectile vascular tissue in which the blood Fig. 2.1 Female external genitalia (premenopausal vessels are dispersed within a fi brous matrix, woman) (Image from http://commons.wikimedia.org/ with only a minimal amount of smooth muscle. wiki/File:Womans_vulva.jpg . Under GNU Free Docu- mentation License.) Nonerectile, sexually responsive vascular tissue is also found surrounding the urethral lumen and within the walls of the vagina (see below). Clitoris The structure of the clitoris has been “redis- covered” many times over the years by anatomists due to the frequent omission or misrepresenta- tion of the organ in historical and contemporary anatomical texts. The clitoris is comprised of two erectile bodies, the corpora cavernosa. The body is the convergence of two corpora, and the crura are the extensions of the corpora beneath the descending pubic rami. Each of the corpora cavernosa is surrounded by a thick fi bro-elastic tunica albuginea (Fig. 2.5 ). Because the majority Fig. 2.2 Axial view of female genitalia, magnetic reso- of the clitoris is hidden by the mons pubis, there nance image. The high intensity signal of the clitoris and is a lack of appreciation for the substantial nature the bulbs is a function of the high blood fl ow within these of these erectile bodies in the vulva (Fig. 2.4 ). structures. (The high signal intensity of the mons and sur- rounding tissue is artifact.) Body body of clitoris; Crus The clitoral body and the crura can be 10 cm or clitoral crus; U urethra; R rectum; IT ischial tuberosity more in length with the body measuring 5–7 cm (image courtesy of Kenneth R. Maravilla, MD) in length. 2 Functional Anatomy of the Female Sex Organs 15 Fig. 2.3 Female internal genitalia, sagittal view (based on drawing by Robert Holmberg, Seattle, WA) Fig. 2.4 Clitoris and bulb. ( a ) Gross sagittal view of indicates the approximate level of the cross-section seen external genitalia. Clitoris is inked blue , with body extend- in adjacent image 1.4b . Centimeter ruler. ( b ) Axial view ing anteriorly into mons pubis and prepuce (glans not vis- of genital cross-section. The darker colored tissue is erec- ible), and right crus amputated. Skin appears dark brown , tile tissue with residual pooled blood. The large, dilated and subcutaneous tissue is tan . Pars intermedia, a collec- vascular spaces are apparent in the clitoral body and the tion of vessels between the clitoral body and urethra, is bulbar tissue draped over the urethral lumen. The rugae of stained green . Right clitoral bulb stained red . Metal probe the anterior vaginal wall is seen beneath the urethral placed through urethral lumen (not visible). Vertical line lumen. Width of clitoral body approximately 1.5 cm 16 V.A.T. Ginger and C.C. Yang Fig. 2.5 Midshaft cross-section of clitoral body, Dilated vascular spaces comprise the erectile tissue of the Masson’s trichrome stain. Collagen/connective tissue corpora, some fi lled with residual corpuscles. Surrounding stained dark blue , smooth muscle (and residual pooled the tunica, particularly on its dorsal surface, is a collec- blood) stained red . The two corpora cavernosa converge to tion of arteries, veins, nerves, and supporting tissue. form the clitoral body. They are surrounded by a fi brous Arrowhead artery; arrow nerve fascicles, asterisk vein. tunica albuginea and separated by a fi brous septum. Bar = 1 mm The glans clitoris rests as a fi brovascular cap at the tip of the clitoral body. Unlike its male counterpart, the glans clitoris lacks smooth mus- cle within its fi brovascular cap, thus differentiat- ing it from the erectile tissue of the clitoris and bulbs. Despite its diminutive size, the glans clitoris is richly innervated and is an important mediator of sensory input for sexual arousal (Fig. 2.6 ). The clitoris appears to be a purely sen- sory organ [ 2 ] . On histological examination, the clitoral tis- sue is composed of large vascular spaces with mainly vascular epithelium and smooth muscle Fig. 2.6 Longitudinal section of clitoral glans and body, interspersed throughout. The trabecular nature S100 antibody stain. Neural elements are stained brown . of the erectile tissue allows for engorgement and The bulbous head of the glans is surrounded by preputial expansion during sexual arousal. tissue. Just beneath the epithelium of the glans is an abun- dance of neural elements, including nerve fi bers and receptors. Bar = 1 mm Bulbs inferior to the clitoral body (Fig. 2.4 ). The bulbs are also referred to as the vestibular bulbs, but The clitoral bulbs drape over the urethra (similar are more closely related to the clitoris than the to a saddle bag) with the two globular bulbs lying vestibule [ 3 ] . The bulbs lie fl anking the urethra 2 Functional Anatomy of the Female Sex Organs 17 and vagina and sit recessed within the vestibule. of the bulbs and their subsequent encroachment They do not surround the introitus as suggested on the introital opening may contribute to a sen- by some texts. There is no tunica albuginea sation of genital engorgement. enveloping the erectile tissue. The size of the bulb varies between individuals and may be dependent on age and estrogenization. Labia Majora The bulbs are considered the equivalent of the male spongiosum, but the bulbs do not com- The labia majora are two elongated folds of skin pletely encircle the urethra. The histologic that extend between the mons pubis and the features of the bulb are similar to that of the perineal body (Fig. 2.1 ). They enclose the labia clitoris. The erectile tissue of the clitoris has minora, glans clitoris, and the vaginal introitus. slightly less smooth muscle and interstitial They are hair bearing, pigmented skin folds. The fi bro-elastic tissue, while the bulbs have more subcutaneous tissue of the labia majora consists prominent fi bro-elastic tissue and smooth mus- mostly of fat.