Medical brain notes 42

  Embryology and Anatomy Knowledge of the embryology and anatomy of the   female genital system is helpful in understandingboth normal anatomy and the congenital anom-alies that occur. Embryology may be useful in many areas  of gynecologic and obstetric practice. For example, in gynecologic oncology, embryology can assist clinicians in predicting the growth and routes of spread of gynecologic cancers; in urogynecology and pelvic reconstructive surgery, it can enhance a surgeon’s comprehension of the compo-nents of pelvic support and possible defects. It can also play a key role in understanding and diagnosing various aspects of sexual dysfunction.   The ovaries, fallopian tubes, uterus, and upper portion of the vagina are derived from the intermediate mesoderm, while the external genitalia develop from genital swellings in the pelvic region. Beginning in the 4th week (postfertil-ization) of development, the intermediate mesoderm forms the  urogenital ridges ...

  Development of the Ovary   Ovaries are homologous to the testes in the male.  Bothtypes of gonads begin development as  gonadal  or  genitalridges  that form during the 5th week of gestation from theurogenital ridges.  Fingerlike bands of epithelial cells pro-ject from the surface of the gonad into each gonadal ridge, forming irregularly shaped  primary sex cords.  Growth of these cords into the gonadal ridge results in the creation of an outer cortex and an inner medulla in the indifferent gonad.   Primordial germ cells  that give rise to gametes appearin the wall of the yolk sac (now called the umbilical vesicle) during the 3rd week of development (see Fig. 4.1). From this location, primordial germ cells migrate along the allantois in the connecting stalk to the dorsal mesentery of the hindgut and then into the gonadal ridges, where they become associated with the primary sex cords by the 6th week. In the female, the primor...

  Development of the Genital Ducts   In both male and female embryos, two pairs of ducts develop—the  mesonephric (wolffian)  and  parameso-nephric (müllerian) ducts.  As with the gonad, these ducts passthrough an indifferent stage in which both pairs of ducts are pres-ent in both the male and the female embryo.  Differentiation ofthe female ductal system is not dependent on development of the ovaries (Fig. 4.4).   In the male embryo, the mesonephric ducts, which drain the embryonic mesonephric kidneys, eventually form the epididymis, ductus deferens, and ejaculatory ducts.  Inthe female embryo, the mesonephric ducts disappear. The para-mesonephric ducts persist to form major parts of the female reproductive tract (the fallopian tubes, uterus, and upper por-tion of the vagina).  Paramesonephric ducts begin as invagi-nations of the epithelium covering the urogenital ridges, eventually forming longitudinally oriented tubes. The cranial end...

  Development of the External Genitalia   The  cloaca  is formed from a dilatation of the caudal end of the hindgut and is covered exteriorly by the cloacal mem-brane. Eventually, the cloaca is separated into the urogeni-tal sinus anteriorly and the anorectal canal posteriorly by the  urorectal septum.  This septum forms from a collec-tion of mesoderm in the pelvic floor that grows downward during the 5th to the 8th weeks of gestation to reach the  cloacal membrane.  At the same time, the  genital tuber-cle  develops at the cranial end of the cloacal membrane,while  labioscrotal swellings  and  urogenital folds  appear on each side (Fig. 4.5A). The genital tubercle enlarges in both the male and the female (Fig. 4.5B). In the presence of estrogens and the absence of androgens, external genitalia are feminized. The genital tubercle develops into the cli toris (Fig. 4.5C). The unfused urogenital folds form the labia mino...

  ANATOMY   Bony Pelvis   The bony  pelvis  is composed of the paired innominate bones and the sacrum. The innominate bones are joined anteriorly to form the  symphysis pubis,  and each is artic-ulated posteriorly with the sacrum through the sacroiliac joint (Fig. 4.6). The  sacrum  is composed of five or six sacral vertebrae, which are fused in adulthood. The sacrum articulates with the coccyx inferiorly and with the fifth lumbar vertebra superiorly. In obstetrics, it is important to assess the size of the pelvis to determine whether it is of adequate capacity for vaginal birth.  This evaluation is based on the diameters of the pelvic outlet, pelvic inlet, and midpelvis. Measurement of these diameters is called  pelvimetry  and can be made radiograph-ically, with computed tomography (the most accurate method), or during a pelvic examination. One of the most important measurements is that of the  obstetrical conju-gate...

  Vulva and Perineum   The  perineum  comprises the area of the surface of the trunk between the thighs and the buttocks, extending from the coccyx to the pubis.  Anatomists also use the term “perineum” torefer to the shallow  compartment  that lies deep to this area and inferior to the pelvic diaphragm. The  vulva  contains the labia majora, labia minora, mons pubis, clitoris, vestibule, and ducts of glands that open into the vestibule (Fig. 4.9). The  labia majora  are folds of skin with underlying adipose tissue, fused anteriorly with the mons pubis and posteriorly at the perineum. The skin of the labia majora contains hair follicles as well as seba-ceous and sweat glands. The  labia minora  are narrow skin folds lying inside the labia majora. The labia minora merge anteriorly with the prepuce and frenulum of the clitoris, and posteriorly with the labia majora and the perineum. The labia minora contain sebaceous and swe...

  The Vagina   The lumen of the  vagina  is lined by a stratified squamous epithelium and surrounded by three layers of smooth mus-cle. Beneath the smooth muscle layers is a submucosal layer of connective tissue containing a rich supply of veins and lymphatic vessels. In children and young women, the ante-rior and poster walls of the vagina are in contact due to the presence of submucosal rugae.  Because the vagina is collapsed,it appears H-shaped in cross section.  The underlying rugaeconnect to the tendinous arch of the pelvic fascia, which is the major support of the walls of the vagina and help maintain its normal architecture. With age and childbirth, the connection between the vaginal walls and the muscu-lar pelvis may weaken or deteriorate, weakening the pelvic floor and causing the surrounding structures (bladder, rec-tum, urethra, and uterus) to become less stable. The  cervix  joins the vagina at an angle between 45 °  and 90 ° . Th...

  Uterus and Pelvic Support   The  uterus  lies between the  rectum  and the  bladder  (Fig. 4.11). Various pelvic ligaments help support the uterus and other pelvic organs. The  broad ligament  overlies the structures and connective tissue immediately adjacent to the uterus. Because it contains the uterine arteries and veins and the ureters, it is important to identify the broad ligament during surgery. The  infundibulopelvic lig-ament  connects the ovary to the posterior abdominal wall and is composed mainly of the ovarian vessels. The  uterosacral ligament  connects the uterus at the level of thecervix to the sacrum and is therefore its primary support. The  cardinal ligament  is attached to the side of the uterus immediately inferior to the  uterine artery.  The  sacro-spinous ligament  connects the sacrum to the iliac spine andis not attached to the uterus. This ligament is frequentl...

  Uterine Tubes   The  fallopian (uterine) tubes (oviducts)  are approxi-mately 7 to 14 cm in length, and are divided into three portions: a narrow and straight isthmus, which adjoins the opening into the uterus; the  ampulla,  or central portion; and the  infundibulum,  which is fringed by the finger-shaped fimbriae. The fallopian tubes surround the ovary and collect the oocyte at the time of ovulation. The fallopian tubes are supplied by the ovarian and uter-ine arteries. The epithelial lining of the fallopian tube is ciliated columnar; the cilia beat toward the uterus, assist-ing in oocyte transport.

  Ovaries   Each ovary is approximately 3 to 5 cm long, 2 to 3 cm wide, and 1 to 3 cm thick in the menstrual years. The size decreases by approximately two-thirds after menopause, when follicular development ceases. The ovary is attached to the broad ligament by the  mesovarium,  to the uterus by the ovarian ligament, and to the side of the pelvis by the sus-pensory ligament of the ovary (infundibulopelvic ligament), which is the lateral margin of the broad ligament. The outer ovarian cortex consists of follicles embedded in a connective tissue stroma. Embryologically, this stroma is the medulla that originated as the gonadal ridge, while the cortex origi-nated as coelomic epithelium. The medulla contains smooth muscle fibers, blood vessels, nerves, and lymphatics.   The ovaries are mainly supplied by the ovarian arteries, which are direct branches of the abdominal aorta, but there also is a blood supply from the uterine artery, a branch of the hypogastric arter...

  ANOMALIES OF THE FEMALE REPRODUCTIVE SYSTEM   Anatomic anomalies are infrequent and arise from defects during embryologic development.  Ovarian dysgenesis  or congenital absence is rare except in cases of chro-mosomally abnormalities. In Turner syndrome (45XO), there are streaks of abnormal ovarian tissues in the pelvis. In the anatomically female patient with a male chromosome compliment (46XY), the gonads only partially descend and can usually be found in the pelvis or even in the inguinal canal.   Much more common are müllerian (paramesonephric) abnormalities, most of which stem from incomplete or anom-alous fusion of the müllerian ducts.  Absence of the uterusoccurs when the müllerian ducts degenerate, a condition called  müllerian agenesis  (Fig. 4.14). This condition is associated with vaginal anomalies (such as absence of the vagina), because vaginal development is stimulated by the developing uterovaginal primordium. Since the vulva and ...