Descent of the gonads is considerably less in the female than in the male, and the ovaries finally settle just below the rim of the true pelvis. The cranial genital ligament forms the suspensory ligament of the ovary, whereas the caudal genital ligament forms the ligament of the ovary proper and the round ligament of the uterus (Fig. 14.24). the latter extends into the labia majora.
The urinary and genital systems both develop from mesodermal tissue. Three urinary systems develop in a temporal sequence from cranial to caudal segments:
The pronephros, which forms in the cervical region, is vestigial.
The mesonephros, which forms in the thoracic and lumbar regions, is large and is characterized by excretory units (nephrons) and its own collecting duct, the mesonephric or wolffian duct. In the human it may function briefly, but most of the system disappears. Ducts and tubules from the mesonephros form the conduit for sperm from the testes to the urethra. In the female, these ducts regress.
The metanephros, or permanent kidney, develops from two sources. It forms its own excretory tubules or nephrons like the other systems, but its collecting system originates from the ureteric bud, an outgrowth of the mesonephric duct. This bud gives rise to the ureter, renal pelvis, calyces, and the entire collecting system (Fig. 14.5). Connection between the collecting and excretory tubule systems is essential for normal development (Fig. 14.6). WT1, expressed by the mesenchyme, makes this tissue competent to respond to induction by the ureteric bud. Interactions between the bud and mesenchyme occur through production of GDNF and HGF by the mesenchyme with their tyrosine kinase receptors RET and MET, respectively, produced by the ureteric epithelium. PAX2 and WNT4, produced by the ureteric bud, cause epithelial-ization of the metanephric mesenchyme in preparation for excretory tubule differentiation (Fig. 14.7). Early division of the ureteric bud may lead to bifid or supernumerary kidneys with ectopic ureters (Fig. 14.9). Abnormal positions of the kidney, such as pelvic and horseshoe kidney, are also well known (Fig. 14.11).
The genital system consists of (a) gonads or primitive sex glands, (b) genital ducts, and (c) external genitalia. All three components go through an indifferent stage in which they may develop into either a male or a female. The SRY gene on the Y chromosome produces testes-determining factor and regulates male sexual development. Genes downstream from SRY include steroidogenesis factor (SF1) and SOX9 that stimulate differentiation of Sertoli and Leydig cells in the testes. Expression of the SRY gene causes (a) development of the medullary (testis) cords, (b) formation of the tunica albuginea, and (c) failure of the cortical (ovarian) cords to develop. In the absence of the SRY gene, the combination of DAX1 expression, to downregulate SF1, and continued expression of WNT4 in the gonadal ridge, causes formation of ovaries with (a) typical cortical cords, (b) disappearance of the medullary (testis) cords, and (c) failure of the tunica albuginea to develop (Fig. 14.21). When primordial germ cells fail to reach the indifferent gonad, the gonad remains indifferent or is absent.
The indifferent duct system and external genitalia develop under the influence of hormones. Testosterone produced by Leydig cells in the testes stimulates development of the mesonephric ducts (vas deferens epididymis), while MIS produced by Sertoli cells in the testes causes regression of the paramesonephric ducts (female duct system). Dihydrotestosterone stimulates development of the external genitalia, penis, scrotum, and prostate (Fig. 14.26). Estrogens influence development of the paramesonephric female system, including the uterine tube, uterus, cervix, and upper portion of the vagina. They also stimulate differentiation of the external genitalia, including the clitoris, labia, and lower portion of the vagina (Fig. 14.26). Errors in production of or sensitivity to hormones of the testes lead to a predominance of female characteristics under influence of the maternal and placental estrogens.
Problems to Solve
1. During development of the urinary system, three systems form. What are they, and what parts of each, if any, remain in the newborn?
2. At birth an apparently male baby has no testicles in the scrotum. Later it is determined that both are in the abdominal cavity. What is the term given to this condition, and can you explain the embryological origin of this defect?
3. It is said that male and female external genitalia have homologies. What are they, and what are their embryological origins?
4. After several years of trying to become pregnant, a young woman seeks consultation. Examination reveals a bicornate uterus. How could such an abnormality occur?
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