Nos

Corpus uteri

Cervix Fornix Vagina

Figure 14.24 A. Genital ducts in the female at the end of the second month. Note the paramesonephric (mullerian) tubercle and formation of the uterine canal. B. Genital ducts after descent of the ovary. The only parts remaining from the mesonephric system are the epoophoron, paroophoron, and Gartner's cyst. Note the suspensory ligament of the ovary, ligament of the ovary proper, and round ligament of the uterus.

Figure 14.25 Androgen action at the cellular level. Receptor complexes with testosterone (T) and dihydrotestosterone (D) interact with DNA to control differentiation of the wolffian duct and external genitalia, respectively. R, androgen receptor; R*, transformed androgen receptor hormone complex.

separated by a septum but later fuse to form the uterine canal (Fig. 14.24A). The caudal tip of the combined ducts projects into the posterior wall of the urogenital sinus, where it causes a small swelling, the paramesonephric or mullerian tubercle (Fig. 14.24A). The mesonephric ducts open into the urogenital sinus on either side of the mullerian tubercle.

Molecular Regulation of Genital Duct Development

SRY is the master gene for testes development and appears to act directly on the gonadal ridge and indirectly on the mesonephric ducts. Thus, it induces the testes to secrete a chemotactic factor that causes tubules from the mesonephric duct to penetrate the gonadal ridge and stimulate further testicu-lar development. In fact, without penetration by these tubules, differentiation of the testes fails. SRY also upregulates steroidogenesis factor 1 (SF1), which acts through another transcription factor, SOX9, to induce differentiation of Sertoli and Leydig cells. Sertoli cells then produce mullerian inhibiting substance (MIS, also called antimullerian hormone, AMH) that causes regression of the paramesonephric (mullerian) ducts. Leydig cells produce testosterone, which enters cells of target tissues where it may remain or be converted to di-hydrotestosterone by a 5a reductase enzyme. Testosterone and dihydrotestosterone bind to a specific high-affinity intracellular receptor protein, and ultimately this hormone receptor complex binds to DNA to regulate transcription of tissue-specific genes and their protein products (Fig. 14.25). Testosterone receptor complexes mediate virilization of the mesonephric ducts, whereas dihydrotestosterone receptor complexes modulate differentiation of the male external genitalia (Fig. 14.26).

Sexual differentation in females was once thought to be a default mechanism that occurred in the absence of a Y chromosome, but it now appears that there are specific genes that induce ovarian development. For example, DAX1, a member of the nuclear hormone receptor family, is located on the short arm of the X chromosome and acts by downregulating SF1 activity, thereby

Ovary Wnt4
Figure 14.26 Influence of the sex glands on further sex differentiation.

preventing differentiation of Sertoli and Leydig cells. The secreted growth factor WNT4 also contributes to ovarian differentiation, and its early expression in the gonadal ridge is maintained in females but downregulated in males. In the absence of MIS production by Sertoli cells, the paramesonephric (mullerian) ducts are stimulated by estrogens to form the uterine tubes, uterus, cervix, and upper vagina. Estrogens also act on the external genitalia at the indifferent stage to form the labia majora, labia minora, clitoris, and lower vagina (Fig. 14.26).

Genital Ducts in the Male

As the mesonephros regresses, a few excretory tubules, the epigenital tubules, establish contact with cords of the rete testis and finally form the efferent ductules of the testis (Fig. 14.27). Excretory tubules along the caudal pole

Epigenital Tubule

Figure 14.27 A. Genital ducts in the male in the fourth month. Cranial and caudal (paragenital tubule) segments of the mesonephric system regress. B. Genital ducts after descent of the testis. Note the horseshoe-shaped testis cords, rete testis, and efferent ductules entering the ductus deferens. The paradidymis is formed by remnants of the paragenital mesonephric tubules. The paramesonephric duct has degenerated except for the appendix testis. The prostatic utricle is an outpocketing from the urethra.

Figure 14.27 A. Genital ducts in the male in the fourth month. Cranial and caudal (paragenital tubule) segments of the mesonephric system regress. B. Genital ducts after descent of the testis. Note the horseshoe-shaped testis cords, rete testis, and efferent ductules entering the ductus deferens. The paradidymis is formed by remnants of the paragenital mesonephric tubules. The paramesonephric duct has degenerated except for the appendix testis. The prostatic utricle is an outpocketing from the urethra.

of the testis, the paragenital tubules, do not join the cords of the rete testis (Fig. 14.27B). Their vestiges are collectively known as the paradidymis.

Except for the most cranial portion, the appendix epididymis, the mesonephric ducts persist and form the main genital ducts (Fig. 14.27). Immediately below the entrance of the efferent ductules, the mesonephric ducts elongate and become highly convoluted, forming the (ductus) epididymis. From the tail of the epididymis to the outbudding of the seminal vesicle, the mesonephric ducts obtain a thick muscular coat and form the ductus deferens. The region of the ducts beyond the seminal vesicles is the ejaculatory duct. The paramesonephric ducts in the male degenerate except for a small portion at their cranial ends, the appendix testis (Fig. 14.27B).

Genital Ducts in the Female

The paramesonephric ducts develop into the main genital ducts of the female. Initially, three parts can be recognized in each duct: (a) a cranial vertical portion that opens into the abdominal cavity, (b) a horizontal part that crosses the mesonephric duct, and (c) a caudal vertical part that fuses with its partner from the opposite side (Fig. 14.24A). With descent of the ovary, the first two parts develop into the uterine tube (Fig. 14.24B) and the caudal parts fuse

Urogenital ridge

Urogenital ridge

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Responses

  • SEBASTIAN LILJA
    What is virilization of mesonephric ducts?
    6 years ago

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