Djh

FIGURE 11 Anti-mullerian hormone (AMH) signaling pathway. AMH binds to its specific primary receptor (I) which then forms a heterodimer with and phosphorylates the secondary signal transducing subunit (II). The activated receptor complex then catalyzes phosphor-ylation ofSmad proteins on serine and threonine residues, causing them to bind Smad 4, which carries them into the nucleus, where transcription of specific genes results in expression of an apoptotic program and resorption of the Mullerian duct cells.

FIGURE 11 Anti-mullerian hormone (AMH) signaling pathway. AMH binds to its specific primary receptor (I) which then forms a heterodimer with and phosphorylates the secondary signal transducing subunit (II). The activated receptor complex then catalyzes phosphor-ylation ofSmad proteins on serine and threonine residues, causing them to bind Smad 4, which carries them into the nucleus, where transcription of specific genes results in expression of an apoptotic program and resorption of the Mullerian duct cells.

androgen in male embryos, or too much androgen in female embryos, differentiation is incomplete and the external genitalia are ambiguous. Differentiation of the masculine external genitalia depends on dihydrotestos-terone rather than testosterone. The 5a-reductase type II responsible for conversion of testosterone to dihydro-testosterone is present in tissues destined to become external genitalia even before the testis starts to secrete testosterone. In contrast, this enzyme does not appear in tissues derived from the wolffian ducts until after they differentiate, indicating that testosterone rather than dihydrotestosterone was the signal for differentiation of the wolffian derivatives.

The importance of androgen action in sexual development is highlighted by a fascinating human syndrome called testicular feminization, which can be traced to an inherited defect in the single gene on the X chromosome that encodes the androgen receptor. Afflicted individuals have the normal female phenotype but have sparse pubic and axillary hair and no menstrual cycles. Genetically, they are male and have intra-abdominal testes and circulating concentrations of testosterone and estradiol that are within the range found in normal men, but their tissues are totally unresponsive to androgens. Their external genitalia are female because, as already mentioned, the primordial tissues develop in the female pattern unless stimulated by androgen. Because AMH production and responsiveness are normal and their wolffian ducts are unable to respond to androgen, both of these duct systems regress and neither male nor female internal genitalia develop. Secondary sexual characteristics including breast development appear at puberty in response to unopposed action of estrogens formed extragonadally from testosterone.

Postnatal Development

Aside from a brief surge in androgen production during the immediate neonatal period, testicular function enters a period of quiescence, and further development of the male genital tract is arrested until the onset of puberty. Increased production of testosterone at puberty promotes growth of the penis and scrotum and increases pigmentation of the genitalia as well as the depth of rugal folds in scrotal skin. Further growth of the prostate, seminal vesicles, and epididymes also occurs at this time. Although differentiation of the epididymes and seminal vesicles was independent of dihydrotestosterone during the early fetal period, later acquisition of 5a-reductase type II makes this more active androgen the dominant form stimulating growth and secretory activity during the pubertal period. Increased secretion of FSH at puberty stimulates multiplication of Sertoli cells and growth of the seminiferous tubules, which constitute the bulk of the testicular mass.

The importance of some of the foregoing information is highlighted by another interesting genetic disorder that has been described as ''penis at twelve.'' Affected individuals have a deletion or inactivating mutation in the gene that codes for 5a-reductase type II and cannot convert testosterone to dihydrotestosterone in derivatives of the genital tubercule. Although testes and wolffian derivatives develop normally, the prostate gland is absent, and external genitalia at birth are ambiguous or overtly feminine. Affected children have been raised as females. With the onset of puberty, testosterone production increases, and there is an increase in the expression of 5a-reductase type I in the skin. Significant growth of the penis occurs at this time in response to the 5a-dihydro-testosterone produced in the liver and skin by the catalytic activity of 5a-reductase type I.

Get Rid of Gallstones Naturally

Get Rid of Gallstones Naturally

One of the main home remedies that you need to follow to prevent gallstones is a healthy lifestyle. You need to maintain a healthy body weight to prevent gallstones. The following are the best home remedies that will help you to treat and prevent gallstones.

Get My Free Ebook


Post a comment