interstitial tissue unknown factors unknown factors
seminiferous tubules testosterone
FIGURE 14 Negative feedback regulation of testicular function; + , stimulation; -, inhibition. Direct effects of testosterone on the pituitary gland are still uncertain.
gonadotropic hormones. The negative feedback effect of inhibin appears to be exerted exclusively on gonado-tropes to inhibit FSHp transcription and FSH secretion in response to GnRH. Some evidence indicates that inhibin may also exert local effects on Leydig cells to enhance testosterone production.
Testicular function is critical for development of the normal masculine phenotype early in the prenatal period. All of the elements of the control system are present in the early embryo. GnRH and gonadotropins are detectable at about the time that testosterone begins stimulating wolffian duct development. The hypothalamic GnRH pulse generator and its negative feedback control are functional in the newborn. Both the frequency and amplitude of GnRH and LH pulses are similar to those observed in the adult. After about the sixth month of postnatal life and for the remainder of the juvenile period, the GnRH pulse generator is restrained and gonadotropin secretion is low. The amplitude and frequency of GnRH pulses decline but do not disappear, and responsiveness of the gonadotropes to GnRH diminishes. It is evident that negative feedback regulation remains operative, however, because blood levels of
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