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This chapter briefly reviews the embryology of the male reproductive system, whose knowledge is required to understand the physiopathology of cryptorchidism and of hypospadias. One distinctive feature of hormone secretion through the hypothalamus-pituitary-gonadal axis is that they regulate their own secretion through negative feedback inhibition. Androgens are essential for spermatogenesis, maturation of secondary sexual characteristics, masculine settlement of the bone-muscle apparatus, and libido. Testosterone is the most important circulating androgen in men's blood. Sperm progression in the seminal tract during ejaculation and contractions of the epididymis are supported by oxytocin and guided by sympathetic and parasympathetic nerves. Sperm-egg interaction is a specialized process that leads to fertilization. The occurrence of acrosomal exocytosis facilitates sperm penetration through the zona pellucida, and exposure of certain molecules on the sperm equatorial segment that participate in fusion with the oolemma.
The male hypothalamic-pituitary-gonadal (HPG) axis is a finely controlled system whose role is to promote spermatogenesis and androgen biosynthesis. Testosterone is thought to feed back to restrain activity of the gonadotropin-releasing hormone (GnRH)-gonadotrope secretory unit. GnRH is released from the hypothalamus in a pulsatile pattern, and the stimulation of gonadotropin biosynthesis and secretion by GnRH is dependent on the pulsatile nature of GnRH delivery to the anterior pituitary. Gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are glycoproteins consisting of a common a subunit and a hormone-specific β subunit that are associated through noncovalent interactions. GnRH stimulates in vitro the synthesis of gonadotropin subunits and increases a, LH-β, and FSH-β subunit mRNA levels as well as the transcriptional activity of corresponding gene promoters. Testosterone seems to exert a direct feedback control of LH secretion, while its action on FSH secretion is mostly indirect.
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