Reproductive physiology is tightly regulated via hormonal control by the steroidal sex hormones. The process starts with puberty at around 10–13 years in females and slightly later in males.
In males, puberty is associated with maturation of the reproductive organs, with enlargement of the testes and penis. This is regulated via the release of glycoprotein gonadotrophins from the anterior pituitary gland in response to gonadotrophin-releasing hormone (GnRH) from the hypothalamus (Figure 50.1). The gonadotrophin, luteinising hormone (LH) acts on Leydig cells of the testes to stimulate the release of the androgens, including testosterone. The release of testosterone leads to the development of secondary sexual characteristics in males (body and facial hair, voice, muscular physique) and libido. Another gonadotrophin, follicle-stimulating hormone (FSH), acts on the Sertoli cells in the semininferous tubules of the testes to promote spermatogenesis. Testosterone also feeds back to inhibit both the hypothalamus and the anterior pituitary. The Sertoli cells exert negative feedback on FSH release via the release inhibin, which acts at the anterior pituitary.
In females, adrenache is the maturation of the adrenals, leading to the production of various steroid hormones (e.g. dehydroepiandrostone [DHEA] and androstenedione), which sensitise towards gonadotrophin stimulation. This then leads to gonarche with breast bud development in response to oestradiol. Puberty in females is similarly regulated via LH and FSH, such that increased pulses of the hormones lead to the initiation of the menstrual cycle with the regular development and release of ova (Figure 50.2). The female sex hormones lead to the development of secondary sexual characteristics in females, such as breast development.
Hormonal regulation of the menstrual cycle
The human female reproductive cycle is characterised by the menstrual cycle, which typically lasts 28 (+/−4) days and coordinates ovulation at its midpoint (Figures 49.3 and 49.4). The cycle is under tight hormonal regulation and divided into four phases.
- Follicular phase (from the start of menstrual bleeding to day 14): during this time there is maturation of a primordial follicle. Many primordial follicles are present in the ovaries from the fetal stage of the female and they contain an ovum or oocyte, arrested in meiosis and surrounded by a single epithelial cell layer (precursor granulosa cells). Many of the primordial follicles undergo atresia but some are rescued by pulses of LH and FSH. Follicles mature during the reproductive years and this is influenced by LH, which acts on outer theca cells of the developing follicles, which then release androgens. The androgens are then converted by aromatase in the granulosa cells to oestrogens, and these levels rise throughout the phase. The maturation process is supported by FSH acting on the granulosa cells. The maturation leads to the formation of a Graafian follicle, one of which per cycle becomes dominant.
- Ovulation: the Graafian follicle grows substantially and ruptures mid-cycle to release the single oocyte into the peritoneal cavity to enter the oviduct. Ovulation is preceded by the appearance of LH receptors on the granulosa cells and by a surge in LH release due to oestradiol, which at high levels has a positive feedback effect.
- Luteal phase (day 14–28): post-ovulation the ruptured follicle becomes the corpus luteum, in which the granulosa cells respond to LH by producing progesterone and the theca cells produce oestrogen to provide the optimal conditions for implantation for a fertilised ovum. The progesterone causes negative feedback on GnRH, so that LH and FSH levels become minimal and there is no further follicular development.
- Luteolysis (day 28): in the absence of fertilisation the corpus luteum starts to break down, with decreased production of progesterone. This leads to menstruation with the breakdown of the vascular uterine wall, which is shed and leads to menstrual period bleeding.
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