Biology of Disease

division again ceases. The second meiotic division of the secondary oocyte is
completed at fertilization.

In males and females, the hypothalamus secretes gonadotrophin releasing
hormone (GnRH) which regulates secretion of LH and FSH from the basophil
cells of the anterior pituitary. Secretion of GnRH, LH and FSH occurs in pulses.
Follicle stimulating hormone and LH act cooperatively to stimulate the ovaries
and testes to secrete sex hormones and to develop germ cells.

The testes are stimulated by LH to release testosterone from their Leydig
cells (Figure 7.37). Testosterone is the principal androgen and its secretion
inhibits further release of LH by a negative feedback mechanism. Follicle
stimulating hormone and testosterone are required by Sertoli cells in the
basement membrane of the seminiferous tubules to produce inhibin that,
in turn, inhibits the secretion of FSH by negative feedback (Figure 7.37).
Testosterone is also required for sexual differentiation, the development of
secondary sexual characteristics and spermatogenesis. It is transported in the
blood, bound to sex hormone binding globulin, SHBG (Figure 7.38 (A)) and to
a lesser extent albumin. Typically, only its free fraction is metabolically active.
Testosterone enters target cells and is converted to the potent androgen,
dihydrotestosterone by the enzyme 5@-reductase. Testosterone is also found
in the plasma of normal females, half of which is secreted by the ovaries, the
remainder arising from the peripheral conversion of androstenedione and
DHEAS, both of which are secreted by the adrenal cortex.

The ovaries produce estrogens, of which estradiol is required for the develop-
ment of female secondary sexual characteristics and normal menstruation.
Circulating estradiol is bound mostly to SHBG (Figure 7.38 (B)), although the
blood concentration of estradiol varies widely with the menstrual cycle.

REPRODUCTIVE HORMONES

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Corpus luteum

Degeneration of

corpus luteum

Graafian

follicle

Primary follicles

Oocyte

Zona

pellucida

Secondary follicles

Ovulation,

release of

secondary

oocyte

Figure 7.36 Overview of oogenesis and release

of oocyte. See text for details.

Hypothalamus

GnRH

Anterior

pituitary

FSH LH

Inhibin Sertolicells Leydigcells

Testosterone

Figure 7.37 The regulation of testosterone

(Figure 7.3 (C)) secretion.