MALE REPRODUCTIVE PHYSIOLOGY
The Hypothalamic-Pituitary-Gonadal Axis
The hypothalamus is the integrative center of the reproductive axis and receives messages from both the central nervous system and the testes to regulate the production and secretion of gonadotropin releasing hormone (GnRH). Neurotransmitters and neuropeptides have both inhibitory and stipulatory influence on the hypothalamus. The hypothalamus releases GnRH in a pulsatile nature which appears to be essential for stimulating the production and release of both luteinizing hormone (LH) and follicle stimulating hormone (FSH). Interestingly and paradoxically, after the initial stimulation of these gonadotropins, the exposure to constant GnRH results in inhibition of their release. LH and FSH are produced in the anterior pituitary and are secreted episodically in response to the pulsatile release of GnRH. LH and FSH both bind to specific receptors on the Leydig cells and Sertoli cells within the testis. Testosterone, the major secretory product of the testes, is a primary inhibitor of LH secretion in males. Testosterone may be metabolized in peripheral tissue to the potent androgen dihydrotestosterone or the potent estrogen estradiol. These androgens and estrogens act independently to modulate LH secretion. The mechanism of feedback control of FSH is regulated by a Sertoli cell product called inhibin. Decreases in spermatogenesis are accompanied by decreased production of inhibin and this reduction in negative feedback is associated with reciprocal elevation of FSH levels. Isolated increased levels of FSH constitute an important, sensitive marker of the state of the germinal epithelium.
The Testes
Leydig; Cells
Testosterone is secreted episodically from the Leydig cells in response to LH pulses and has a diurnal pattern, with the peak level in the early morning and the trough level in the late afternoon or early evening. In the intact testis, LH receptors decrease or down-regulate after exogenous LH administration. Large doses of GnRH or its analogs can reduce the numbers of LH receptors and therefore inhibit LH secretion. This has been applied clinically to cause medical castration in men with prostate cancer. Estrogen inhibits some enzymes in the testosterone synthetic pathway and therefore directly effects testosterone production. There also appears to be an intratesticular ultra short loop feedback such that exogenous testosterone will override the effect of LH and inhibit testosterone production. In normal males, only 2% of testosterone is free or unbound. 44% is bound to testosterone-estradiol-binding globulin or TeBG, also called sex hormone-binding globulin. 54% of testosterone is bound to albumin and other proteins. These steroid-binding proteins modulate androgen action. TeBG has a higher affinity for testosterone than for estradiol, and changes in TeBG alter or amplify the hormonal milieu. TeBG levels are increased by estrogens, thyroid administration and cirrhosis of the liver and may be decreased by androgens, growth hormone and obesity. The biological actions of androgens are exerted on target organs that contain specific androgen receptor proteins. Testosterone leaves the circulation and enters the target cells where it is converted to the more potent androgen dihydrotestosterone by an enzyme 5-alpha-reductase. The major functions of androgens in target tissues include 1) regulation of gonadotropin secretion by the hypothalamic-pituitary axis; 2) initiation and maintenance of spermatogenesis; 3) differentiation of the internal and external male genital system during fetal development; and 4) promotion of sexual maturation at puberty.
Male Infertility Overview
Assessment, Diagnosis, and Treatment
Stephen F. Shaban, M.D. Clinical Assistant Professor
Department of Surgery, Division of Urology
University of North Carolina School of Medicine
Chapel Hill, NC.
Hormonal Control of Spermatogenesis
An intimate structural and functional relationship exists between the two separate compartments of the testis, i.e. the seminiferous tubule and the interstitium between the tubules. LH effects spermatogenesis indirectly in that it stimulates androgenous testosterone production. FSH targets Sertoli cells. Therefore, testosterone and PSH are the hormones that are directed at the seminiferous tubule epithelium. Androgen-binding protein which is a Sertoli cell product carries testosterone intracellularly and may serve as a testosterone reservoir within the seminiferous tubules in addition to transporting testosterone from the testis into the epididymal tubule. The physical proximity of the Leydig cells to the seminiferous tubules and the elaboration by the Sertoli cells of androgen-binding protein, cause a high level of testosterone to be maintained in the microenvironment of the developing spermatozoa. The hormonal requirements for initiation of spermatogenesis appear to be independent of the maintenance of spermatogenesis. For spermatogenesis to be maintained like for instance after a pituitary obliteration, only testosterone is required. However, if spermatogenesis is to be re-initiated after the germinal epithelium has been allowed to regress completely, then both FSH and testosterone are required.
Physical Examination
A careful examination of the testes is an essential part of the examination. Normal adult testes are on the average about 4.5 cm long and 2.5 cm wide with a mean volume of about 20 cc. A caliper or orchidometer may be used to measure testicular size. If the seminiferous tubules were damaged before puberty, the testes are small and firm. With postpubertal damage, they are usually small and soft.
!!!!GYNECOMASTIA!!!! is a consistent feature of a feminizing state. Men with congenital hypogonadism may have associated midline defects such as anosmia, color blindness, cerebellar ataxia, hair lip, and cleft palate. Hepatomegaly may be associated with problems of hormonal metabolism. Proper neck examination may help rule out thyromegaly, a bruit or nodularity associated with disease. Neurologic exam should test the visual fields and reflexes.
!!!PRE-TESTICULAR CAUSES OF INFERTILITY!!!
Isolated LH deficiency ("Fertile eunuch")
Isolated FSH deficiency
Hyperprolactinemia
Exogenous hormones (estrogen-androgen excess,
glucocorticoid excess, hyper and hypothyroidism).
HYPOTHALAMIC DISEASE
Kallmann's syndrome which is an isolated gonadotropin (LH and FSH) deficiency occurs in both a sporadic and familial form and although uncommon i.e. 1 in 10,000 men, it is second to Klinefelter's syndrome as a cause of hypogonadism. The syndrome is often associated with anosmia, congenital deafness, hair lip, cleft palate, craniofacial asymmetry, renal abnormalities, color blindness. The hypothalamic hormone GnRH appears to be absent. If exogenous GnRH is administered, both LH and FSH are released from the pituitary. Except for the gonadotropin deficiency, anterior pituitary function is intact. The syndrome appears to be inherited either as an autosomal recessive trait or an autosomal dominant trait with incomplete penetrance. The differential diagnosis should include delayed puberty. Kallmann's syndrome distinguishing features though are testes less than 2 cm in diameter and positive family history with the presence of anosmia. "Fertile eunuch" are individuals with isolated LH deficiency. They have eunuchoid proportions with variable degrees of virilization and gynecomastia. They characteristically have large testes and semen containing a few sperm. Plasma FSH levels are normal but both the serum LH and testosterone concentrations are low normal. The cause appears to be a partial gonadotropin deficiency in which there is adequate LH to stimulate testosterone production with resultant spermatogenesis but insufficient testosterone to promote virilization. In isolated FSH deficiency which is rare, patient's are normally virilized and have normal testicular size and baseline levels of LH and testosterone. Sperm counts range from O to a few sperm. Serum FSH levels are low and do not respond to GnRH stimulation. Congenital hypogonadotropic syndromes are associated with secondary hypogonadism and a multitude of other somatic findings. Prader-Willi syndrome is characterized by hypogonadism, hypomentia, hypotonia at birth and obesity. Laurence-Moon-Bardet-Biedel syndrome is an autosomal recessive trait characterized by mental retardation, retinitis pigmentosa, polydactyly and hypogonadism. These syndromes are felt to be due to a defect in hypothalamic deficiency of GnRH.
!!!!PROLACTIN!!!! also has a complex inter-relationship with the gonadotropins, LH and FSH. In males with hyperprolactinemia, the prolactin tends to inhibit the production of GnRH. Besides inhibiting LH secretion and testosterone production, elevated prolactin levels may have a direct effect on the central nervous system. In individuals with elevated prolactin levels who are given testosterone, libido and sexual function do not return to normal as long as the prolactin levels are elevated.
!!!!!!HYPERPROLACTEMIA!!!!! can cause both reproductive and sexual dysfunction. Prolactin-secreting tumors of the pituitary gland whether from a microadenoma (less than 10 mm) or a macroadenoma, can result in loss of libido, impotence, galactorrhea, gynecomastia and alter spermatogenesis. Patients with a macroadenoma usually first present with visual field abnormalities and headaches. They should undergo CT or MRI scanning of the pituitary and laboratory testing of anterior pituitary, thyroid and renal function. These patients have low serum testosterone levels but basal serum levels of LH and FSH are either low or low normal and reflect an inadequate pituitary response to depressed testosterone.
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