Endocrinology of Male Reproduction in Animals: Hormonal Control

The endocrinology of male reproduction involves a complex interaction between the hypothalamus, pituitary gland, and testes.

Primordial germ cells migrate to the gonads, differentiate into spermatogonia located in the center of the primitive testes. Development of medullary cords occurs earlier in males than in females.

The primitive testes produce or secrete an androgen or androgen like substance which causes development of mesonephric duct system and male external genetalia and at the same time degeneration and atrophy of the paramesonephric ducts.

Excessive and prolonged dosages of androgens given early in foetal life may cause masculinization of the external genitalia organs of the female fetuses.

Prior to the onset of puberty releasing hormones are secreted by the hypothalamus which is carried via the hypothalamo hypophyseal portal system into the anterior pituitary and causes the release of FSH and LH. FSH acts on the seminiferous tubules of the testes and is responsible for spermatogenesis i.e., from the initial division of the spermatogonia through the formation of secondary spermatocytes.

Thereafter the testosterone is responsible for sperm cell development. LH stimulates the growth and development of interstitial cells or Leydig cells in between the seminiferous tubules of the testes and the secretion of the testosterone and androgens from these cells.

These androgens are secreted into the blood stream where they cause the development of the secondary sexual characteristics in the male and development and maintenance of male reproductive tract. The androgens suppress GnRH, LH, FSH secretion by a negative feedback on the pituitary and hypothalamus.

FSH also interacts with receptors on sertoli cells to cause production of androgen binding protein (ABP), conversion of testosterone to dihydrotestosterone and estrogen completion of sperm release (spermiation) and secretion of inhibin.

The inhibin secreted into the blood stream has a negative feedback effect on FSH but on LH secretion. The ABP forms a complex with androgens and is carried along with the spermatozoa into the epididymis. The epithelial cells of the epididymis require relatively high levels of androgen for normal function.

Although much of the testosterone secreted into the seminiferous tubules is converted into dihydrotestosterone (DHT) by the enzyme 5alpha reductase, some of the testosterone gets converted to estrogens by the enzyme aromatase.

High levels of testosterone, estrogen or progesterone maintained by continuous injections, feeding or steroid producing tumors of the testes and adrenal gland causes and atrophy of the seminiferous epithelium and interstitial cells. This process can be reversed by withdrawal of the exogenous source of steroids resulting in testicular rebound phenomenon.

High levels of estrogens found in stallion urine are mainly due to estrogen produced by interstitial cells and or sertoli cells of the testes. Sertoli cell tumor of canine testes is common and there produce high levels of estrogens which cause feminization and testicular atrophy.

Androgen Binding Protein (ABP)

ABP is produced by sertoli cells under the influence of FSH and forms a complex with androgen. It is carried along with spermatozoa into the epididymus. The epithelial cells of the epididymus require high levels of androgen for normal function-Epididymal transport, sperm maturation and storage.

Testosterone

Testosterone is a steroid hormone soluble in oil and alcohol but not in water. acetate is converted to cholesterol which in turn is converted to progesterone-androstenedione-testosterone (16 times more active than androstenedione in bulls). In stallion the androstenodione is the major androgen.

Functions of Testosterone

• Sexual differentiation of external male genetalia and descent of the testes into scrotum in fetuses or neonates.
• Keratinization of the preputial epithelium, separation of glans penis from prepuce, growth of the penis and prepuce at puberty.
• Growth and maintenance of accessory sex glands.
• Sexual desire or libido, ability for normal erection and ejaculation.
• Secondary sexual characteristics like hair or horn growth male attitudes, voice, increased bone thickness, increased muscle tissue with different distribution of fat from female.
• Maintenance of secretory and absorptive activities and structure of efferent ducts, epididymis and ductus deferens including ampulla.
• Spermiogenesis- development and maturation of spermatids and spermatozoa.