The Male Reproductive System / The Testes

The testes, or testicles, are the male gonads. They contain germ cells that differentiate into mature spermatozoa, supporting cells called Sertoli cells, and testosterone-producing cells called the Leydig cells. The germ cells migrate to the fetal testes from the embryonic yolk sac. The Sertoli cells are analogous to the granulosa cells in the ovary, and the Leydig (interstitial) cells are analogous to the stromal cells of the ovary.

The embryonic differentiation of the primitive, indifferent gonad into either the testes or ovaries forms a fascinating chapter in fetal development. Testosterone and its potent derivative, dihydrotestosterone, play key roles in the formation of male genitalia in the fetus in the first trimester of pregnancy. During the first four weeks after birth, they sensitize the genitalia to respond appropriately to androgens when puberty begins. The testes are formed in the abdominal cavity and descend into the scrotum during the seventh month of pregnancy. Stimulation of testicular descent is provided by androgens, along with a protein hormone called Müllerian-inhibiting substance. It is not uncommon in normal males for the testes to be incompletely descended and easily retracted into the abdomen, but this condition usually corrects itself by the age of three months.

The adult testis consists largely of a series of tubules with a central cavity. Sperm cells are continuously maturing as they move from the outer edge of the tubule into the central lumen; the most primitive forms, called spermatogonia, differentiate first into spermatocytes and then spermatids. They eventually mature into spermatozoa and are released into the lumen. Spermatozoa travel through the tubular network to be stored in seminal vesicles and, finally, to be ejaculated with the semen. Interspersed among the seminiferous tubules are Sertoli cells, and in the area between tubules (interstitium) are located the hormone-secreting Leydig cells.
 

Regulation of hormone secretion

Androgen levels in the circulation are regulated by the classical hypothalamic-pituitary-target gland axis. The secretion of pituitary LH (sometimes referred to in the male as interstitial cell stimulating hormone, or ICSH) is secreted following stimulation by gonadotropin-releasing hormone (GnRH) from the hypothalamus. Luteinizing hormone stimulates the Leydig cells to secrete testosterone. When testosterone levels rise above normal, GnRH and LH secretion are inhibited. In the normal course of events, therefore, testosterone levels remain within normal bounds.

The hypothalamic component of this axis comes into play when it is appropriate to override the usual constraints. It has been shown in primates, for example, that serum testosterone levels rise when males are placed in proximity to receptive females, but the level falls when these same males are caged with unreceptive, hostile males to whom they are strangers. It is thought by some that the reduction in serum testosterone levels in such an alien environment is accompanied by a decrease in aggressive behaviour, which, literally, may have survival value. A relation between androgen levels and aggressive behaviour in humans remains uncertain; complex social and interpersonal factors make interpretation difficult.

Like other steroid hormones, testosterone is transported in the plasma bound to a testosterone-binding globulin (TeBG) and to albumin. Only about 2 percent of testosterone is transported unbound in the plasma. Free testosterone is in equilibrium with that which is bound so that when the free steroid enters the cell some bound testosterone is freed simultaneously.

Hormones

Testosterone serves as a circulating prohormone for an important steroidal metabolite, dihydrotestosterone, that performs most of the androgenic functions in the body. Testosterone may also be converted into the potent estrogen estradiol in tissues, particularly adipose tissue. Furthermore, testosterone is interconvertible with androstenedione, which, again in adipose tissue, may be converted to the estrogen estrone.

Testosterone has two major actions: it serves as the feedback inhibitor of GnRH secretion from the hypothalamus and LH secretion from the pituitary, and it directs the development of embryonic Wolffian ducts into the formation of seminiferous tubules. Dihydrotestosterone is responsible for ongoing sperm maturation (spermatogenesis), for the virilization of the embryonic genitalia, and for sexual maturation at puberty. In addition, androgens are powerful anabolic hormones; that is, they enhance the growth of body tissues, particularly muscle.

Normal spermatogenesis requires the secretion of LH and FSH. Luteinizing hormone stimulates testosterone secretion from Leydig cells in the stroma of the testis; the testosterone is converted to dihydrotestosterone, and it must be present locally in high concentration for normal generation of sperm to proceed. Follicle-stimulating hormone acts directly on the seminiferous tubules to stimulate the normal maturation of sperm. Finally, as indicated previously, androgens stimulate Sertoli cells to secrete inhibin. When released into the blood, inhibin dampens pituitary FSH secretion, an additional component of the feedback control mechanism.

The following reflexes may be active when the ovaries are involved:

• Metabolic Bladder Sub-Reflex
• Metabolic Kidney Sub-Reflex
• Metabolic Liver Sub-Reflex
• Metabolic Testicle Sub-Reflex
• Blood Quality Reflex
• Hemoglobin Reflex
• Intestinal Flora Pus Sub-Reflex
• Liver-Metal Overlad Pus Sub-Reflex
• Gut Sub Reflex
• Pituitary Brain Sub Reflex Page

These reflexes should be tested and treated with the proper supplementation.

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