Testicular physiology

The testicles (testes) of a healthy adult person are paired, ovoid, with dimensions of 3.6-5.5 cm in length and 2.1-3.2 cm in width. The mass of each is about 20 g. Because of their location in the scrotum, these glands have a temperature of 2-2,5 C below the abdominal temperature, which promotes heat exchange between the blood. Spermatica and superficial venous system. Venous outflow from testicles and their appendages forms a plexus, the blood from which is left to the kidney, and to the right - into the lower genital vein. The testicle is surrounded by a thick capsule consisting of 3 layers: visceral, tunica vaginalis, belly coat and inner, tunica vasculosa. The white membrane has a fibrous structure. In the shells smooth muscle fibers are located, the reduction of which facilitates the movement of sperm into the epididymis (epididymis). Under the capsule there are approximately 250 pyramidal lobules separated from each other by fibrous septa. In each lobule there are several convoluted vas deferens 30-60 cm in length. These canals account for more than 85% of the testicle volume. Short straight tubules connect the tubules directly to the rete testis, from where the sperm enters the duct of the epididymis. The latter in its straightened form reaches 4-5 m in length, and in the folded state forms the head, body and tail of the appendage. In the epithelium surrounding the lumen of the tubule, Sertoli cells and spermatocytes are located. In interstitial tissue, the Leydig cells, macrophages, blood and lymphatic vessels lie between the tubules.

Cylindrical cells Sertoli perform many functions: barrier (due to close contacts with each other), phagocytic, transport (participation in the movement of spermatocytes to the lumen of the tubule) and, finally, endocrine (synthesis and secretion of androgen-binding protein and inhibin). Polygonal Leydig cells have an ultrastructure (pronounced smooth endoplasmic reticulum) and enzymes characteristic of steroid-producing cells.

Testicles play a major role in the physiology of reproduction in men. Thus, the acquisition of the male phenotype by the fetus is largely determined by the production of embryonic testes of the Müller's inhibitory substance and testosterone, and the appearance of secondary sexual characteristics during puberty and the ability to reproduce by the steroidogenic and spermatogenic activities of the testicles.

Synthesis, secretion and metabolism of androgens. In their products testicles have a more important role than the adrenal cortex. Suffice it to say that only 5% of T is formed outside the testicles. Leydig cells are able to synthesize it from acetate and cholesterol. The synthesis of the latter in testicles probably does not differ from the process occurring in the adrenal cortex. The key stage in the biosynthesis of steroid hormones is the conversion of cholesterol to pregnenolone, which involves the cleavage of the side chain in the presence of NADH and molecular oxygen. Further conversion of pregnenolone to progesterone can occur in various ways. In humans, apparently predominant importance D ⁵ -path in which pregnenolone is converted into 1 7a-hydroxypregnenolone and further into dehydroepiandrosterone (DHEA) and T. However, possible and A ⁴ is a path through the 17-hydroxyprogesterone and androstenedione. Enzymes of such transformations are Zbeta-hydroxysteroid dehydrogenase, 17a-hydroxylase, etc. In tet-ticules, as in the adrenal glands, conjugates of steroids (mainly sulphates) are produced. Enzymes cleaving the side chain of cholesterol are localized in the mitochondria, while enzymes for the synthesis of cholesterol from acetate and testosterone from pregnenolone are in microsomes. In testicles, there is a substrate-enzyme regulation. Thus, the hydroxylation of steroids in the 20th position is very active in humans, and the 20a-oxymetabolites of progesterone and pregnenolone inhibit 17a-hydroxylation of these compounds. In addition, testosterone can stimulate its own formation, affecting the conversion of androstenedione.

Adult's testicles produce from 5 to 12 mg of testosterone per day, as well as the weak androgens dehydroepiandrosterone, androstenedione and androstene-3beta, 17beta-diol. In the testicles tissue small amounts of dehydrotestosterone are formed, and aromatization enzymes are also present, as a result of which small amounts of estradiol and estrone enter the blood and seminal fluid. Although the main source of testicular testosterone are Leydig cells, but steroidogenesis enzymes are also present in other testicle cells (tubular epithelium). They can participate in creating a local high T level, required for normal spermatogenesis.

Testicles secrete T not constantly, but occasionally, and this is one of the reasons for the wide fluctuations in the level of this hormone in the blood (3-12 ng / ml in a healthy young man). The circadian rhythm of testosterone secretion ensures its maximum content in the blood in the early morning (about 7 am) and minimal after noon (about 13 hours). T is present in the blood mainly as a complex with sex hormone-binding globulin (GGSG), which connects T and DHT with a greater affinity than estradiol. Concentration of GGSG decreases under the influence of T and growth hormone and increases under the action of estrogens and thyroid hormones. Albumin binds androgens weaker than estrogens. In a healthy person in the free state, approximately 2% of serum T is found, 60% are bound to SHGG and 38% to albumin. Metabolic transformations undergo both free T and T, bound to albumin (but not SGHG). These transformations are mainly reduced to the reduction of the D ⁴ -keto group with the formation of 3alpha-OH- or 3beta-OH-derivatives (in the liver). In addition, the 17β-hydroxy group is oxidized to the 17β-keto form. About half of the testosterone produced is excreted from the body in the form of androsterone, etiocholanolone and (to a much lesser extent) epiandrosterone. The level of all these 17-ketosteroids in the urine does not allow us to judge the production of T, since similar adrenal androgens undergo similar metabolic transformations. Other excreted metabolites of testosterone are its glucuronide (whose level in the urine of a healthy person is well correlated with the production of testosterone), as well as 5α-and 5β-androstane-Zalph, 17β-diols.

Physiological effects of androgens and the mechanism of their action. In the mechanism of physiological action of androgens, there are features that distinguish them from other steroid hormones. Thus, in organah- "target" reproductive system, kidney and skin T influenced by intracellular enzyme D ⁴ -5a-reductase is converted to DHT, which, in fact, causes androgenic effects: increase in the size and functional activity of accessory sex organs in male hairiness type and increased secretion of the apocrine glands. However, in skeletal muscles, T itself without the additional transformations is able to increase protein synthesis. The receptors of the vas deferens have, apparently, equal affinity for T and DHT. Therefore, individuals with insufficiency of 5a-reductase retain active spermatogenesis. Turning into 5beta-androsten- or 53-pregneneoids, androgens, like progestins, can stimulate hemopoiesis. The mechanisms of the influence of androgens on linear growth and ossification of metaphyses have not been studied enough, although the growth acceleration coincides with the increase in secretion of T in puberty.

In target organs, the free T penetrates into the cytoplasm of the cells. Where there is 5a-reductase in the cell, it becomes DHT. T or DHT (depending on the target organ) binds to the cytosolic receptor, changes the configuration of its molecule and, accordingly, the affinity for the nuclear acceptor. The interaction of the hormone receptor complex with the latter leads to an increase in the concentration of a number of mRNAs, which is due not only to the acceleration of their transcription, but also to the stabilization of molecules. In the prostate, T also enhances the binding of methionine mRNA to ribosomes, where large amounts of mRNA enter. All this leads to the activation of translation with the synthesis of functional proteins that change the state of the cell.