Testes: structure & endocrine function
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The document outlines the organization and physiological functions of the testes, focusing on the roles of Sertoli cells and hormones such as androgen-binding protein (ABP) and inhibin in spermatogenesis. It explains the regulatory mechanisms, including the interaction of the hypothalamus, pituitary gland, and Leydig cells, along with hormonal influences like FSH and testosterone. The text emphasizes the importance of ABP for concentrating testosterone and details the feedback regulation of FSH by inhibin during reproductive development.
Testes: structure & endocrine function
- 1. Fish Reproduction & Conservation Biol. Res. Lab. Department of Zoology H.N.B. Garhwal University Campus Badshahithaul Tehri Garhwal –249199, Uttarakhand, INDIA e-mail: agarwalnareshk3@rediffmail.com Dr. N. K. Agarwal Professor of Zoology The Testes: Organisation & Physiological Function Androgen binding protein (ABP), Inhibin. Neurendocrine control of testicular functions (Gn RH regulation, FSH- effects on germinal epitheluim, LH-effects on Leydig cells, negative feed back regulation).
- 2. The Testes: Organisation & physiological actions Androgen binding protein (ABP), Inhibin. Neurendocrine control of testicular functions (Gn RH regulation, FSH- effects on germinal epitheluim,LH- effects on Leydig cells, negative feed back regulation).
- 3. Organisation: Human testis with genital duct system It includes tubuli recti, rete testis, ductuli efferentes, ductus epididymidis and ductus deferens
- 5. Part of seminiferous lobule showing spermatogenic cells and sertoli cells
- 6. • At all stages of differentiation, the spermatogenic cells are in close contact with Sertoli cells which are thought to provide structural and metabolic support to the developing sperm cells. Sertoli cells •A single Sertoli cell extends from the basement membrane to the lumen of the seminiferous tubule, although the cytoplasmic processes are difficult to distinguish at the light microscopic level.
- 7. Testosterone is needed in very high quantities for maintenance of the reproductive tract, and ABP allows a much higher level of fertility Sertoli cells serve a number of functions during spermatogenesis, they support the developing gametes in the following ways: •Phagocytose residual cytoplasm left over from spermiogenesis •They release Antimullerian hormone which prevents formation of the Mullerian Duct / Oviduct. •Maintain the environment necessary for development and maturation via the blood-testis barrier •Secrete substances initiating meiosis •Secrete supporting testicular fluid •Secrete androgen-binding protein, which concentrates testosterone in close proximity to the developing gametes •Secrete hormones effecting pituitary gland control of spermatogenesis, particularly the polypeptide hormone, inhibin
- 8. Androgen-binding protein (ABP) • Androgen-binding protein (ABP) is a glycoprotein (beta-globulin) produced by the Sertoli cells in the seminiferous tubules of the testis that binds specifically to testosterone (T), dihydrotestosterone (DHT), and 17-beta-estradiol. • Because ABP binds to T and DHT, these hormones are made less lipophilic and become concentrated within the luminal fluid of the seminiferous tubules. The higher levels of these hormones enable spermatogenesis in the seminiferous tubules and sperm maturation in the epididymis. • ABP has the same amino acid sequence as sex hormone-binding globulin (SHBG); the difference is the site of production and the addition of different sugar moieties. ABP contains 403 amino acids, resulting in a molecular weight of 44,533. Its gene is located on chromosome 17. • ABP’s production is regulated under influence of FSH on Sertoli cell, enhanced by insulin, retinol, and testosterone.
- 9. Inhibin Inhibin has been defined as a gonadal hormone that exerts a specific negative feedback action on the secretion of FSH from the gonadotropic cells of the pituitary gland. The existence of inhibin was postulated as early as 1923. However, only after reliable and sensitive bioassay systems for the detection and estimation of inhibin had been developed and an ample source of inhibin was found in the form of ovarian follicular fluid . It is apparent now that inhibin, which itself consists of a dimer of two different subunits, alpha and beta, is a member of a much larger family of (glyco)protein hormones and growth factors, which also includes Müllerian-inhibiting substance, transforming growth factor- beta, erythroid differentiation factor, an insect protein that plays an important role in differentiation, and the dimer of two inhibin beta- subunits, activin. The latter substance was reported to counteract the effects of inhibin in pituitary cells. Interactions between these regulatory substances will certainly be a field of major interest in the near future. Inhibin molecules, similar to those in the gonads, have been detected in placental tissue as well.
- 10. Inhibin The postulate of inhibin in seminal plasma, prostate, and gastric juice is apparently due to detection of substances that differ from gonadal inhibin. Most likely these substances, the amino acid sequences of which have been elucidated, have a biological function that differs from the suppression of peripheral levels of FSH in vivo. Inhibin is produced in the Sertoli cells in the testis and in the granulosa cells in the ovary. The production of inhibin is stimulated by FSH, but presently a lot of controversy exists about other factors that might play a role in the regulation of the production of inhibin. Because of the lack of reliable methods for estimation of peripheral levels of inhibin in humans and experimental animals, almost all evidence for the physiological importance of inhibin in the regulation of reproductive processes is derived from circumstantial evidence. From these indirect results, it appears likely that inhibin plays an important role in the feedback regulation of peripheral concentrations of FSH during the period in which Sertoli cells and granulosa cells, the target cells for FSH, divide, i.e., during puberty in male animals and during the development of ovarian follicles in female animals.
- 11. • Hormonal control of spermatogenesis varies among species. • In humans initiation of spermatogenesis occurs at puberty due to the interaction of the Hypothalamus, Pituitary gland and Leydig cells. • Follicle stimulating hormone stimulates both the production of ABP (Androgen Binding Protein) by Sertoli cells, and the formation of the blood-testis barrier. • Androgen binding protein is essential to concentrating testosterone in levels high enough to initiate and maintain spermatogenesis, which can be 20-50 times higher than the concentration found in blood. Hormonal Control of Testicular function
- 12. • Follicle stimulating hormone may initiate the sequestering of testosterone in the testes, but once developed only testosterone is required to maintain spermatogenesis. However, increasing the levels of follicle stimulating hormone will increase the production of spermatozoa by preventing the apoptosis of type A spermatogonia. • The hormone inhibin acts to decrease the levels of follicle stimulating hormone. • The Sertoli cells themselves mediate parts of spermatogenesis though hormone production. They are capable of producing the hormones estradiol and inhibin. • The Leydig cells are also capable of producing estradiol in addition to their main product testosterone Hormonal Control of Testicular function
- 13. Neuro- Endocrine control of testicular function (Gn RH regulation, FSH- effects on germinal epitheluim, LH- effects on Leydig cells, negative feed back regulation)
- 14. March 25, 2020 Prof. N.K.Agarwal agarwalnareshk3@rediffmail.com