Thursday, February 24, 2011

HORMONAL CONTROL OF SPERMATOGENESIS IN FARM ANIMALS



-       The hormones control the reproduction even at the embryonic stage. In the developing embryo the gonads remain in different.

-       Differentiation of gonads into the testis in male occur earlier compare to the differentiation of gonad into ovary in female.

-       The primitive testis apparently secrets androgen like substance which is responsible for,

(1)          The development of mesonephric tubules and ducts into permanent genital duct system of the male.
(2)          The formation of the male’s external genitalia and
(3)          The degeneration of the paramesonephric duct system.

ü Normal reproductive activity in male animals comprises the production of semen containing normal spermatozoa in the adequate numbers, together with the desire and the ability to mate.
o  These sexual functions are under the intrinsic control through the hormones and the central nervous system but environmental factors such as temperature, length of day light, nutrition, change of surrounding and systematic disease may influence reproductive performance.
o  The same pituitary gonads tropic hormones occur as in the female and they act on testis as follows:
o  Follical stimulating hormone promotes development of the epithelium of the seminiferous tubules and there by it controls spermatogenesis.
o  Luteinizing hormone, or interstitial cell  stimulating hormone, promotes growth and function of the interstitial or leydig cells and thus the control the secretion of the male sex hormones, testosterone.
o  Chemically testosterone is steroid in nature it is closely related to estrogen and progesterone in females and is excreted as androsteron.
o  It causes development of outward male characteristic as well as promoting growth and the function of accessory sexual organs, by its influence on the accessory glands it affects the composition of the semen.
o  Therefore, the functions of the testicals are spermatogenesis and endocrine, since both are under pituitary control there is a close relationship between them. 
o  LH is required for the spermatogenesis because it is required for the production of the testosterone hormone.
o  However, testosterone alone is not adequate for completion of spermatogenesis and a block on spermatogenesis occurs at meiosis.
o  FSH is required for initial spermatogenesis: FSH is also important for completion of meiosis and for final morphological differentiation of the spermatid to spermatozoa through its influence on the activity of sertoli cells.
o  FSH also playing role in the release of spermatids from the syncytium of spermatids surrounding the sertoli cells.
o  There is some evidence that a third gonadotropin (prolactin) also plays an important role in the process of spermatogenesis, prolactin facilitates the inter action of LH with its receptors located on leydig cells.
o  The sertoli cells provide nutrition and regulate the spermatogenesis.



o    Spermatogenesis is the process by which male primary germ cells undergo division, and produce a number of cells termed spermatogonia, from which the primary spermatocytes are derived.
o    Each primary spermatocyte divides into two secondary spermatocytes, and each secondary spermatocyte into two spermatids or young spermatozoa.
o    These develop into mature spermatozoa, also known as sperm cells. Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, and the two secondary spermatocytes by their subdivision produce four spermatozoa.
o    Spermatozoa are the mature male gametes in many sexually reproducing organisms. Thus, spermatogenesis is the male version of gametogenesis.
o    In mammals it occurs in the male testes and epididymis in a stepwise fashion, and for humans takes approximately 64 days.
o    Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly, and is essential for sexual reproduction.
o    It starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age.
o    The entire process can be broken up into several distinct stages, each corresponding to a particular type of cell.
§  Sertoli cells serve a number of functions during spermatogenesis, they support the developing gametes in the following ways:
ü Maintain the environment necessary for development and maturation via the blood-testis barrier
ü Secrete substances initiating meiosis
ü Secrete supporting testicular fluid
ü Secrete Androgen-Binding Protein, which concentrates testosterone in close proximity to the developing gametes
ü Testosterone is needed in very high quantities for maintenance of the reproductive tract, and ABP allows a much higher level of fertility
ü Secrete hormones affecting pituitary gland control of spermatogenesis, particularly the polypeptide hormone, Inhibin.
ü Phagocytose residual cytoplasm left over from spermiogenesis.
ü They release Antimullerian hormone which prevents formation of the Mullerian Duct / Oviduct.
ü Protect spermatids from the immune system of the male.

Hormonal control of spermatogenesis
§  Hormonal control of spermatogenesis varies among species.
§  Initiation of spermatogenesis occurs at puberty due to the interaction of the hypothalamus, pituitary gland and Leydig cells.
§  If the pituitary gland is removed, spermatogenesis can still be initiated by follicle stimulating hormoneand testosterone.
§  Follicle stimulating hormone stimulates both the production of 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.
§  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.
§  Studies from rodent models suggest that gonadotropin hormones (both LH and FSH) support the process of spermatogenesis by suppressing the proapoptotic signals and therefore promote spermatogenic cell survival. 
§  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.
§  In mammals, pituitary control of spermatogenesis varies with age. In the rat, before puberty FSH is highly active whereas LH is not; after puberty it seems that LH alone is able to support spermatogenesis.
Hormonal regulation of spermatogenesis in experimental animals:
§  In the rat, the roles of follicle stimulating hormone (FSH) and luteinizing hormone (LH) vary with the age of the animal.
§  FSH is effective in the prepubertal rat in the support of spermatogonial multiplication.
§  LH is active in the hypophysectomized adult.
§  FSH acts by binding specifically to receptor sites on cellular membranes of the seminiferous epithelium inducing biochemical changes. The development of a system which limits the response to FSH according to age is suggested.
§  Spermatogenesis proceeds only to primary spermatocytes. Morphological maturation of Sertoli cells occurs as long as LH-like activity exceeded FSH-like activity.
§  In hypogonadotropic hypogonadism, FSH has been shown to be inactive on germ cells while LH allows resumption of meiosis after hypophysectomy.

Hormone
Secretion From
Function
GnRH
Hypothalamus
§  Control release of LH and FSH.
FSH         
Anterior pituitary gland
§  Stimulates maturation of seminiferous tubules and spermatogenesis
LH (Luteinizing hormone)
Anterior pituitary gland
§  Produced by anterior pituitary increases testosterone production by Leydig cells
Testosterone

Leydig cells

§  Necessary for normal sperm development.
§  It activates genes in Sertoli cells, which promote differentiation of spermatogonia.
Activin 
Sertoli Cells
§  Enhances FSH biosynthesis and secretion
Inhibin

Sertoli Cells
§  Regulates FSH synthesis and inhibits FSH secretion.

1 comment:

  1. describe the role of hormones in the process of spermatogenesis post puberty in the male farm animals

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