Lecture 16: Endocrinology of the Male and Spermatogenesis

Spermatogenesis

-production of normal numbers of spermatogonia requires=adequate secretion of GnRH from the hypothalamus, FSH and LH secretion from the anterior pituitary, secretion of gonadal steroids, mitotic divisions resulting in haploid spermatids, morphologic transformation of spermatids into spermatogonia

-takes place within the seminiferous tubules

-in domestic animals, the nucleus is oval and flattened and surrounded by a nuclear membrane

-the shape of the head is unique for each species

-the anterior 2/3 is covered by the acrosome

-sperm are released continuously=ability to turn off and on in seasonal breeders

-evaluation of sperm numbers in the ejaculate does not always reflect normal or abnormal spermatogenesis=2-4 week delay before deleterious effects are observed, 6-12 weeks are required before restoration of normal spermatogenesis

Male Endocrinology

-LH acts on the leydig cells within the testes=analogous to the theca interna, binding of LH causes leydig cells to produce testosterone

-high concentrations of testosterone are necessary for spermatogenesis=intratesticular levels of T are 100-500x greater than that of systemic blood

-testosterone is released in a pulsatile pattern like GnRH

-sertoli cell function is FSH dependent=convert T to E2 utilizing the same mechanism as the granulosa cells (the role of E2 in the male is not fully understood, does supply negative feedback to hypothalamus), also produce inhibin

Proliferation

-1st phase of spermatogenesis

-consists of all mitotic divisions of spermatogonia

-several generations undergo division=generates large numbers of spermatogonia

Stem Cell Renewal

-some spermatogonia revert back to a more primitive type, and thus replacing the more primitive cells

Meiotic Phase

-2nd phase of spermatogenesis

-cells in this stage are called spermatocytes

-spermatocytes undergo meiotic division

-genetic diversity is guaranteed through replication and crossing over=no two sperm are identical in their genetic makeup

-produces haploid spermatids

Differentiation

-3rd phase of spermatogenesis

-no cell divisions

-”spermatogenesis”

-spermatid transforms into a spermatozoan containing a head, midpiece, and principal piece=transformation of a round cell into the typical sperm form

Spermiation

-release of the spermatozoa from the sertoli cells into the lumen of the seminiferous tubules

-analogous to ovulation except it occurs continually

Head

-nucleus+acrosome+postnuclear cap

Tail

-middle piece+principal piece+terminal piece

Acrosome

-membrane bound lysosome that contains hydrolytic enzymes=acrosin, hyaluronidase, zona kysin, esterases, enzymes, and acid hydrolases

-required for the penetration of the cellular investments and zona pellucida of the ovulated oocyte

Sperm Production

-testicular size is a good indicator of sperm producing ability

-the bigger the factory the greater the output

-circumference in bulls and rams

-length or width in stallions and boars

Sperm Viability

-motility=the ability of sperm to swim progressively forward

-the relationship between motility and fertility is not a good one=egg can’t be fertilized if the sperm doesn’t get there

-morphology=structure of the sperm cell, abnormal is defined as any shape or characteristic deviating from normal, every ejaculate will contain 5-15% abnormal sperm, reduced fertility may result when abnormal >20%

-unseen abnormalities=abnormal nuclear composition, surface protein deficiencies, abnormal biochemical composition

Semen Sorting

-sperm contains X or Y chromosome

-2.8-4.2% more DNA in X

-DNA of sperm cells is stained with flourochrome=dye emits light at specific wavelengths

-sperm pass single file in front of a laser

-amount of reflectance is measured and related to the amount of DNA in the cell

-method called flow cytometry