When do spermatogonia undergo mitosis

The number of stages within a spermatogenic cycle and the number of cycles required for the completion of spermatogenesis varies between species. There are 12 different stages of the cycle in the bull of about 14 days each; approximately four cycles within a given region of the tubule occur before an A1 spermatogonia is transformed into a spermatozoa.

Six stages have been noted in man; four day cycles are needed to complete spermatogenesis. The linear pattern of the spermatogenic cycle is less ordered in man than in farm animals or rodents.

Hormonal regulation. Spermatogonia continue to divide, but in reduced numbers, after hypophysectomy. Spermatocytogenesis is completely arrested at the primary spermatocyte stage in hypophysectomized animals; this step is restored by testosterone.

Androgen-binding protein the testicular counterpart of SHBG sequesters testosterone within the seminiferous tubule and caput epididymis. Meiosis II is hormonally-independent.

Follicle-stimulating hormone participates in spermiogenesis. Estradiol and DHT are also involved in the spermatogenic process. Hormonal effects on sperm cells are not direct, but are mediated through Sertoli cells.

Biochemical and biophysical facets of sperm-Sertoli interactions in spermatogenesis are largely unknown. Rate of production of spermatozoa is not influenced by endocrine therapy. Blood-testis barrier. As sperm cells mature they move between Sertoli cells from the basal toward the adluminal compartment of the seminiferous tubule.

Because nucleotide recombinations can occur during meiosis I, the genetic code of chromosomes of gametes can differ from that of somatic parent cells ie. Occluding junctions that interconnect adjacent Sertoli cells shield secondary spermatocytes, spermatids, and spermatozoa from autoimmune recognition Figure The blood-testis barrier also acts to conserve certain products of Sertoli cells within the seminiferous tubule, such as ABP.

The epithelial syncytium of this barrier extends through the epididymis. Vasectomy can lead to a breakdown in the blood-testis barrier in laboratory animals and subhuman primates; as a result, an autoimmune response is mounted against sperm antigens released into the periphery. Immune complexes can lodge within the kidneys and adhere to walls of blood vessels causing renal damage and atherosclerosis; possible complications of this nature, although not detected thusfar, need to be monitored closely in long-term vasectomized men.

Effect of temperature. Sperm cells will not mature at core body temperature in most mammals spermatogenic DNA polymerase b and recombinase activities exhibit unique temperature optima ; to adapt, the testes assume an external position.

Testicular descent from the abdomen normally transpires during fetal or neonatal life. If the testes fail to descend into the scrotum, a condition called cryptorchidism, the male will be sterile; gone uncorrected by surgery or androgen treatment spermatogonia will eventually degenerate.

At the end of the second meiotic division, each secondary spermatocyte produces two spermatids. Each spermatid receives 23 chromatids from the parent cell. In the spermatids, the chromatids represent single strands of DNA and become the chromosomes of the gametes.

Subsequently, each spermatid undergoes a radical morphological alteration to become a spermatozoon. This process of morphological change is called spermiogenesis. Note that half of the spermatids will development into spermatozoa carrying an X chromosome and half will develop into spermatozoa carrying a Y chromosome.

This page has been archived and is no longer updated. Meiosis, Genetic Recombination, and Sexual Reproduction. Meiotic timelines for humans. Keywords Keywords for this Article. Save Cancel. Share Cancel.

Revoke Cancel. Flag Inappropriate The Content is. Flag Content Cancel. Delete Content. This cell will start the first meiotic division and be arrested in its progress in the first prophase stage. At the time of birth, all future eggs are in the prophase stage. At adolescence, anterior pituitary hormones cause the development of a number of follicles in an ovary.

This results in the primary oocyte finishing the first meiotic division. The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell.

This second cell is called a polar body and usually dies. A secondary meiotic arrest occurs, this time at the metaphase II stage. At ovulation, this secondary oocyte will be released and travel toward the uterus through the oviduct.

If the secondary oocyte is fertilized, the cell continues through the meiosis II, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm.