Spermatogenesis
|
|
Spermatogenesis refers to the creation, or genesis, of sperm cells, which occurs in the male gonads or testes. Sperm cells develop initially from germ stem cells known as spermatogonia. As these differentiate they become spermatocytes, spermatids (after meiosis) and then finally the fully mature spermatozoa. The female counterpart to spermatogenesis is oogenesis.
The process of spermatogenesis is highly regulated via hormonal messengers and also the cell-cell comunication between the developing germ cells (sperm cells) and the nurse-like Sertoli cells.
The Sertoli cell is essential for spermatogenesis as it provides support for the developing sperm cells - moving them towards the lumen of the semiferous tubule as they develop until maturity when they are released. The Sertoli cell also reduces motility and capacitation (initiation of the acrosome reaction) of the sperm cells so viability is maintained.
Spermatozoa are produced in seminiferous tubules in the testes. They start off as spermatogonia, undergoing mitosis becoming a type A spermatogonium or a type B spermatogonium. Type B spermatogonia become primary spermatocytes. Primary spermatocytes go through a meiotic division to become secondary spermatocytes, which undergo another meiotic division to become spermatids. Type A spermatogonia stay as spermatogonia, and do not change. They act as stem cells and will divide again producing more Type A and B cells.
The primary spermatocytes contain twice the DNA of a normal body cell (2 × 2N). Each primary spermatocyte divides into two secondary spermatocytes containing two sets of chromosomes (2 × 1N). The secondary spermatocytes then divide into two spermatids, each containing just one set of chromosomes (1N), half the DNA needed to make a human being. (The other half will come from the ovum at fertilisation).
The spermatids at this point are still round cells. During the process called spermiogenesis the sperm extrude tails, repackage DNA into a head, cap the head with a lysosome-like acrosome and construct an energy transducing neck region of tightly packed mitochondria to finally become mature testicular spermatozoa. They are then released into the lumen of the seminiferous tubule and travel with fluid from the testis into the epididymis for further maturation processes allowing them to swim forward and fertilize an ovum.de:Spermatogenese fr:Spermatogenèse su:Spérmatogenesis