Spermatogenesis is the process of sperm production, which takes place in the seminiferous tubules of the testes. It includes events which spermatogonia, stem cells diploid (2n chromosomes), lead to sperm, haploid male gametes (n chromosomes).
Spermiogenesis in turn is the differentiation of spermatids into spermatozoa.
By the 1970s, scholars have noted a deterioration of spermatogenesis in rich countries (Europe, USA, Japan). They were first observed significant differences in sperm quality by region studied, and comparisons over time have shown that - in all cases studied - average number of sperm per milliliter of semen decreases from year to year. He has been halved in 50 years.
Spermatogenesis begins at puberty, peak around 20-30 years, gradually slowing to quarantine but may continue into old age. Spermatogenesis is a slow and continuous process requiring approximately 74 days in humans, and runs in spurts or waves along the seminiferous tubule.
The tubule is limited by a tubular sheath formed of thin blade or basement membrane of fibroblasts. Some of these include fibroblast contractile filaments and fibers of collagen and are the myoid cells or myofibroblasts -
* The tubular sheath is called basal membrane of the seminiferous tubule or membrana propria.
* These tubes are surrounded by connective tissue ("filler") and also formed interstitial cells of Leydig which is, of course, shot through the arterioles and venules providing oxygen. (See testis) These cells or endocrine cells of Leydig secrete mostly testosterone (and dihydrotestosterone), they form the interstitial gland of the testis.
* The wall of the tubule consists of a stratified epithelium consisting of two types of cells
o germline cells arranged in 4-8 layers
o Sertoli cells, cells based on the high basal membrane and reaching the lumen by their apical pole;
* In the adult seminiferous tubules, the different layers of germ cells are the stages of spermatogenesis that occurs centripetally, from the spermatogonia located against the basement membrane to sperm released into the light: Under the microscope, there that these tubes are "filled" cells apparently different, organized rings. At the periphery, there are stem cells (spermatogonia, while the center are the sperm. In fact, undifferentiated cells grow and eventually become male gametes:
o spermatocyte I
o spermatocyte II
Firstly, through multiple mitoses from a spermatogonia, we obtain a stock. Then the spermatogonia grows and becomes spermatocyte I. At this stage, are still stem cells with chromosomes 2n. Meiosis I produces spermatocytes spermatocytes II, which have more than n chromosomes (see meiosis). Following meiosis led to spermatids, which migrate to the "light" of the tube. The differentiation of these eventually gives the sperm in the center of the tube. It said then that spermatogenesis is "centripetal".
This process is continuous, that is to say that the cells become successively and are replaced by new spermatogonia obtained by mitosis.
In the seminiferous tubules, we also note the presence of large cells, Sertoli cells, which support and nourish the spermatogonia and are therefore essential for spermatogenesis.
The tubules lead by a drainage system, which becomes the epididymis vas deferens leaving testis. The cells are propelled into the channel, and mixed with seminal fluid in the prostate to form semen during ejaculation.
Three hormones play an important role in spermatogenesis:
* LH (luteinizing hormone)
* FSH (follicle stimulating hormone)
LH and FSH are produced by the pituitary gland while testosterone is produced in the Leydig cells.
FSH acts directly on spermatogonia and Sertoli cells to speed spermatogenesis (assimilation plus testosterone). LH, in turn, increases the production of testosterone, which also acts on spermatogonia (and in particular permits the final differentiation) and the Sertoli cells. If testosterone production reaches a certain threshold, the Leydig cells send negative feedback to the pituitary gland that inhibits the production of LH and FSH. It is therefore a self-regulating system that does not allow the excessive production of testosterone, and therefore sperm.
General decrease in the number of sperm produced during spermatogenesis?
By the 1970s, experts were astonished significant differences in sperm quality, according to the region from the patient or donor.,
10 years later, first whistleblowers confirm these disparities, but note that the analysis of sperm counts for years 1935 to 1980 all indicate a decline in sperm counts in all regions studied.
Over sixty studies suggest that at least in Western countries, men face a general problem of degradation of spermatogenesis. The magnitude of the phenomenon, its causes and its translation in terms of male fertility are still controversial;
In France: For example, according to the sperm counts of 1351 sperm donors CECOS Paris, there has been no change in the volume of semen produced by about 20 years, from 1973 to 1992, but average concentration of spermatozoa has cons decreased significantly (- 2.1% / year from 89 x106 / ml of semen 1973 60x106 per ml in 1992.
Meanwhile, the percentage of normal sperm Mobile fell 0.6% per annum and that of normal form sperm decreased by 0.5% per year.
After making adjustments for age and duration of sexual abstinence, on these 20 years, each new generation (by calendar year of birth) has lost 2.6% of sperm from the cohort born in the year before, and the rate of motile sperm decreased by 0.3% per year, and the sperm of normal shape decreased 0.7% / year.
See also Pregnancy