Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubules.
These cells are called spermatogonial stem cells. The mitotic division of these produces two types of cells. Type A cells replenish the stem cells, and type B cells differentiate into spermatocytes. The primary spermatocyte divides meiotically Meiosis I into two secondary spermatocytes; each secondary spermatocyte divides into two equal haploid spermatids by Meiosis II.
The spermatids are transformed into spermatozoa sperm by the process called Spermiogenesis. These develop into mature spermatozoa, also known as sperm cells. Spermatozoa are the mature male gametes in many sexually reproducing organisms. Thus, spermatogenesis is the male version of gametogenesisof which the female equivalent is oogenesis.
In mammals Developing sperm cells begin the process of meiosis they occurs in the seminiferous tubules of the male testes in a stepwise fashion.
Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly, and is essential for sexual reproduction. DNA methylation and histone modification have been implicated in the regulation of this process. Spermatogenesis produces mature male gametes, commonly called sperm but more specifically known as spermatozoawhich are able to fertilize the counterpart female gamete, the oocyteduring conception to produce a single-celled individual known as a zygote.
This is the cornerstone of sexual reproduction and involves the two gametes both contributing half the normal set of chromosomes haploid to result in a chromosomally normal diploid zygote. To preserve Developing sperm cells begin the process of meiosis they number of chromosomes in the offspring — which differs between species — one of each gamete must have half the usual number of chromosomes present in other body cells.
Otherwise, the offspring will have twice the normal number of chromosomes, and serious abnormalities may result. In humans, chromosomal abnormalities arising from incorrect spermatogenesis results in congenital defects and abnormal birth defects Down syndromeKlinefelter syndrome and in most cases, spontaneous abortion of the developing foetus.
Spermatogenesis takes place within several structures of the male reproductive system. The initial stages occur within the testes and progress to the epididymis where the developing gametes mature and are stored until ejaculation. The seminiferous tubules of the testes are the starting point for the process, where spermatogonial stem cells adjacent to the inner tubule wall divide in a centripetal direction—beginning at the walls and proceeding into the innermost part, or lumen —to produce immature sperm.
For humans, the entire process of spermatogenesis is variously estimated as taking 74 days   according to tritium-labelled biopsies and approximately days  according to DNA clock measurements.
Including the transport on ductal system, it Developing sperm cells begin the process of meiosis they 3 months. Testes produce to million spermatozoa daily. The entire process of spermatogenesis can be broken up into several distinct stages, each corresponding to a particular type of cell in humans. The primary spermatocyte is arrested after DNA synthesis and prior to division. Spermatocytogenesis is the male form of gametocytogenesis and results in the formation of spermatocytes possessing half the normal complement of genetic material.
In spermatocytogenesis, a diploid spermatogoniumwhich resides in the basal compartment of the seminiferous tubules, divides mitotically, producing two diploid intermediate cells called primary spermatocytes. Each primary spermatocyte then moves into the adluminal compartment of the seminiferous tubules and duplicates its DNA and subsequently undergoes meiosis I to produce two haploid secondary spermatocyteswhich will later divide once more into haploid spermatids.
This division implicates sources of genetic variation, such as random inclusion of either parental chromosomes, and chromosomal crossoverto increase the genetic variability of the gamete. Each cell division from a spermatogonium Developing sperm cells begin the process of meiosis they a spermatid is incomplete; the cells remain connected to one another by bridges of cytoplasm to allow synchronous development.
It should also be noted that not all spermatogonia divide to produce spermatocytes; otherwise, the supply of spermatogonia would run out. Instead, spermatogonial stem cells divide mitotically to produce copies of themselves, ensuring a constant supply of spermatogonia to fuel spermatogenesis. Spermatidogenesis is the creation of spermatids from secondary spermatocytes.
Secondary spermatocytes produced earlier rapidly enter meiosis II and divide to produce haploid spermatids.
The brevity of this stage means that secondary spermatocytes are rarely seen in histological studies. During spermiogenesis, the spermatids begin to form a tail by growing microtubules on one of the centrioles, which turns into basal body.
These microtubules form an axoneme. Later the centriole is modified in the process of centrosome reduction. Spermatid DNA also undergoes packaging, becoming highly condensed.
The DNA is packaged firstly with specific nuclear basic proteins, which are subsequently replaced with protamines during spermatid elongation. The resultant tightly packed chromatin is transcriptionally inactive.
The Golgi apparatus surrounds the now condensed nucleus, becoming the acrosome. Maturation then takes place under the influence of testosterone, which removes the remaining unnecessary cytoplasm and organelles.
The excess cytoplasm, known as residual bodiesis phagocytosed by surrounding Sertoli cells in the testes. The resulting spermatozoa are now mature but lack motility, rendering them sterile. The mature spermatozoa are released from the protective Sertoli cells into the lumen of the seminiferous tubule in a process called spermiation.
The non-motile spermatozoa are transported to the epididymis in testicular fluid secreted by the Sertoli cells with the aid of peristaltic contraction. While in the epididymis the spermatozoa gain motility and become capable of fertilization.
However, transport of the mature spermatozoa through the remainder of the male reproductive system is achieved via muscle contraction rather than the spermatozoon's recently acquired motility. At all stages of differentiation, the spermatogenic cells are in close contact with Sertoli cells which are thought to provide structural and metabolic support to the developing sperm cells.
A single Sertoli cell extends from the basement membrane to the lumen of the seminiferous tubule, although the cytoplasmic processes are difficult to distinguish at the light microscopic level.
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Sertoli cells serve a number of functions during spermatogenesis, they support the developing gametes in the following ways:. The process of spermatogenesis is highly sensitive to fluctuations in the environment, particularly hormones and temperature. Testosterone is required in large local concentrations to maintain the process, which is achieved via the binding of testosterone by androgen binding protein present in the seminiferous tubules.
Testosterone is produced by interstitial cells, also known as Leydig cellswhich reside adjacent to the seminiferous tubules.
Seminiferous epithelium is sensitive to elevated temperature in humans and some other species, and will be adversely affected by temperatures as high as normal body temperature. Consequently, the testes are located outside the body in a sack of skin called the scrotum.
This is achieved by regulation of blood flow  and positioning towards and away from the heat of the body by the cremasteric muscle and the dartos smooth muscle in the scrotum.
Dietary deficiencies such as vitamins B, E and Aanabolic steroidsmetals cadmium and leadx-ray exposure, dioxinalcohol, and infectious diseases will also adversely affect the rate of spermatogenesis.
Hormonal control of spermatogenesis varies among species. In humans the mechanism is not completely understood; however it is known that initiation of spermatogenesis occurs at puberty due to the interaction of the hypothalamuspituitary gland and Leydig cells.
If the pituitary gland is removed, spermatogenesis can still be initiated by follicle stimulating hormone FSH and testosterone. FSH stimulates both the production of androgen binding protein ABP by Sertoli cellsand the formation of the blood-testis barrier. ABP is essential to concentrating testosterone in levels high enough to initiate and maintain spermatogenesis.
Intratesticular testosterone levels are 20— or 50— times higher than the concentration found in blood, although there is variation over a 5- to fold range amongst healthy men. The hormone inhibin acts to decrease the levels of FSH. Studies from rodent models suggest that gonadotropins 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 through 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.
Estrogen has been found to be essential for spermatogenesis in animals. From Wikipedia, the free encyclopedia. Spermatogenesis Seminiferous tubule with maturing sperm.
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