Spermatogenesis is a process in which haploid spermatozoa develops from germ cells in the testicular seminal tube tubules. This process begins with the division of mitosis of stem cells located close to the basal membrane of the tubule. These cells are called spermatogonial stem cells. This division of mitosis produces two types of cells. Cells type A fills the stem cells, and cell type B differentiates into spermatocytes. Primary spermatocytes divide meiotically (Meiosis I) into two secondary spermatocytes; each secondary spermatocyte divides into two haploid spermatids similar to Meiosis II. Spermatids are converted into spermatozoa (sperm) by a process called Spermiogenesis. It develops into a mature spermatozoa, also known as sperm cells. Thus, primary spermatocytes cause two cells, secondary spermatocytes, and two secondary spermatocytes by their subdivisions to produce four spermatozoa.
Spermatozoa are adult male gametes in many sexually reproducing organisms. Thus, spermatogenesis is a male version of gametogenesis, which is equivalent to women being oogenesis. In the mammal it occurs in the seminiferous tubules of the male testes gradually. Spermatogenesis depends on the optimal conditions for the process to occur properly, and is essential for sexual reproduction. DNA methylation and histone modification have been involved in setting up this process. It begins at puberty and usually continues uninterrupted until death, although a slight decrease can be seen in the quantity of sperm produced with increasing age (see Infertility Men).
Video Spermatogenesis
Destination
Spermatogenesis produces an adult male gamete, commonly called sperm but more specifically known as spermatozoa, which is able to fertilize the counterpart female gabon, oocyte, during fertilization to produce one-cell known as zygote. This is the basis of sexual reproduction and involves two gametes that contribute half a set of normal (haploid) chromosomes to produce a normal zygote (diploid) chromosome.
To maintain the number of chromosomes in offspring - different between species - each gamete must have half the number of chromosomes common to other body cells. Otherwise, the offspring will have twice the normal number of chromosomes, and serious abnormalities can occur. In humans, chromosomal abnormalities arise from faulty spermatogenesis results in congenital defects and abnormal birth defects (Down Syndrome, Klinefelter's Syndrome) and in many cases, spontaneous abortion in developing fetuses.
Maps Spermatogenesis
Locations in humans
Spermatogenesis occurs in some structures of the male reproductive system. The initial stage takes place inside the testes and progresses to the epididymis where the gametes grow up and are stored until ejaculation. The testis seminiferous tubule is the starting point for the process, in which the spermatogonial stem cells adjacent to the inner tubular wall divide in centripetal direction - starting on the wall and continuing to the deepest part, or
Duration
For humans, the entire process of spermatogenesis is thought to require a range of 74 days (according to a tritium-labeled biopsy) and about 120 days (according to DNA clock measurements). Including transportation on the ductal system, it takes 3 months. Testes produce 200 to 300 million spermatozoa daily. However, only about half or 100 million of them become sperm worthy.
Stages
The whole process of spermatogenesis can be broken down into several different stages, each corresponding to a particular cell type in humans. In the following table, ploidi, copy number and chromosome/chromatid number are for one cell, generally before synthesis and division of DNA (in G1 if applicable). Primary spermatocytes are captured after DNA synthesis and before cleavage.
Spermatocytogenesis
Spermatocytogenesis is a male form of gametocytogenesis and results in the formation of spermatocytes that have half of the normal complement of genetic material. In spermatocytogenesis, the diploid spermatogonium, which is in the basal compartment of the seminiferous tubule, divides mitotically, producing two diploid intermediate cells called the primary spermatocytes. Each primary spermatocyte then moves into the adluminal compartment of the seminiferous tubule and doubles its DNA and then undergoes iiosis I to produce two haploid secondary spermatocytes, which then divide once again into a haploid spermatid. This division has implications for sources of genetic variation, such as random inclusion of parental chromosomes, and crossover chromosomal, to increase genetic variability of gametes.
Any cell division from spermatogonia to spermatid is incomplete; the cells remain connected to each other by cytoplasmic bridges to enable synchronous development. It should also be noted that not all spermatogonia are cleaved to produce spermatocytes; otherwise, the spermatogonia supply will be exhausted. In contrast, spermatogonial stem cells divide mitotically to produce copies of themselves, ensuring a constant supply of spermatogonia for spermatogenesis fuel.
Spermatidogenesis
Spermatidogenesis is the formation of spermatids from secondary spermatocytes. The previously produced secondary spermatocytes rapidly enter meiosis II and divide to produce haploid spermatids. In short this step means that secondary spermatocytes are rarely seen in histologic studies.
Spermiogenesis
During spermiogenesis, spermatids begin to form the tail by growing microtubules in one of the centrioles, which turns into a basal body. This microtubule forms an axon. Then centriole is modified in centrosome reduction process. The anterior portion of the tail (called the midpiece) thickens because the mitochondria are arranged around the axon to ensure the energy supply. Spermatid DNA also undergoes packaging, becomes very thick. DNA is packed first with certain nuclear base proteins, which are then replaced with protamines during spermatid elongation. Strictly produced chromatin by transcription is inactive. The Golgi apparatus surrounds the now condensed core, becoming an acrosome.
Maturation then occurs under the influence of testosterone, which removes unnecessary cytoplasmic and organelle residues. Excess cytoplasm, known as the residual body , is phagocytosed by surrounding Sertoli cells in the testes. The resulting spermatozoa are now ripe but lacking in motility, making them sterile. Mature spermatozoa are released from protective Sertoli cells into the lumen of the seminiferous tubules in a process called spermiation.
Non-motile spermatozoa is transported to the epididymis in testicular fluid secreted by Sertoli cells with the aid of peristaltic contractions. While in the epididymis spermatozoa get motility and become capable of fertilization. However, transport of mature spermatozoa through the rest of the male reproductive system is achieved through muscle contraction rather than the motility obtained by recent spermatozoa.
Sertoli cell role
At all stages of differentiation, spermatogenic cells are in close contact with Sertoli cells that are thought to provide structural and metabolic support in developing sperm cells. A single Sertoli cell extends from the basement membrane to the seminiferous tubular lumen, although the cytoplasmic process is difficult to distinguish at the microscopic level of light.
Sertoli cells serve a number of functions during spermatogenesis, they support a growing gamete in the following ways:
- Maintain the necessary environments for development and maturation, through a blood-testis barrier
- Squeezer starts meiosis
- Secrete supports testicular fluid
- The instantaneous androgen binding protein (ABP), which concentrates testosterone near the growing gametes
- Testosterone is required in very high amounts for maintenance of the reproductive tract, and ABP allows a much higher fertility rate
- The secret hormones that affect the control of the pituitary gland spermatogenesis, especially the polypeptide hormone, inhibin
- remaining Phagocytose cytoplasm remaining from spermiogenesis
- The secretion of the anti-MÃÆ'¼llerian hormone causes the deterioration of the MÃÆ'¼llerian channel
- Protect spermatids from the male immune system, through a blood-testis barrier
- Contribute to spermatogonial sperm niches
The intercellular ICAM-1 and ICAM-1 intercellular adhesion molecules have an antagonistic effect on the tight intersections that form the blood-testicular barrier. The ICAM-2 molecule regulates spermatid adhesion on the apical side of the barrier (towards the lumen).
Influencing factor
The process of spermatogenesis is very sensitive to environmental fluctuations, especially hormones and temperature. Testosterone is required in large local concentrations to maintain the process, which is achieved by binding of testosterone by the androgen-binding protein present in the seminiferous tubules. Testosterone is produced by interstitial cells, also known as Leydig cells, which are adjacent to the seminiferous tubules.
The bile epithelium is sensitive to high temperatures in humans and some other species, and will be affected by temperatures as high as normal body temperature. As a result, the testes are located outside the body in a leather sack called the scrotum. The optimum temperature is maintained at 2 Ã, Â ° C (male) -8Ã, Â ° C (mouse) below body temperature. This is achieved by regulating blood flow and positioning toward and away from body heat by the cremasteric muscles and smooth muscle dartos in the scrotum.
Dietary deficiencies (such as vitamins B, E and A), anabolic steroids, metals (cadmium and lead), x-ray exposure, dioxin, alcohol, and infectious diseases will also affect spermatogenesis rates. In addition, the male germ line is susceptible to DNA damage caused by oxidative stress, and this damage is likely to have a significant impact on fertilization and pregnancy. Exposure to pesticides also affects spermatogenesis.
Hormonal control
Hormonal control of spermatogenesis varies among species. In humans the mechanism is not fully understood; It is known however that spermatogenesis initiation occurs at puberty due to the interactions of the hypothalamus, pituitary gland and Leydig cells. If the pituitary gland is removed, spermatogenesis can still be started by follicle stimulating hormone (FSH) and testosterone. In contrast to FSH, LH appears to have little role in spermatogenesis beyond stimulating the production of gonadal testosterone.
FSH stimulates the production of the androgen-binding protein (ABP) by Sertoli cells, and the formation of a blood-testicular barrier. ABP is very important to concentrate testosterone in a high enough level to start and maintain spermatogenesis. The level of intratesticular testosterone is 20-100 or 50-200 times higher than the concentrations found in the blood, although there are variations over the range of 5 to 10-fold among healthy men. FSH may initiate testosterone secretion in the testes, but after development only testosterone is required to maintain spermatogenesis. However, increasing FSH levels will increase spermatozoa production by preventing apoptosis type A spermatogonia . Inhibin hormone serves to decrease FSH levels. Studies of the mouse model suggest that gonadotropin (both LH and FSH) supports the spermatogenesis process by suppressing the proapoptotic signals and therefore improves spermatogenic cell survival.
Source of the article : Wikipedia
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