Spermatogenesis
From Wikipedia, the free encyclopedia
Spermatogenesis is the process by which stem germ cells develop into mature spermatozoa (sperm cells). It is one of the most important and delicate processes that occur in the male body and is essential for sexual reproduction. This process occurs in four stages: multiplication, growth, maturation or meiosis, and differentiation.
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[edit] The point?
Since sexual reproduction involves the union of two sex cells, or gametes, then it will also involve the union of the chromosomal material present in each cell's nucleus. To preserve the number of chromosomes of the offspring, which is characteristic of each species (e.g. normal humans have 46 chromosomes), 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.
In males, spermatogenesis solves this problem by allowing the production of sperm cells with n chromosomes (haploid cells) from stem cells with 2n chromosomes (diploid cells). Oogenesis, on the other hand, allows the creation of haploid sex cells, called ova, in females.
[edit] Where and when?
Spermatogenesis takes place inside a male’s testes, specifically in the seminiferous tubules, in a centripetal direction—beginning at the walls and proceeding into the innermost part, or lumen. It starts at puberty and usually continues uninterrupted till death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age.
[edit] Stages: from stem cell to sperm cell
[edit] Multiplication
The first phase of spermatogenesis is also known as the "spermatogonial phase" in which stem cells divide to replace themselves and to produce a population of cells destined to become mature sperm. Three cell types are recognized on the basis of the appearance of the nuclei: Type A dark spermatogonia (Ad), Type A pale spermatogonia (Ap), and Type B spermatogonia (B).
Type Ad spermatogonia are the stem cells of spermatogenesis and will divide to produce either two identical Type Ad spermatogonia or two Type Ap spermatogonia. Type Ad spermatogonia do not participate in spermatogenesis and are meant to ensure that stem cells never run out of supply. This is due to the fact that stem cells are needed in large quantities since the average male produces trillions of sperm cells throughout his lifetime.
On the other hand, Type Ap (pale) spermatogonia are the ones that participate in spermatogenesis and undergo mitotic divisions. Eventually, after numerous divisions, they are committed to Type B spermatogonia, which enter the "spermatocyte phase" of spermatogenesis.
The Type B spermatogonium initiates another mitotic division that results in two daughter cells, each equipped with 2n chromosomes. At this point in the process, the daughter cells of spermatogonium start moving away from the basal lamina.
[edit] Growth
The two produced daughter cells initially have chromosomes with just one chromatid. One of these cells enter into a period of growth, known as interphase, which allows the cell to replenish its chromosomal material. The end result is a cellswith double-chromatid chromosomes, known as a primary spermatocytes. The other cell is left as a spermatogonium and does not take part in the process.
[edit] Maturation
The primary spermatocytes enter into meiosis, which is a double division. The first division, meiosis I, is called the reductional division. It produces two daughter cells each with n chromosomes (half the starting number, 2n). These daughter cells are dubbed secondary spermatocytes. It is worth noting that the chromosomes of these cells have two chromatids.
The second division, meiosis II, is called the equational division, and is nearly identical to mitosis. However, it is very important to note that there is no period of "intermission," or interphase, between the meiosis I and meiosis II. The previously obtained daughter cells each produce two new daughter cells, which brings the total of daughter cells (or granddaughter cells, to be exact) to four. The newly formed daughter cells have n chromosomes but with one chromatid. They are called spermatids.
In short, what was accomplished during meiosis was first, dividing the chromosomes in half, then second, breaking up the double-chromatid chromosomes into single-chromatid chromosomes. All of these progeny cells remain attached to each other by cytoplasmic bridges. The bridges remain until sperm are fully differentiated.
[edit] Differentiation
Once all the previous divisions are done, and spermatids are formed, differentiation starts. Differentiation is essentially the process of transforming spermatids into bona fide, mature sperm cells, with all the features necessary for the task they’re designed to do—most important of all, mobility.
Differentiation is also known as spermiogenesis. During differentiation, the spermatid is gradually molded into an elongated shape, and large portions of the cytoplasm (the residual cytoplasm) are shed off. Most notably, the spermatid develops an acrosome, produced by the Golgi apparatus, and a flagellum that looks roughly like a long tail and is responsible for motility. The flagellum is produced by the one of the centrioles present in the spermatid. The nucleus is squeezed into an elongated shape and forms with the acrosome the head of the sperm, the acrosome occupying the topmost part of the head. The mitochondria which act as power plants for the cell are arranged in the middle piece of the sperm along with the remaining centriole.
Once differentiation is complete, the sperm cell separates from the Sertoli cell to which it was bound during spermatogenesis and migrates into the lumen of the seminiferous tubules.
[edit] Duration
The entire process of spermatogenesis takes 64 to 72 days. The maturation phase and the differentiation phase last for two and three weeks repectively.
[edit] Sertoli cells and developing sperm cells
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 its cytoplasm is difficult to distinguish at the light microscopic level. They are characterized by the presence of a vesicular, oval, basally positioned nucleus which contains a prominent nucleolus. The nuclear envelope often contains a definite fold. The significance of the very close association of the two types of cells is unknown. Sertoli cells are endocrine cells - they secrete the polypeptide hormone, inhibin. Inhibin acts at the level of the pituitary to reduce the secretion of follicle stimulating hormone.
Sertoli cells serve as "nurse" cells for spermatogenesis, nourishing germ cells as they develop. These specialized support cells also participate in germ cell phagocytosis. High-affinity FSH receptors exist on Sertoli cells and FSH binding induces the production of androgen-binding protein, which is then secreted into the tubular luminal fluid. By binding testosterone, androgen-binding protein ensures that high levels of androgen (20-50 × that of serum) exist within the seminiferous tubules. Evidence also suggests that inhibin is Sertoli cell-derived. Ligand-receptor complexes, such as c-kit and kit ligand, may also mediate communication between germinal and Sertoli cells.
[edit] Conditions and regulators
[edit] Testosterone
A kind of sex-hormone that has steroid structure. In men, it provides the development of male secondary sexual characteristics. Testosterone is also present in women.It is produce by interstitials cell that appear in between seminiferous tubules.
1. Vitamains B, E & A as well as some proteins are important for spermatogenesis.
2. Sperms are only produced at around 33°C (which is about 4°C-5°C less than our normal body temperature.) If the testes are held too close to the body, for example, by tight cloth binders, this may cause sterilization.
3. Testosterone & FSH (Follicle Stimulating Hormone) promote spermatogenesis.
[edit] See also
[edit] References
- BARDIN CW: Pituitary-testicular axis. In: YEN SS , JAFFEE RB , eds: Reproductive Endocrinology, 3rd ed. Philadelphia: WB Saunders, 1991
- CHAMBERS CV , SHAFER MA , ADGER H , et al: Microflora of the urethra in adolescent boys: relationships to sexual activity and nongonococcal urethritis. J Ped 110:314-321, 1987
- CZYBA JC , GIROD C: Development of normal testis. In: HAFEZ ESE , ed: Descended and Cryptorchid Testis. The Hague, Martinus Nijhoff, 1980.
- Whitmore WF, Kars L, Gittes RF: The role of germinal epithelium and spermatogenesis in the privileged survival of intratesticular grafts. J Urol 1985;134:782.
[edit] External links
- Animation of Spermatogenesis Flash
- Spermatogenesis - Male reproductive physiology Am Fam Physician 2000;62:1095.
