Meiosis is not the only step in the formation of the gametes. These cells must become specialized for their role in fertilization. The overall process of gamete formation is called gametogenesis. In animals, there are two types of gametogenesis: spermatogenesis, which is sperm production, and oogenesis, which is production of the ovum or egg.
The sperm cell, or spermatozoon (plural: spermatozoa), is highly specialized for travel, allowing it to seek out an egg to fertilize. Most spermatozoa have flagella (tail-like structures) that allow them to swim, although there are animals that have sperm which have no flagella and crawl like an amoeba. All spermatozoa derive from immature premeiotic stem cells called spermatogonia (singular: spermatogonium), which reproduce mitotically. Some spermatogonia begin to differentiate into primary spermatocytes, which then undergo the first meiotic division. This division produces two haploid secondary spermatocytes from each primary spermatocyte. Secondary spermatocytes undergo meiosis II, producing two spermatids from each secondary spermatocyte. Overall, therefore, each primary spermatocyte divides by meiosis to produce four haploid spermatids. The spermatids must undergo further differentiation and maturation to become spermatozoa.
The ovum (plural: ova), or egg, is the largest cell type in the body of most animals. This is because the ovum must store large quantities of materials needed for the development of the new individual after fertilization. The stem cell that produces ova is called an oogonium (plural: oogonia). Like spermatogonia, oogonia reproduce by mitosis, although some differentiate into primary oocytes, which begin meiosis. These oocytes only get as far as prophase I, at which point they arrest. The oocytes stay in this dormant state until they are about to be ovulated. In humans, the prophase arrest occurs before the female is even born, and the oocytes stay dormant until at least puberty, which is approximately a dozen years later. Some oocytes remain dormant for over 40 years! Prior to ovulation, the primary oocyte completes meiosis I, producing two haploid cells. Cytokinesis in this case is unequal, such that one of the haploid cells receives almost all of the cytoplasm, while the other receives very little. The cell that receives the cytoplasm is called a secondary oocyte; the cell receiving little is called a polar body. The secondary oocyte undergoes meiosis II (often the polar body does as well). Once again, cytokinesis is unequal, with one cell receiving almost all of cytoplasm. This cell becomes the ovum; the other cell, which receives little cytoplasm, is another polar body. Oogenesis, therefore, produces one haploid ovum and three haploid polar bodies from each primary oocyte. Only the ovum is capable of being fertilized; the polar bodies eventually die. Oogenesis works this way so that important materials can be stockpiled more easily for later in development.
Fertilization is the union of egg and sperm. Since each of these cells is haploid, fertilization accomplishes the reconstitution of a diploid genome in the new individual, which at this early stage is called a zygote.
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