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The generation of ova or eggs, the female gametes. Primordial germ cells, once they have populated the gonads, proliferate and differentiate into sperm (in the testis) or ova (in the ovary). The decision to produce either spermatocytes or oocytes is based primarily on the genotype of the embryo. In rare cases, this decision can be reversed by the hormonal environment of the embryo, so that the sexual phenotype may differ from the genotype. Formation of the ovum most often involves substantial increases in cell volume as well as the acquisition of organellar structures that adapt the egg for reception of the sperm nucleus, and support of the early embryo. In histological sections, the structure of the oocyte often appears random but as the understanding of its chemical and structural organization increases, an order begins to emerge. See Ovum, Spermatogenesis
Among lower vertebrates and invertebrates, mitotic divisions of the precursor cells, the oogonia, continue throughout the reproductive life of the adult; thus extremely large numbers of ova are produced. In the fetal ovary of mammals, the oogonia undergo mitotic divisions until the birth of the fetus, but a process involving the destruction of the majority of the developing ova by the seventh month of gestation reduces the number of oocytes from millions to a few hundred. Around the time of birth, the mitotic divisions cease altogether, and the infant female ovary contains its full complement of potential ova. At puberty, the pituitary hormones, follicle stimulating hormone (FSH), and luteinizing hormone (LH) stimulate the growth and differentiation of the ova and surrounding cells (see illustration). See Mitosis
One important feature of oocyte differentiation is the reduction of the chromosome complement from the diploid state of the somatic cells to the haploid state of gametes. Fusion with the haploid genome of the sperm will restore the normal diploid number of chromosomes to the zygote. The meiotic divisions which reduce the chromosome content of the oocyte occur after the structural differentiation of the oocyte is complete, often only after fertilization. Unlike the formation of sperm, in which the two divisions of meiosis produce four equivalent daughter cells, the cytoplasm of the oocyte is divided unequally, so that three polar bodies with reduced cytoplasm and one oocyte are the final products. Generally, each fertilized oocyte produces a single embryo, but there are exceptions. Identical twins, for example, arise from the same fertilized egg. See Meiosis
The provision of nutrients for the embryo is a major function of the egg, and this is accomplished by the storage of yolk in the cytoplasm. Yolk consists of complex mixtures of proteins (vitellins), lipids, and carbohydrates in platelets, which are membrane-surrounded packets dispersed throughout the egg cytoplasm (ooplasm). The amount of yolk in an egg correlates with the nutritional needs of the embryo. Although the eggs of mammals are extremely small as compared to the fetus, the bulk of the nutrition is supplied by the placenta; yolk is required only until implantation in the uterine wall.
Egg cytoplasm also contains large stores of ribonucleic acid (RNA) in the form of ribosomal, messenger, and transfer RNA. These RNAs direct the synthesis of proteins in the early embryo, and may have a decisive influence on the course of development. The mechanism by which the RNA is supplied to the egg is the basis for a major classification of ovary types. Panoistic ovaries, in which the egg nucleus is responsible for the production of all the stored RNA in the ooplasm, are typical of vertebrates, primitive insects, and a number of invertebrates. The amounts of RNA produced during the meiotic prophase in such ovaries are much larger than those produced by a somatic cell, and thus special mechanisms seem to be involved in the synthetic process. See Deoxyribonucleic acid (DNA), Ribonucleic acid (RNA)
the development of the female sex cell, or ovum.
Oogenesis is divided into three periods: multiplication, growth, and maturation. The duration of each period varies among animals. During multiplication the female sex cells, which are called oogonia at this stage, multiply by a series of mitoses (seeMITOSIS). An oogonium is small, with a relatively large nucleus, and does not contain many organoids in its cytoplasm. After mitosis stops, the oogonia enter a period of growth and are called primary oocytes. In the first growth period an oocyte enlarges only slightly, while the nucleus prepares for the reduction of genetic material (seeMEIOSIS). A phase of slow growth follows, which can last for years in certain animals and in humans. Next, the oocytes undergo vitellogenesis—a phase of rapid growth during which yolk accumulates; this phase can be as short as a few days or even a few hours. During vitellogenesis the volume of the oocyte may increase tens of thousands of times.
Maturation involves meiosis and consists of two successive divisions, during which the chromosome number is halved. As a result of the first division, a secondary oocyte and the first polar body are formed. The second division gives rise to the second polar body and the mature ovum, which is haploid and capable of further development. As the oocyte matures, the properties of the cytoplasm change.
A distinction is made between diffuse and localized oogenesis. Diffuse oogenesis, in which the ova form anywhere in the organism’s body, is found in sponges and some flatworms. In localized oogenesis, which is characteristic of all other animals, the ovum develops in specialized organs called ovaries. In both types of oogenesis, the oocyte may grow independently or it can be nourished by special auxiliary cells—nurse cells or follicular cells.
Nurse cells, also called trophocytes, develop from oogonia and are usually connected to the oocyte by cytoplasmic bridges. They supply the oocyte mainly with ribosomal ribonucleic acid. Such auxiliary cells, which reach their maximum development before the beginning of vitellogenesis and then degenerate, are found in the ovaries of many insects and some worms. Another type of auxiliary cell—the cells of the follicular epithelium—are characteristic of oogenesis in many invertebrates and all vertebrates. These cells develop from the somatic cells of the ovary and surround the growing oocyte with a compact layer. The ovum and the follicular cells make up the follicle. The follicular cells regulate the transfer by pinocytosis of yolk proteins from the blood into the oocyte. In the late stages of oogenesis, the follicular cells secrete the materials of the secondary ovular membranes.
In vertebrates, the follicular cells under the control of the pituitary gland secrete steroid hormones that bring about the maturation of the oocyte. Ovulation—release of the oocyte from the follicular membranes—occurs at various stages of maturation in different animals. For example, in most vertebrates and in humans it occurs during metaphase of the second meiotic division.
REFERENCERaven, C. Oogenez. Moscow, 1964. (Translated from English.)
T. B. AIZENSHTADT