Oogenesis

Oogenesis (ovogenesis), the process of development of female germ cells—eggs, is usually divided into three successive periods: 1) the period of multiplication, 2) the period of growth, 3) the period of maturation. Oogenesis can be traced particularly instructively when studying the ovaries of certain animals (Ascaris), where each of the periods listed above corresponds to a specific morphological zone. The first period—the period of multiplication—is characterized by the intensified multiplication of so-called primary egg cells (oogonia). This period covers an extremely different amount of time in various groups of animals; for instance, in vertebrates, it is observed only during a limited time coinciding with the development of the embryo, and in humans, for example, it ends in the first years of postnatal life; in invertebrates, on the contrary, the formation of oogonia occurs throughout the entire life of the animal. Oogonia are small, rounded cells with a clearly formed nucleus and a large nucleolus (the 'germinal spot' of old authors). The question of the origin of primary egg cells is to a significant extent obscure and, in any case, is resolved very differently for different zoological groups. For the majority of higher animals, the most typical origin of oogonia is from the epithelium of the internal body cavity (coelomic epithelium), i.e., from the middle mesodermal layer. However, for some of the most primitive representatives of the animal world—sponges, coelenterates, etc.—the emergence of oogonia from other layers (endoderm and, as an exception, from the ectoderm) has been described. In some particularly favorable cases (Ascaris—Boveri; vertebrates—Nussbaum; some insects, crustaceans, etc.), it was possible to recognize the primordium of the future gonad at extremely early stages of development—at the moment of the formation of axial organs, the appearance of germ layers, gastrulation, at the stage of a few blastomeres, and according to Boveri, even at the stage of two blastomeres, where the cell giving rise to the germ elements, in contrast to the progenitor of somatic cells, is characterized by the preservation in a normal form of the entire supply of chromatin received during fertilization (Ascaris). All these data on the early differentiation of the germ primordium led to the creation of a special germ-track theory. One should distinguish between diffuse and localized oogenesis depending on whether the site of development of the germ elements is sufficiently fixed. We encounter the first type of oogenesis at the lower rungs of the zoological ladder—in sponges, some coelenterates, and flatworms, where, apparently, any parenchyma cell can, under certain conditions, turn into an oogonium. In the majority of cases, however, a more or less precisely localized development of germ cells takes place, and sometimes this site is secondary (e.g., in the manubrium of medusae), i.e., it appears as an accumulation of cells that have actively (by means of amoeboid movements) arrived from other parts (sometimes even other layers) of the animal's body. The second period—the period of growth—is characterized by an increase in the volume of the egg and the accumulation of nutrient substance—yolk—within it. It is precisely as a result of this stage of oogenesis that the egg sometimes assumes relatively large dimensions (e.g., the egg of birds). In the processes of yolk accumulation, an extremely active role apparently belongs to the nucleus (germinal vesicle) and, in particular, its chromatin apparatus. In particular, the so-called accessory nuclei, rich in chromatin substances, which have been repeatedly described, are considered true yolk-formers. Often, the exit of chromatin in the form of a small cloud from the nucleus has also been described, with its granules, which ended up in the protoplasm, playing the role of centers for newly forming yolk lumps. The formation of the yolk itself occurs in many cases concentrically in the direction from the periphery to the center, which is obviously connected with the gradual impregnation of the plasma with special substances of an enzymatic nature. Large, ready-to-use cells with large nuclei are called oocytes of the first order at this stage. Depending on the special features of the growth period, three main types of oogenesis are distinguished: 1) solitary, 2) alimentary, 3) nutrimentary. In the first case, there is isolated growth and formation of the oocyte; in both of the latter, additional, sometimes quite complex formations appear, which apparently have significance for creating the most favorable conditions for the accumulation of nutrients. In the case of alimentary oogenesis, we are dealing with a formation very common among representatives of almost all zoological groups: a single- or multi-layered follicle—a sheath around the egg, built from indifferent cells of the gonad, which transfer nutrients to the growing egg cell. In the case of nutrimentary oogenesis, we encounter special nutritive cells, which are usually located around the egg in the form of an irregular group and are, by their origin, germ cells (abortive oocytes). An example of nutrimentary oogenesis is represented by the so-called yolk sacs, or vitellaria, of flatworms—special organs which, as a comparison of related forms shows, are an accumulation of abortive oocytes that gradually disintegrate and are used for the nutrition of the growing egg. The third period of oogenesis—the period of maturation of germ cells—is characterized by the reduction of chromatin and a corresponding decrease in the number of chromosomes, occurring as a result of two successive 'maturation divisions' (reduction divisions). The reduction is carried out differently in different cases and is invariably connected with the appearance of complex chromosomal formations consisting of four parts (so-called tetrads). In the so-called meiotic maturation (studied in Ascaris), the decrease in the number of chromosomes takes place before the beginning of division. In the second, pseudomitotic type of maturation, the decrease in the number of chromosomes occurs during one of the two successively occurring maturation divisions. In this, a post-reductional division is distinguished, in which the first division (equational) occurs by way of longitudinal splitting of the chromosomes, and the second—the reductional one proper—is connected with the transverse division of the chromosomes. In some few cases, a pre-reductional division is observed, in which the transverse division (reductional) precedes the longitudinal (equational) splitting of the chromosomes. The biological significance of the phenomena of maturation is very great and comes down both to the preservation of a certain number of chromosomes unchanged (during fertilization, two cells with half the number of chromosomes unite) and to the creation of new hereditary combinations. Histologically, reduction divisions in eggs proceed quite peculiarly and begin with the formation of a spindle at the periphery of the cell and the release of the first reductional, polar, or directing body (the latter name is connected with the fact that the place of its release coincides with the direction of the subsequent first cleavage furrow), which represents an abortive cell with a minimal amount of protoplasm. The anaphase of the first reductional division in the egg directly, bypassing the resting stage, passes into the equatorial plate of the 2nd reductional division, with the spindle being positioned at an angle of 90° to the previous position. The result of this division is the appearance of the second directing body. The first directing body often divides into 2, and as a result, 4 cells arise: the egg and 3 abortive cells, which are usually incapable of development. In some cases, the reductional bodies reach significant sizes and then can not only be fertilized by a spermatozoon but also undergo the first stages of cleavage. Parthenogenetically developing eggs either do not release directing bodies at all or release only one—a circumstance that becomes understandable for those cases where the reduction of the number of chromosomes is absent (parthenogenesis). During the phenomena of maturation, peculiar changes also occur in the protoplasm of the egg. Thus, Delage, in attempts to fertilize enucleated parts of eggs, managed to do so only in the case of a matured egg. Reduction divisions in the cells of different species occur at different times: sometimes at the moment preceding fertilization (sea urchin), sometimes immediately after fertilization (Ascaris). Phenomena extremely close to the processes of maturation of germ cells in animals have also been described for the processes of maturation of egg cells in plants. Literature—see the literature for the article Fertilization. S. Zalkind.