Eggs and Oogenesis in Animal (1956 Words) | Biology

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Eggs:

The egg in all animals is a larger cell in comparison to other type of cells.

All contain reserve material of various kinds such as yolk, glycogen, nucleic acids (DNA, RNA, etc.) and various species of proteins.

Eggs are produced as large as possible, compatible with the number needed to ensure adequate variation, to cope of with the general enormous mortality rate of eggs, embryos and juveniles, and to distribute developing progeny beyond the terrestrial limits of the parental populations.

The number of eggs produced becomes greatly reduced and individual egg size correspondingly increased only in those cases, where special means are evolved to ensure survival whether by hiding, securing or otherwise protecting eggs, or above all by retaining developing eggs, within the maternal body as in mammals.

The egg cell or ovum has three basic functions: (i) to supply a nucleus containing half of the chromosomal component of the future embryo, (ii) to supply almost all the cytoplasm to the zygote, and (iii) to supply food reserves that will enable the embryo to develop up to a stage where it can begin to feed upon exogenous materials.

Therefore, for these future needs, an egg cell becomes immotile, large-sized, well-specialized, packaged and programmed during the process of oogenesis. Packaging i.e. the growth of the primary oocyte, and the accumulation of condensed food reserve, yolk, glycogen etc. within, it relates mainly to the number of cells that the developing egg can become, while, programming of egg relates to the determinative or directional information that egg may possess as a specialized type of reproductive cell.

Oogenesis:

The process of oogenesis is somewhat more complicated and different than spermatogenesis. Besides, the production of four unequal sized haploid cells, there is acquisition of food reserves in the egg cytoplasm for the development of embryo. Further, before the occurrence of meiosis, enormous amount of growth and differentiation of egg-cytoplasm takes place. The oogenesis is more or less similar in all vertebrate groups.

During oogenesis, the cells of germinal epithelium detach from the surface epithelium and enter the cortex of the ovary. These germinal cells are diploid and are called primordial germ cells. They pass through the three stages to form a fully formed egg which are:

(1) Phase of multiplication

(2) Phase of growth

(3) Phase of maturation.

Oogenesis in insects:

In insects, eggs are produced in the ovarioles of an ovary. The thinner part of an ovariole is called germanum, which contains oogonia enveloped by mesodermal cells. The stouter part of the ovariole is called the vitellarium, which contains oocytes in a longitudinal row. Each oocyte is surronded by a single layer of cuboidal follicle cells.

In the germanum, oogonia produce oocytes by mitosis. One oogonium divides into two cells—one primary oogonium and a cytoblast. The blast divides by four consecutive mitosis into 16 cells (cytocysts). One of these becomes an oocyte and the other 15 becomes nurse cells (trophocytes). All the 16 cells are surrounded by a single layer of flattened follicle cells.

The oocyte and the nurse cells, with their follicular covering, come to the vitellarium, where the oocyte gets yolk granules from the haemolymph and the nurse cells. As deposition of yolk continues, the oocyte grows. Then by meiosis the oocyte forms two cells, one big cell—the ovum and the other very small cell—the first polar body. The latter degenerates after sometime. In this way, the ovum gets half the number of chromosomes of the species.

When the ovum is ready for the transfer to the oviduct, the follicle cells secrete the chorion or outer shell around it. In cockroach, the nurse cells are absent.

Oogenesis in mammals:

In mammals the process of oogenesis shows the following three phases:

1. Phase of multiplication:

The primordial germ cells become the oogonia—the egg mother cell. The oogonial cells eventually undergo proliferation by repeated mitotic divisions, giving rise to the eggs and become primary oocytes when cell division ceases. Now they enter into a period of growth.

2. Phase of growth:

Owing to the fact that the egg contributes the greater part of the substance used in the development, growth plays a much greater role in oogenesis than in spermatogenesis. The period of growth in the female gametes is very prolonged and tremendous growth of oocyte occurs during this phase.

Most of the primordial germ cells are approx., 10 µm (0.01 mm) in diameter. The young oocyte of amphibians may be about 50 µm (0.05 mm) and the mature amphibian egg is rather large about 1000 to 2000 µm (1.0 to 2.0 mm) in diameter. In birds the diameter of ovum is as large as 40,000 µm and in mammals it is only 200 µ.m.

The rate of growth of oocytes also varies; it may be slow or fast. The young oocyte start growing after the tadpole metamorphoses into the young frog and by the third year the eggs mature and the frog spawn for the first time.

In other animals, the growth of oocyte may proceed at a much higher rate and takes shorter time for completion. In hen, the last rapid growth of oocyte occurs in 10 to 14 days preceding ovulation, and during this time the volume of the oocyte increases 200 fold.

The progressive growth increase in nuclear as well as cytoplasmic substances) of oocytes may be divided into two stages—(a) previtellogenesis growth period and (b) vitellogenesis growth period.

(a) Previtellogenesis growth period:

During this phase, no synthesis and accumulation of food reserve material, the yolk, takes place, but tremendous increase in the volume of nucleus and cytoplasm of primary oocyte occurs. There is qualitative and quantitative increase in the amount of cytoplasm. The mitochondria increase in number, the network of endoplasmic reticulum with ribosomes becomes more complicated, the Golgi bodies manufacture cortical granules, besides performing their normal function.

(i) Growth of nuclear substances:

During this phase due to the production of the large amount of nuclear sap, the nucleus of the growing oocyte increase in size. A dark body appears at one place outside the nucleus, usually near the Golgi complex and is known as yolk nucleus of Balbiani. This large sized oocyte inflated with the fluid is now called germinal vesicle.

The nucleus of the occyte enters the prophase of meiotic division. Synapsis occurs between homologous chromosomes but the subsequent stages of meiosis are postponed and each chromosomes increases in its length, but the amount of DNA in each choromosomes does not increase in proportion to the enlargement of the nucleus. The increased chromosomes look like bottle brush; hence they are called the lamp-brush chromosomes.

It is believed that the loop of chromosomes represent actual site for the main activity of the genes, i.e. transcription of mRNA, which inturn controls translation process in which synthesis of proteins in the cell cytoplasm takes place.

During the growth period of ooctye, all mRNA molecules are not utilized during translation but some are inactivated by the wrapping of proteins around them and stored as informosomes to be used during early cleavage of egg, when chromosomal DNA remains more actively engaged in its own transcription of mRNA (messenger RNA) r- RNA (ribosomal RNA) or t-RNA (transfer RNA).

The RNAs are transcribed by r-DNA of ‘nucleolar organizer region’ of chromosomes. The nucleolus has a significant role in the storage and maturation of the ribosomal RNAs. It also synthesizes all the proteins required for the biogenesis of ribosomes.

Therefore, during growth period of primary oocytes, the nucleolus increases greatly in size and becomes very conspicuous. In many animals, particularly in amphibians, instead of one large nucleous, numerous small sized nucleoli are formed in their germinal vesicles. Most of them are localized on the periphery of the nucleus, immediately underneath the nuclear membrane.

The increased transcriptional activity (i.e. RNA synthesis) of chromosomal genes during growth period of oocytes, is called gene amplification (Ephel, 1973) or redundancy (De Robertis et al, 1982). When MRNA molecules are transcribed from DNA then it is known as transcriptional amplification. Each MRNA molecule in turn can be translocated several times into the corresponding proteins known as translational amplification.

This high rate of gene amplification or gene activity is correlated with the fact that gene reduction (meiosis) does not take place until after the growth of the oocytes has been completed. As a result the oocytes remain tetraploid for a long time.

(ii) Growth of cytoplasmic substances:

The amount of cytoplasm of oocyte increases both quantitatively and qualitatively during the Previtellogenesis growth period of oocyte. Young oocytes, in many animals, show a very simple organisation due to poor cytoplasmic inclusions and possess none of the specialized structures found in the adult oocyte and mature egg. The cytoplasm is finely granular having granules of ribonucleo-protein and DNA.

Mitochondria, the carriers of oxidative enzymes are fairly scarce in young oocytes but may increase in number very considerably during the growth of primary oocyte because overall oxygen consumption increases during this time. In amphibians and birds, the mitochondria become aggregated in the form of large ‘Mitrochondrial clouds”. Mitrochondria possess its own circular DNA. So in a growing oocyte, the amount of mitrochondrial DNA far exceeds the amount of nuclear DNA.

The young oocytes have the granular endoplasmic reticulum in the form of numerous, small vesicles. Annulated lamellae are also found in the cytoplasm of growing oocyte. These membranous structures appear in the form of stakes of cistemae, either in parallel or in spiral arrangement.

Sometimes, annulated lamellae are associated with ribosomes and RNA in high concentrations, and there is also an ATPase activity in the pore complexes of these lamellae. The lamella, thus serves as a storage site of RNA in cytoplasm and they are found to break down and disappear during late oogenesis.

In young oocyte the Golgi bodies are found around the Centrosome. In mature oocytes they form a large spherical mass in some mammals, or become located in the sub-cortical cytoplasm of frog and chick, or sometimes may disappear completely. The Golgi complex of oocyte is believed to synthesize cortical granules besides performing its normal function.

In the cortical region, cortical granules are present. These are membrane bound spherical bodies of diameter from 0.8 microns (sea urchin) to 2.0 microns (frog) and contain acid mucopolysaccharides. These mucopolysaccharides are used during fertilization, in the formation of fertilization membrane.

They are present in bivalve molluses, some annelids, fishes, frogs and some mammals (rabbit and man), but are absent in some insects, gastropodes urodeles, birds and some mammals (rat and guine apig). These granules are synthesized’ by cisternae of Golgi complex in the interior of the oocyte and later they move to the periphery where they are arranged in a layer close to the plasma membrane of oocyte.

(b) Vitellogenesis growth period:

The process of formation and deposition of yolk is called vitellogenesis. In amphibians and fishes its synthesis takes place inside the modified mitochondria. In some insects yolk formation occurs in fat bodies whereas in most of the vertebrates yolk is formed in the liver of the mother. From the liver it is carried by blood to the ovary.

In the ovary, this material ‘percolates’ to the follicle cells and from there to the cytoplasm of the oocyte. However, in vertebrates, a very small quantity of the material for the formation of yolk (hardly 1%) is synthesised by the oocyte cytoplasm itself.

Thus a major part of the material forming the yolk is exogenous (formed outside the oocyte). Yolk is a general term that covers the major storage material such as glycogen, certain other carbohydrates, proteins and lipids. Hence, vitellogenesis is the period of rapid growth.