Short notes on the process of oogenesis

Short notes on the process of oogenesis

The process by which the eggs or ovadevelop from the primordial germ cells of the ovary is called oogenesis. The mature ovary consists of cells similar to the primordial cells in the testis. These however give rise to eggs.

The process of the development of the egg can be studied under the fol­lowing stages. These are multiplication phase, growth phase and matura­tion phase.

1. Multiplication phase:

The primordial germ cells divide repeatedly by means of mitosis to produce a large number of daughter cells. These are called oogonia. The oogonia are known to divide again several times mi- totically to produce a large number of cells called the primary oocytes. After the formation of primary oocytes there is no more division. The cells enter into the growth phase.

2. Growth phase:

The growth phase of oogenesis is longer and more com­plicated than in spermatogenesis. There is considerable enlargement in the size of the primary oocyte during the growth phase. For instance the primary oocyte of frog initially has a diameter of only about 50 microns but at the end of the fruit phase the diameter of the matured egg will be as much as 2000 microns.

Growth in the primary oocyte is brought about by the accumulation of a large amount of fats and proteins. These get accu­mulated in the form of yolk. The yolk is usually concentrated towards the lower portion of the egg called the vegetal pole. The upper side of the egg which contains cytoplasm and the egg nucleus remains often separate from the yolk and it is called the animal pole.

The cytoplasm of the oocyte is very rich in RNA, DNA, enzymes etc. Other cytoplasmic organells like mitochondria, golgi complex c ribosomes are also found in large numbers. In Oocytes of some amphibians and brtds the mitochondria accumulate at certain places forming mitochondrial clouds. During this period, the nucleolar genes show increased activity of RNA synthesis and multiply their number. This is known as gene amplifi­cation or redundancy.

The nucleus enlarges in size due to the increased amount of nucleoplas. The nucleolous also becomes large due to the synthesis of ribosomal RNA. At this stage in some of the amphibians the chromosomes change their shape and become giant lampbrush chromosomes. Maturation phase. It is during this phase that the oocyte undergoes reduc­tion division (meiosis) eventually to produce the egg or the ovum.

The maturation division of the primary oocyte differs greatly from that of the spermatocytes. While in spermatogenesis at the end of meiosis four sperms are formed, here at the end of meiosis only one large haploid egg is formed, the remaining three cells forming three small polar bodies. This unequal division results in the formation of a single egg having a large quantity of stored food that is necessary for the development of the em­bryo.

The first maturation division (meiosis I) reduces the chromosomes into half. After the homologus chromosomes undergo chiasma formation and crossing over, the nuclear membrane breaks and the chromosomes move towards the opposite poles. The nuclear division is followed by the cyto­plasmic division.

This division (Cytokinesis) is unequal and results in the formation of a small polar body and large secondary oocyte or ootid. During the second maturation division (meiosis II) the haploid secondary oocyte as well as the polar body undergoes a mitotic division.

As a result of this, the polar body forms two polar bodies, while in the secondary oo­cytes the division results in the formation of a mature egg and a second polar body. Thus at the end of second maturation division there will be a large single egg and three polar bodies. These polar bodies eventually ooze out and degenerate, while the egg is ready for fertilization.