Describe in proper sequence, the development of an ovum from oogonium in a human female? 2
Answers
Oogenesis is the process of the formation of a mature ovum from the oogonia in females. It takes place in the ovaries. During oogenesis, a diploid oogonium or egg mother cell increases in size and gets transformed into a diploid primary oocyte. This diploid primary oocyte undergoes first meiotic division i.e., meiosis I or reductional division to form two unequal haploid cells. The smaller cell is known as the first polar body, while the larger cell is known as the secondary oocyte. This secondary oocyte undergoes second meiotic division i.e., meiosis II or equational division and gives rise to a second polar body and an ovum. Hence, in the process of oogenesis, a diploid oogonium produces a single haploid ovum while two or three polar bodies are produced.
The three stages of Oogenesis :-
Stage # 1. Multiplication Phase:
The primordial germinal cells divide repeatedly to form the oogonia. The oogonia multiply by the mitotic divisions and form the primary oocytes which pass through the growth phase.
Stage # 2. Growth Phase:
The growth phase of the oogenesis is comparatively longer than the growth phase of the spermatogenesis. In the growth phase, the size of the primary oocyte increases enormously.
In the primary oocyte, large amount of fats and proteins becomes accumulated in the form of yolk and due to its heavy weight (or gravity), it is usually concentrated towards the lower portion of the egg forming the vegetative pole.The cytoplasm of the oocyte becomes rich in RNA, DNA, ATP and enzymes. Moreover, the mitochondria, Golgi apparatus, ribosomes, etc., become concentrated in the cytoplasm of the oocyte.During the growth phase, tremendous changes also occur in the nucleus of the primary oocyte. The nucleus becomes large due to the increased amount of the nucleoplasm and is called germinal vesicle.
The chromosomes change their shape and become giant lamp-brush chromosomes which are directly related with increased transcription of messenger RNA (mRNA) molecules and active protein synthesis in the cytoplasm. When the growth of the cytoplasm and nucleus of the primary oocyte is completed, it becomes ready for maturation phase.
Stage # 3. Maturation Phase:
The maturation phase is accompanied by the maturation or meiotic division. The maturation division of the primary oocyte differs greatly from the maturation division of the spermatocyte. Here after the meiotic division of the nucleus, the cytoplasm of the oocyte divides unequally to form a single large-sized haploid egg and three small haploid polar bodies or polocytes at the end.
This type of unequal division has the great significance for the egg. If the equal divisions of the primary oocyte might have been resulted, the stored food amount would have been distributed equally to the four daughter cells and which might prove insufficient for the developing embryo.
Therefore, these unequal divisions allow one cell out of the four daughter cells to contain most of the cytoplasm and reserve food material which is sufficient for the developing embryo.