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The following points highlight the three successive stages of oogenesis. The stages are: 1. Multiplication phase 2. Growth phase 3. Maturation phase.
Oogenesis: Stage # 1. Multiplication Phase:
The primordial germinal cells divide repeatedly to form the oogonia (Gr., oon = egg). The oogonia multiply by the mitotic divisions and form the primary oocytes which pass through the growth phase.
Oogenesis: 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. For instance, the primary oocyte of the frog in the beginning has the diameter about 50µ but after growth phase the diameter of the mature egg reaches about 1000µ to 2000µ.
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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 portion of the cytoplasm containing the egg pro-nucleus remains often separated from the yolk and occurs towards the upper side of the egg forming the animal 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.
In certain oocytes (Amphibia and birds), the mitochondria become accumulated at some place in the occyte cytoplasm and form the mitochondrial clouds.
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 nucleolus becomes large or its number is multiplied due to excessive synthesis of ribosomal RNA (rRNA) by ribosomal DNA (rDNA) of nucleolar organizer region of chromosomes.
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Thus, the nucleus or germinal vesicle of primary oocyte of Triturus has 600 nucleoli, of Siredon has 1000 nucleoli and of Xenopus has 600 to 1200 nucleoli due to synthesis of ribosomal RNA (rRNA).
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.
Oogenesis: 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.
(i) First maturation division:
During the first maturation division or first meiosis, the homologous chromosomes of the primary oocyte nucleus pass through the pairing or synapsis, duplication, chiasma formation and crossing over. Soon after, the nuclear membrane breaks and the bivalent chromosomes move towards the opposite poles due to contraction of chromonemal fibres.
A new nuclear envelope is developed around the daughter chromosomes by the endoplasmic reticulum. After the karyokinesis, the unequal cytokinesis occurs and a small haploid polar body or polocyte and a large haploid secondary oocyte or ootid are formed.
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(ii) Second meiotic division:
The haploid secondary oocyte and first polocyte pass through the second meiotic division. Due to the second meiotic division, the secondary oocyte forms amature egg and a second polocyte. By the second meiotic division, the first polocyte also divides into two secondary polocytes. These polocytes ooze out from the egg and degenerate while the haploid egg cell becomes ready for the fertilisation.
