Oogenesis is the process through which ova or the female gametes are formed and this female gamete is known as an ovum. Sometimes we refer female gametes to eggs but the word egg can involve different stages of development and the significance of egg varies based on types of organism.
For instance, the complete development of birds takes place inside the eggs while in placental mammals once the egg is fertilized and begins to divide, you don’t call it as egg anymore. You need to recall that every ovum has to be haploid and consist of a single copy of every chromosome.
What is Oogenesis?
Oogenesis is the process of formation of female gametes. This process begins inside the foetus before birth. The steps in oogenesis upto the production of primary oocytes occur before birth. Primary oocytes do not divide further. They either become secondary oocytes or degenerate.
Oogenesis occurs in the outermost layers of the ovaries. Oogenesis starts with a germ cell called oogonium and undergoes mitosis to increase in number. The process of oogenesis takes place in the following three stages:
Also Read: Spermatogenesis
Process of Oogenesis
The process of oogenesis is completed in the following three stages:
The primary oocyte grows while being arrested in meiosis-I. The follicular cells proliferate and form a stratified cuboidal epithelium. Such cells are known as granulosa cells. These cells secrete glycoproteins to form zona pellucida around the primary oocyte.
The fluid-filled spaces between granulosa cells combine to form a central fluid-filled space called antrum. These are known as secondary follicles. In every month cycle, these secondary follicles develop under the influence of follicle-stimulating hormone and leutinizing hormone.
This stage is induced by LH surge and meiosis-I completes here. Two haploid cells of unequal sizes are formed within the follicle. One of the daughter cells that receive less cytoplasm forms a polar body. This cell does not participate in ovum formation. The other daughter cell is known as the secondary oocyte. The two daughter cells undergo meiosis-II. The polar body replicates to form two polar bodies, while the secondary oocyte arrests in the metaphase stage of meiosis-II.
Development of oocyte takes place in ovaries. Every oocyte is neighboured by follicle cells to form a follicle. As the menstrual cycle starts, primary oocytes initiate to grow bigger and follicle cells rise in number causing the follicle to grow larger too.
Normally, some nurturing oocytes degenerate and leave just one follicle to mature. Here, fraternal twins may be born which are distinct genetically.
When a follicle attains maturity, the primary oocyte finishes its primary meiotic division and becomes secondary oocyte. Soon after, the follicle breaks and secondary oocyte is liberated in the fallopian tube even when the second meiotic division has not happened. This release of a secondary oocyte from ovaries is known as ovulation.
Meiosis-II is completed on fertilization. This gives off a third polar body. If the fertilization does not occur, the oocyte degenerates 24 hours after ovulation while remaining arrested in meiosis-II.
Also Read: Gametogenesis in Humans
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