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Describe the stages of oogenesis in human females.

Last updated date: 25th Jun 2024
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Hint:Oogenesis occurs in ovaries, which are the female reproductive organs in females. It is initiated in the embryonic stage. Oogenesis is the differentiation of ovum. Ovum is haploid and consists of a single copy of chromosomes.

Complete answer:
During the early fetal development, primordial germ cells migrate from yolk sac to the ovaries. There, the germ cells differentiate to form oogonia. Oogenesis begins with the development of primary oocytes, which occurs through the transformation of oogonia into primary oocytes, a process called oocytogenesis. Once the oocytogenesis is complete, no primary oocytes are created. The primary oocytes enter prophase I of meiosis I, during the fetal stage but do not complete the phase until puberty.

During the arrested stage of development of the primary oocyte, it is covered by a single layer of follicle cells. Such an oocyte, along with the follicle cells are called primordial follicle. A small proportion of primordial follicles start to become developing follicles, before birth. This oocyte is then surrounded by the zona pellucida and is surrounded by layers of follicle cells, called granulosa cells, and a basal lamina that separates the oocyte from the other cells of the ovary.

After puberty, hormones FSH and LH trigger ovulation about once a month. The dominant primary oocyte completes meiosis I to produce a polar body and a large secondary oocyte. The secondary oocyte is then arrested at the metaphase II stage. The follicles grow and break at the surface of the ovary releasing the secondary oocyte, which is still surrounded by the granulosa cells. The released oocyte completes meiosis II if a sperm fertilizes it in a day or so. Otherwise, the large secondary oocyte again divided asymmetrically to produce the mature egg and a small polar body. Eventually, the polar bodies and the ovum degenerates.

Note:Unlike spermatogenesis, oogenesis begins in the females even before they are born. Basically, it has three stages: Pre-natal stage, antral stage, and Pre-ovulatory stage. Generally, the female gametes are referred to as eggs, but the word egg can include various stages of development, hence, the significance of an egg differs on the type of organisms.