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Explain the steps in the formation of an ovum from an oogonium in humans.

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Answer
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Hint: The process by which ovum is formed from oogonium is called oogenesis. It occurs in three phases: multiplication, growth, and then maturation.

Complete answer:
Oogenesis is the process by which an ovum or a secondary oocyte is produced from the oogonia.
Oogonium is the singular form, oogonia are plural.
First, during the embryonic period of a female, the primordial germ cells differentiate to form numerous oogonium or oogonial cells.
The cell pool is maintained by repeated mitotic divisions.

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Now the oogonia grows into the primary oocytes as the onset of meiosis I occur. After completion of meiosis I, diploid primary oocytes produce two types of haploid cells- a secondary oocyte and the first polar body.
The secondary oocyte completes its meiotic division after sperm entry and produces another polar body and a mature ovum. The first polar body also divides to form two new polar bodies.

Note: Oogenesis is a discontinuous process. It is arrested twice.
First, the primary oocyte is arrested in prophase I; it releases after puberty.
Second, the secondary oocyte is arrested at metaphase II; it releases upon sperm entry.