Question

Which is the longest phase in oogenesis?

Answer 1

Hint: The process of formation of a mature female gamete is called oogenesis which is markedly different from spermatogenesis. Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells called oogonia are formed within each fetal ovary; no more oogonia are formed or added after birth.

Complete answer:
An ovarian follicle consists of an oocyte, surrounded by one or more layers of follicular cells, the granulosa cells, which are derived from the germinal epithelium lining the ovary. The oogonial cell starts division and enters into prophase-I of the meiotic division, and gets temporarily arrested at this stage called primary oocytes. The longest phase in oogenesis is the diplotene stage of prophase I, where the first meiotic division gets arrested in the primary oocytes. Diplotene lasts for years in some vertebrates. Each primary oocyte gets surrounded by a layer of granulosa cells and then called primary follicles.
A large number of those follicles degenerate from birth to puberty. Degenerate of ovarian follicles is called follicular atresia and their disposal is done by phagocytes. Therefore, at puberty only 60,000 to 80,000 primary follicles are remaining in each ovary.
With the onset of puberty, a primary follicle begins to mature with each ovarian cycle. The follicular cells become cuboidal, divided by mitosis to form a stratified epithelium, the granulosa layer. So, the primary follicles get surrounded by more layers of granulosa cells and a new theca, called secondary follicles. Granulosa cells rest on a basement membrane and the surrounding stromal form theca folliculi. The secondary follicle soon transforms into a tertiary follicle which is characterized by a fluid-filled cavity antrum, which appears between the granulosa between the granulosa cells. Initially, the antrum is crescent shaped, but with time it greatly enlarges. The fluid of the antrum is called liquor folliculi. As the follicles grow, the theca folliculi get organised into theca interna and theca externa.
The primary oocyte within the tertiary follicle develops and grows in size and finishes its first meiotic division at puberty. It is an unequal division leading to the formation of an outsized haploid secondary oocyte and a small polar cell.

Note:
Generally, only one ovum is liberated in each menstrual cycle by alternate ovaries. Each ovary releases six ova in one year.Only about 400-450.Ova are produced by human females over the entire span of her reproductive life which lasts about 35-40 years.