Question

Give a schematic representation of oogenesis in humans. Mention the number of chromosomes at each stage. Correlate the life phases of the individual with the stages of the process.

Answer 1

Human gametes are of two types, the male sperm and the female ovum. These are produced by reductional division (Meiosis) in the germ cells of their respective organs.

Complete answer:
The oogenesis refers to the production of mature ovum from the germ cells in the ovary. Unlike Spermatogenesis in which spermatogonia are produced regularly from the onset of puberty to the rest of the life, the number of oogonia are fixed from the girl’s birth and is only continued to a point in her life (menopause).
Following are the detailed steps of oogenesis.
(1) The precursor of an oocyte (2n) (the precursor of the ovum), the Oogonium (2n) transforms into primary oocytes, a process known as “oocytogenesis” which happens before or shortly after the girl’s birth. The number of oocytes present in the ovary is thus determined at the time of the birth and no new oocytes are added to this in case of humans.
(2) The oocyte is supposed to go under meiosis to produce the mature ovum however, all the oocytes in the female fetus are arrested at the prophase I stage until the onset of puberty. At which point only a few of these would undergo complete division to mature ovum.
(3) On the onset of puberty, a single primary oocyte (2n) undergoes meiosis I to produce a polar body and a secondary oocyte (2n) during every menstrual cycle. following this division the secondary oocyte is again arrested at metaphase II where it remains until the oocyte is fertilized. If a particular secondary oocyte isn’t fertilized, it disintegrates and is eliminated (menstruation).
(4) When exposed to sperm, the secondary oocyte completes the Meiosis II to produce a mature ootid and the second polar body (the function of this polar body is to eliminate the extra set of the haploid chromosome produced during meiosis).

Note: Spermatogenesis which is the production of sperms in the male testicles and it begins on the onset of puberty never ends and continues even in old age. During menstruation, a single primary oocyte is released from the ovary into the fallopian tubes every month where it transforms into the secondary oocyte.