Question

The oogonia is arrested in which of the following phases of cell division?
A) Anaphase II
B) Anaphase I
C) Interphase
D) Both A and B

Answer 1

The process of oogenesis involves the formation of haploid female gametes, called ova. This is entirely different from the process of spermatogenesis where spermatogenesis commences when the individual attains puberty and continues for the rest of life. Whereas in oogenesis, the cell division process is arrested until the girl attains puberty and has also been found to be arrested until 50 years of age.

Complete answer:
The process of oogenesis involves 2 biological processes-Genetical programmings and packaging. The period of oogenesis varies in different animals. In the first month of fetal development, the human female consists of 1700 primary germ cells in the undifferentiated female gonad. By the end of two months, they proliferate to form about 600,000 oogonia. The ovaries contain over 7 million oogonia, during its fifth month. However many became degenerated.

Like spermatogenesis, oogenesis is formed of three phases:
1.Multiplicative phase: Here primary germ cells of the germinal epithelium of the ovary undergo rapid mitotic divisions to form diploid egg mother cells, oogonia.
2.Growth phase: The duration of this phase is very long i.e, it may take 12-13 years in the human female. Here, one oogonium of the egg nest is transferred to the diploid primary oocyte, while other forms into the follicular epithelium around it. The structure so formed is called the primary follicle. Later, a secondary follicle is formed when the secondary oocyte is surrounded by more layers of granulosa cells and later forms a tertiary follicle. It further changes to the graafian follicle.
3.Maturation phase: It is characterized by meiosis. Here, the fully grown oocyte undergoes meiosis I to form two unequal haploid cells. The smaller is called the first polar body and the larger cell is called a secondary oocyte. The secondary oocyte undergoes meiosis II to form unequal haploid cells. The smaller cell is called the second polar body and the larger cell is called ootid.

In most of the organisms including females, the ovulation occurs at the secondary oocyte stage where meiosis I have been completed and the first polar body has been released. Meiosis II is stopped at metaphase II and is completed only at the time of sperm entry. The prophase I arrested oocyte acquires its ability to resume meiosis when it approaches its full size. With response to LH, the oocyte progresses to metaphase II and stimulates ovulation.

Thus, the correct answer is option ‘B’.

Note: Both (Spermatogenesis and oogenesis) involves meiosis and both maturation division occurs in the testis for males, whereas for females, the first maturation division occurs in the ovaries. And second maturation division occurs outside the ovaries once the fertilization is initiated.