Question

Describe the process of oogenesis in human female.

Answer 1

Oogenesis is a process of formation of the haploid female gamete i.e. egg or ovum from the diploid germinal epithelium. Oogenesis takes place in the ovaries and is initiated prior to birth of the female baby. It involves the process of mitosis and meiosis. This process can be divided into three stages:

1. Multiplication phase: In this stage, the primary germinal cells - PGCs (2n) of ovary undergo repeated mitotic division to form millions of gamete mother cells or oogonial cells (2n). This process is completed in the embryonic stage of human females.
2. Growth phase: Some of the oogonia stop division and begin to increase in size and form the primary oocytes (2n). Cellular organelles like ER, Golgi apparatus and mitochondria increase in number.
3. Maturation phase: The primary oocytes (2n) enter the maturation phase, which includes meiotic division (Meiosis I and Meiosis II). The diploid primary oocytes undergo meiosis I (reduction division) to form two haploid daughter cells having 23 chromosomes. However, due to unequal division of cytoplasm, of the two daughter cells, one is a large cell, called secondary oocyte (n) and another is a small cell, called 1st polar body (n). Generally, the 1st polar body does not enter meiosis II.
   The secondary oocyte (n) proceeds only upto metaphase II of meiosis II and its division is further stopped or arrested at this stage. The secondary oocyte is shed from the Graafian follicle and ovary during ovulation. Meiosis II is completed only if fertilization occurs. This last phase is usually completed in the ampulla of the fallopian tube at the time of fertilization. In this division also, the two unequal daughter cells are formed i.e. the large cell is ovum (n) and the small cell is 2nd polar body (n). The ovum (n) so formed functions as the female gamete and is ready for fertilization. If the secondary oocyte is not fertilized by a sperm/spermatozoa, it is shed off along with menstruum.