12. What is oogenesis? Give a brief account of oogenesis.
The process of formation of mature female gametes is called oogenesis. It starts during embryonic development stage when millions of oogonia (gamete mother cells) are formed in each fetal ovary. The gamete mother cells start division and enter into prophase-I of meiotic division and get temporally arrested. At this stage, they are called primary oocytes. Each primary oocyte gets surrounded by a layer of granulosa cell than it is called the primary follicle. The primary follicle gets surrounded by more layers of granulosa cells called secondary follicle that transform into tertiary follicle that contains a fluid filled cavity called antrum. The tertiary follicles further change into the mature follicle called Graafian follicle, which rapture to release secondary oocytes (ovum) from the ovary by the process of ovulation.