At ovulation, this secondary oocyte will be released and travel toward the uterus through the oviduct. A secondary meiotic arrest occurs, this time at the metaphase II stage. This second cell is called a polar body and usually dies. The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell. This results in the primary oocyte finishing the first meiotic division. At adolescence, anterior pituitary hormones cause the development of a number of follicles in an ovary.

At the time of birth, all future eggs are in the prophase stage. This cell will start the first meiotic division and be arrested in its progress in the first prophase stage. The cell starting meiosis is called a primary oocyte, as shown in Figure 2. As with sperm production, oogenesis starts with a germ cell, called an oogonium (plural: oogonia), but this cell undergoes mitosis to increase in number, eventually resulting in up to about one to two million cells in the embryo. Oogenesis, illustrated in Figure 2, occurs in the outermost layers of the ovaries.

Figure 2. The process of oogenesis occurs in the ovary’s outermost layer.

Gametogenesis oogenesis