Oogenesis is the production of secondary oocytes from the primordial germ cells. It takes place in the ovaries.
As a foetus early in development, a female has around 6 million eggs in her ovaries. This number is steadily reduced throughout gestation, so it’s already down to 1-2 million when the baby girl is born. This decrease in egg number continues through childhood, so by the time that baby girl reaches puberty, she has between 300,000 and 400,000 eggs. Before puberty, more than 10,000 eggs die each month. After starting her menstrual cycle, a woman loses about 1,000 every month.
- Germinal epithelium cells form oogonia. These are produced before birth.
- Oogonia undergo a period of growth and maturation, dividing via mitosis to produce primary oocytes.
- The primary oocytes undergo meiosis but the process is halted at prophase 1. At the same time, the germinal epithelium also produces follicle cells that surround the primary oocytes. These are primary follicles.
- During puberty, hormones stimulate one of the primary follicles. This matures and becomes a Graafian follicle.
- The primary oocytes do not mature until just before ovulation. The primary oocyte completes the first meiotic division to form the haploid secondary oocyte and a small polar body. This polar body can sometimes go on to divide again in meiosis II, forming two polar bodies.
- The mature Graafian follicle moves to the surface of the ovary where it releases the secondary oocyte. This is ovulation.
- The secondary oocyte begins the second meiotic division, but stops at metaphase 2 unless fertilisation takes place. (Read here that women aren’t going to waste energy on something that may not be fertilised and just lost in menstrual fluid).
- Then, as one of my students so aptly put it: it’s like one of those choose your own adventure books: Option a) If fertilisation takes place, meiosis completes and a large ovum and a second polar body is formed. Option b) Fertilisation doesn’t take place and the secondary oocyte is lost with the endometrium during the woman’s period.
- After ovulation, the Graafian follicle becomes the corpus luteum (Latin: yellow body). This produces hormones throughout pregnancy that prevents further ovulation and maintains the endometrium. It degenerates if pregnancy does not occur, becoming the corpus albicans (Latin: whitening body), or a scar on the ovary surface that is a remnant of ovulation.
What is the point of the polar bodies?
Polar bodies serve to eliminate one half of the diploid chromosome set produced by meiotic division in the ovum, leaving behind a haploid cell. They have no biological role in fertilisation or embryo development, but by being far smaller in size to the gametocyte that will become the ovum, it ensures that as much cytoplasm as possible will end up in the final ovum.
Describe the process of oogenesis.
Important: You are expected to be able to compare and contrast the processes of spermatogenesis and oogenesis.
Compare spermatogenesis and oogenesis.