Second day of embryology today. Learning the female anatomy and reproductive system. We’re waiting until next year to go over the hormones, since that takes us into endocrinology, so today it was anatomical and physiological. Much different than the male counterpart, more steps, more name changes, more complexity. Thank goodness I’ve been through this before, or my mind would be boggled!
10 days after conception, in the embryo a pair of cells called oocytes migrate around to the posterior body-surface and are surrounded by follicular cells, becoming a primordial follicle. These remain until the 4th month gestation, when they begin doing divisions by meiosis I. The chromatids pair and pair again, swap info in their tetrad, and then divide in two, but they don’t complete this stage as males do. Instead they remain at this stage until puberty. The original 5-10 million oocytes go through a rigorous selection process, mechanism unknown. By birth the number has dropped to approximately 2 million, and by puberty the number is closer to 400,000. Only about 500 will complete the ovarian cycle in a woman’s lifetime.
Step : The follicular cells that surround the oocyte begin to divide and are now called granulosa cells. These form a layer surrounding the oocyte, with an outer ring called thecal cells and an inner ring called the zone pellucida.
Step : The granulosa cells begin to secrete fluid, called folliculi liquor, into the space between the two rings and the outer ring grows thicker. During this phase granulosa cells grow faster than the primary oocyte. Estrogen is produced by the thecal cells. The whole thing is called a secondary follicle.
Step : By the 10th-14th day, the fluid has increased to fill much of the space between the outer (theca) and inner (zone pellucida) rings. Some granulosa cells from the middle space remain around the oocyte, forming the corona radiata. This tertiary (or vesicular) follicle presses against the wall of the ovary. Luteinizing hormone (LH) levels begin to rise, which prompts the oocyte to complete meiosis I, but rather than simply dividing as usual, the cell kicks out one nucleus (polar body) and keeps the extra cytoplasm, creating one large secondary ooctye. This is another selection process, to ensure that the best possible “egg” completes the cycle. Meiosis II begins with chromatids dividing and lining up, but this phase isn’t completed now, and maybe never unless fertilization occurs.
Step : Around day 14 is ovulation. The secondary oocyte that has pressed up against the wall of the ovary breaks through into the abdominal cavity. By unknown magic it comes in contact with the fimbriae of the fallopian tube.
Step : The granulosa cells left behind in the ovary proliferate again, forming the corpus luteum. While estrogen is still secreted, this phase is marked by production of copious amounts of progesterone to prepare for pregnancy. This stimulates maturation of the uterine lining and secretions of the uterine glands.
Step : At around 28 days, if fertilization hasn’t occurred, the corpus luteum stops producing progesterone and decays, shrinking to a bulge of fibrous scar tissue called the corpus albicans. Lack of progesterone and estrogen now causes the lining of the uterus to shed and menstruation begins. From here the cycle begins again.
Fun times in embryology. Next time we’ll begin talking about what happens if fertilization does occur = pregnancy! :)