Brain BREAK! presents: Oocytes.
Spermatogenesis is fairly straightforward, but the development of oocytes is trickier. Primary oocytes are what a female is born with. Prior to ovulation, Meiosis I is triggered in one of these cells. This is the egg in one menstrual cycle.
Primary oocyte: 46 chromosomes.
It’s prepared for Meiosis I but waiting for a signal.
Then, puberty strikes and the menstrual cycle begins. A series of hormones are released during this process, and many more.
- The hypothalamus releases Gonadotropin-Releasing Hormone.
- From this message, the pituary gland releases Gonadotropin (gonad meaning sex organ, and tropin referring to the stimulation of something.)
- The ovaries therefore release Luteinizing Hormone and Follicle-Stimulating Hormone.
*After ovulation, the corpus luteum can take over with its own Estrogen and Progesterone, of which there are several subtypes of their own.
After this cascade of hormones, a primary oocyte is “selected” and receives the signal to charge into Meiosis I. Instead of two identical cells, however, a polar body and secondary oocyte are formed. They are still haploids, but different.
Under a microscope, polar bodies are actually very tiny. A polar body announces, “We’re leftovers! Humans like to meiose, and why stop ‘em?” For our purposes, the polar bodies are junk. This haploid polar body may go on to split into two polar bodies, but they aren’t important for our purposes. So back to our haploid that’s going to do the real work.
Secondary oocyte: 23 chromosomes.
This is the oocyte that is ovulated! From there, it has some options.
- If it is not fertilized, it doesn’t divide any further and is expelled during menstruation.
- If it is fertilized by a sperm cell, it undergoes its Meiosis II. Like Meiosis I, the oocyte divides into one “junk” polar body, while the haploid is carried on in full to the next stage, which is the…
Ovum: Becoming 46 chromosomes with the addition of the sperm’s haploid DNA.
The structure as a whole is referred to as a zygote, and from there it will divide rapidly, going into its 2-cell, 4-cell, blastocyte, etc., stages.
Remember that oocytes are the portion containing genetic material (nucleus) and follicle refers to the fluid-filled sac. A primary oocyte will be classified as such while its follicle goes through its own stages in the ovary.
So much work! -CNx