Shining a Light on the Oocyte -
From: Edwin Grant Conklin: Facts and factors of development. The Popular science monthly, Volume 84, p524. New York, Popular Science Pub. Co., June 1914.
A lot is happening to the oocyte in advance of fertilization. Ovulation follows the coordinated response of both the follicle cells and the oocytes in each menstrual cycle to signals from the brain in the form of proteins called hormones. Only a few follicles respond to the increase in the level of the main hormone, follicle stimulating hormone (FSH), which is produced by the pituitary (a small pea sized gland in the middle of brain, read more on the pituitary here). The pituitary was thought by ancient anatomists to be the seat of the soul. It’s a pretty important organ coordinating all sorts of key processes, but personally I think the soul resides in the stomach!
The stimulated follicle cells break contact with the oocyte and increase their production and release of big proteoglycans (these are complicated chemicals we make that act sort of like a glue) and tissue plasminogen activator (an important protein that breaks up blood clots but also helps cells move around). At the same time the follicle accumulates fluid, swells, and moves toward the periphery of the ovary where it is ready for final maturation and release.
Ovulation occurs in response to a surge in the level of luteinizing hormone (LH), which is also produced by the pituitary. Following this stimulation, the oocyte undergoes nuclear maturation and meiotic division. [We will discuss cell division later but for now one cell becomes two after all the chromosomes are duplicated, but in meiotic division we will see that the number of chromosomes is reduced to half the normal number in FOUR cells. Download genome.gov’s great illustration of meiosis here for a visual explanation. This is necessary because the sperm will bring a different half set of chromosomes to fertilize the egg, thus restoring the correct number but now with a whole new set of genes from Dad. Wow!]
Genes are shuffled in this process, the great genetic lottery! One set of chromosomes, surrounded by a small amount of cytoplasm, is extruded as a polar body (a teeny tiny cell that disintegrates). So the division results in one really big cell with half of the normal number of chromosomes and the polar body also with a half set of chromosomes. It is in this stage that the oocyte is finally released from the follicle.
Activation stimulates the ovum to complete its interrupted second meiotic division, which completes the process of reducing the number of chromosomes to “haploid” (one copy of each gene instead of the normal two). Mistakes can happen during this process and cause big problems like Down Syndrome.
Fertilization is the union of sperm and egg and the fusion of male and female pronuclei (these are little nuclei that have the half compliment of chromosomes, half from Mom and half from Dad) into one nucleus with a complete set of chromosomes followed by the initiation of a series of cellular transformations (see https://www.ncbi.nlm.nih.gov/books/NBK26843/). Prior to fertilization the sperm are released into the reproductive tract of the female and reach to the ampulla of the oviduct within five minutes; however, these sperm are not capable of fertilization for about an hour. As we discussed previously, they need to mature and prepare for the acrosomal reaction.
In the mouse an active spermatozoa in the ampulla of the oviduct retains its capacity to fertilize the egg for eight hours, and generally all eggs of a female are fertilized within six hours of mating. After fertilization a clear stiff envelope around the egg called the zona pellucida (this is what we have instead of an egg shell) expands and the ovum shrinks, resulting in what is called the perivitelline space between the ovum and the zona pellucida. The first polar body is visible in this space. Both male and female pronuclei may be seen as early as six hours after mating. Fusion of the posterior part of the sperm head with egg membrane triggers a cascade of reactions.
- Before an egg (oocyte) can be fertilized it must go through a series of transformations before it is released through ovulation.
- Follicle Stimulating Hormone triggers some of the follicle cells in the cloud around the oocyte to separate and release proteins that help bind cells together and proteins to help cells to move around.
- The follicles begin to accumulate fluid and swell up to prepare for ovulation.
- Luteinizing Hormone levels escalate and start ovulation, meanwhile the oocyte’s chromosomes are reduced by half through meiotic division where half the chromosomes form the female pronucleus, and the other half are expelled in a polar body.
- As the chromosomes assort themselves, the opportunity for mistakes can have drastic effects, like Down Syndrome.
- The haploid oocyte is fertilized when the activated sperm penetrates the cloud of follicular cells and its male pronucleus joins the oocyte’s female pronucleus to form a complete set of chromosomes.
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