Day 5: What Came First: the Chicken or the Egg?

Anatomy of the Ovarian Follicle

The egg cell within the ovary

The yolk of an egg is actually a single cell called an oocyte. The nucleus of the cell will eventually undergo meiosis, and if the mature oocyte is fertilized by a sperm, the fused nuclei of the oocyte and sperm will become an embryo. The cytoplasm of the cell contains organelles, granules, and vacuoles of stored nutrients. When a hen reaches the age at which she will start laying eggs, the oocyte begins to store yolk in its cytoplasm. The yolk contains all the nutrients than a growing embryo would need. Click here to find out more about what is in yolk. The oocyte accumulates so much yolk that the nucleus of the cell and most of the cytoplasmic contents ( particularly those needed by the nucleus to divide) are pushed to one end of the cell. This end of the oocyte is referred to as the animal pole. The opposite end of the oocyte, where the yolk is stored, is referred to as the vegetal pole.

The entire oocyte is enclosed in a cell membrane. While the oocyte is still in the ovarian follicle, the surface of its cell membrane is covered with closely packed microvilli. Under a microscope, the microvilli look like delicate lines radiating from the cell; thus the cell membrane of the oocyte was given the name zona radiata (zone of radiating lines). By the time the oocyte is ready to be released from the ovary, the cell membrane will have lost most of its microvilli. Just outside the zona radiata lies the vitelline membrane. The vitelline membrane is a thin layer of protein fibers which separates the oocyte from its surrounding follicular cells.

The follicular cells

Surrounding the oocyte are the specialized cells of the ovarian follicle responsible for nuturing the growing egg. The outer layers of the follicle are called the thecal layers. The inner thecal layer has a network of capillaries which supplies the developing oocyte with nutrients. Immediately surrounding the egg cell is a layer of follicular cells. These cells pass nutrients from the capillaries to the growing egg. The vitelline membrane lies between the follicular cells and the cell membrane of the oocyte.

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