An egg cell (ovum) is one of the largest cells in the human body.
Egg cells are produced in the ovaries, and a woman is born with 500,000 potential eggs (follicles) in each ovary.
The zona pellucida (egg wall) is an outer membrane of the egg. This structure helps the sperm enter the egg through its hard outer layers.
The corona radiata surrounds an egg and consists of two or three layers of cells from the follicle. They are attached to the zona pellucida – the outer protective layer of the egg – and their main purpose is to supply vital proteins to the cell.
The cytoplasm is a gel-like substance that holds all the cell’s other internal structures, called organelles. It is in the cytoplasm that all the cell’s activities take place to keep it alive and functioning properly. Amongst the more important organelles are structures called mitochondria, which supply most of the energy for the cell.
The nucleus is the heart of the egg cell; it contains most of the genetic material in the form of chromosomes. This is where the genes are situated. An egg, like a sperm, contains half the number of chromosomes as a normal cell, i.e. 23 each.