Zoology · Ch 2 — Human Reproduction
Structure of the Human Ovum
Structure of the Human Ovum
The human ovum is described as alecithal (it has essentially no stored yolk) and non-cleidoic (it is never enclosed in a hard protective shell); it is microscopic in size, yet is nonetheless the largest single cell in the human body. Its cytoplasm, called the ooplasm, contains a large nucleus known as the germinal vesicle. The ovum is wrapped in three successive coverings. Nearest the ooplasm is a thin, transparent inner layer, the vitelline membrane. Separated from this by a narrow perivitelline space is a thick middle layer, the zona pellucida — the barrier a fertilising sperm must digest its way through using the acrosome's hyaluronidase. And outermost is a thick coat of leftover follicular cells, the corona radiata, that surrounded the oocyte inside its follicle and is still attached when the egg is released at ovulation. These layers do more than just protect the egg before fertilisation: after a sperm successfully fuses …
What this figure shows. A labelled cross-section through the human ovum showing, at the centre, the ooplasm (cytoplasm) containing a large germinal-vesicle nucleus; immediately around the ooplasm a thin inner vitelline membrane; a narrow perivitelline space separating this from a thick middle layer, the zona pellucida; and, as the outermost covering, a thick layer of retained follicular cells forming the corona radiata — the three coverings drawn as concentric rings ar …