Microlecithal and Isolecithal Ovum of Human
The ovum or the female gamete is much larger than the sperm in size. It is non - motile and laden with different types of energy rich materials like yolk, glycogen and proteins accumulated in its cytoplasm. It is enclosed by one on more egg envelops. Size of ovum varies in different animals and depends upon the amount of yolk.
On the basis of amount of yolk, the ovum can be microlecithal (small sized egg with very small amount of yolk) mesolecithal (egg containing moderate amount of yolk) and macrolecithal (containing very large amounts of yolk).
Human ovum is microlecithal. The cytoplasm is differentiated into outer transparent exoplasm or egg cortex and inner opaque endoplasm or ooplasm. Nucleus is excentric in position, so human ovum has polarity. The side of the ovum with nucleus and polar body is called animal pole and the opposite side is called vegetal pole. The line passing through the 2 poles is called primary axis or animal vegetal axis.
The human ovum is surrounded by a number of egg envelopes.
•vitelline membrane - A thin, inner transparent layer secreted by the ovum itself.
•Zona pellucida - A thick middle layer which is transparent and non - cellular.
•Corona radiata - An outer, thicker coat formed of radially elongated follicular cells.
Between the vitelline membrane and zona pellucida, there is a narrow space called perivitelline space.