Oocyte. Light micrograph of a section through an oocyte within a early antral follicle in an ovary. Oocytes are immature ova, or egg cells. It has a very large active nucleus (large, round, centre) compared with those seen in the many surrounding granulosa cells of the follicle. Its cytoplasm contains a growing supply of organelles, proteins and messenger RNA required for metabolic and synthetic functions should the oocyte be fertilised and become the activated zygote. The surface of the oocyte is surrounded by a layer of special proteins called the zona pellucida which is an important barrier recognising species-specific sperm and preventing multiple sperm from fertilisation. Magnification: x140 when printed 10 centimetres wide.