Eizell

An Eizelle, or oocyte, is the female gamete in animals and plants. In humans, oocytes form in the ovaries as part of the female reproductive system. During development, primordial germ cells give rise to oogonia, which proliferate before birth and enter meiosis to form primary oocytes that arrest in prophase I until puberty. At each menstrual cycle, several follicles begin to mature; usually one follicle completes the first meiotic division just before ovulation, producing a secondary oocyte and a first polar body. The secondary oocyte is released during ovulation and travels through the reproductive tract, where it is arrested at metaphase II until fertilization. If a sperm fertilizes the oocyte, meiosis II completes, yielding a mature ovum and a second polar body. The fusion of sperm and oocyte genomes forms a zygote.

An oocyte is a relatively large cell, about 100–120 micrometers in humans, with a nucleus surrounded by a cytoplasm rich in nutrients, maternal RNAs, and proteins. It is surrounded by the zona pellucida and, in the ovarian follicle, by cumulus cells. The oocyte provides most of the cytoplasmic material necessary for early embryo development before embryonic genome activation.

Development and reproduction: Oogenesis begins with oogonia and produces primary oocytes that pause before birth and resume during puberty. Folliculogenesis and ovulation lead to the release of a secondary oocyte, which remains arrested at metaphase II until fertilization. Fertilization triggers completion of meiosis II and the formation of a zygote.

Clinical and research relevance: Oocytes are central to human reproduction. In assisted reproductive technologies, oocytes can be collected for IVF, donor eggs may be used, and oocyte cryopreservation allows future use. Research investigates meiotic errors, maternal age effects, and strategies to preserve or enhance oocyte quality.

Variation across species: Eggs vary considerably in size, yolk content, and composition. In birds and many fish, eggs are large and yolk-rich; in most mammals, eggs are smaller with limited yolk, and patterns of maturation differ by species.