ova

Ova, the plural of ovum, are the female gametes of animals. They are typically large, nutrient-rich cells produced by oogenesis in the ovaries. When an ovum is fertilized by a sperm, a zygote forms and begins embryonic development.

In mammals, oogenesis begins before birth with oogonia that develop into primary oocytes arrested in prophase I. After puberty, hormonal cycles promote recruitment of a few follicles each cycle; ovulation releases a secondary oocyte that completes meiosis I and arrests at metaphase II, ready to be fertilized.

An ovum is surrounded by protective layers, including the zona pellucida and, in many species, the cumulus oophorus; the surrounding follicle provides the initial cytoplasmic and nutritional resources. The ovum contributes most of the cytoplasm and organelles to the zygote.

Across animals, ova vary in size, yolk content, and mode of development. Some species lay eggs that develop outside the body (oviparity), while others retain eggs internally or give birth (viviparity). In humans and many mammals, the egg is relatively small but rich in cytoplasm, whereas eggs of birds and some reptiles are often large with substantial yolk.

Clinical relevance: In humans, the number and quality of ova decline with age, increasing risks of chromosomal abnormalities. Ova are retrieved for assisted reproduction techniques such as in vitro fertilization and can be preserved by cryopreservation for fertility preservation.