Egg cell

Lore & Background

In mammals, ova are produced by ovaries; a number of ova are present at birth and mature via oogenesis.

Reader's Guide

The egg cell is central to sexual reproduction across animals, plants, and other organisms. In animals, it is the non-motile female gamete that fuses with sperm to form a zygote. The historical doctrine 'ex ovo omne vivum' shifted understanding away from spontaneous generation. In plants, egg cells form within archegonia or embryo sacs, and fertilization leads to seed development. The egg cell's large size, visible to the naked eye in humans, and its role in carrying mitochondria and cytoplasm for early development underscore its biological significance. Ongoing research continues to explore whether mature mammals can generate new egg cells.

Did You Know?

• In humans, maternal DNA recombines approximately 40–45 times on average during meiosis, while paternal DNA recombines approximately 25–30 times.
• Most mammalian ova contain only a tiny amount of nutritive yolk, sufficient only for early embryonic development, unlike bird eggs which nourish the chick through incubation. However, monotremes (egg-laying mammals) lay
• In the moss Physcomitrella patens, the Polycomb protein FIE is expressed in the unfertilised egg cell but is inactivated soon after fertilisation.

Definition and Biological Role

The egg cell, also called the ovum, serves as the female reproductive gamete in the vast majority of anisogamous organisms—those that reproduce sexually through a larger female cell and a smaller male one. The specific term "egg cell" applies when the female gamete lacks the ability to move on its own. When the male counterpart, the sperm, is motile, the overall mode of sexual reproduction is classified as oogamous. In certain algae, fungi, oomycetes, and bryophytes, a nonmotile female gamete produced within a specialized structure called the oogonium takes the name oosphere, and once it is fertilized it becomes known as an oospore. The central biological event involving the egg cell is fertilization: when it fuses with a sperm, the resulting diploid cell, the zygote, begins rapid growth and ultimately gives rise to an entirely new organism. This single union of two gametes represents the foundational step in the life cycle of sexually reproducing species across the tree of life.

A Century of Discovery

For centuries, the existence of eggs in non-mammalian animals was plainly visible, yet the idea that mammals also relied on eggs for reproduction was a radical proposition. The doctrine "ex ovo omne vivum," linked to William Harvey in the early seventeenth century, boldly asserted that every living animal originated from an egg. This stance simultaneously rejected two prevailing theories of the era: spontaneous generation, which held that life could arise from non-living matter, and preformationism, which proposed that a tiny pre-made organism resided within the reproductive cell. These three milestones collectively transformed the egg cell from a philosophical concept into a documented biological reality.

Mammalian Oogenesis and the Finite Oocyte Question

In all vertebrates, egg cells are generated within the ovaries, the female gonads. In mammals, a finite pool of ova is already present at birth and subsequently matures through the process of oogenesis. Research conducted in the 1870s on humans, dogs, and cats indicated that the production of immature egg cells, or oocytes, ceases at or very shortly after birth. Whether mature mammals can genuinely generate new egg cells remains an open and actively debated question in reproductive biology.

Egg Cells Across the Tree of Life

The egg cell manifests in strikingly different forms depending on the organism. In oviparous species such as birds, most fish, amphibians, and reptiles, the ovum develops protective outer layers and travels through the oviduct to the exterior. Fertilization may occur internally, as in birds and reptiles, or externally, as in many fish and amphibians, after which the embryo grows using nutrients stored within the egg before hatching outside the mother. Ovoviviparous animals occupy an intermediate niche: the embryo is nourished by the egg just as in oviparous species, but it hatches inside the mother's body shortly before or just after the egg is expelled. In the plant kingdom, nearly all land plants alternate between diploid and haploid generations, with the female gametophyte producing egg cells within archegonia. In flowering plants, the gametophyte has been reduced to eight cells inside the ovule, and a pollen tube delivers the sperm nucleus to the egg. In algae, the egg cell is commonly termed an oosphere, and in Drosophila, oocytes develop within individual egg chambers supported by nutrient-rich nurse cells.