An ovum (plural ova, from the Latin word ovum meaning egg or egg cell) is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization. In lower plants and algae, the ovum is also often called oosphere.
In higher animals, ova are produced by female gonads (sexual glands) called ovaries and all of them are present at birth in mammals, and mature via oogenesis.
Human and mammal ova
In the viviparous animals (which include humans and all other placental mammals), the ovum is fertilized inside the female body, and the embryo then develops inside the uterus, receiving nutrition directly from the mother.
The human ova grow from primitive germ cells that are embedded in the substance of the ovaries. Each of them divides repeatedly to give rise to several smaller cells, the oogonia. The oogonia then develop into the ova, the primary oocytes.
The ovum is one of the largest cells in the human body, typically visible to the naked eye without the aid of a microscope or other magnification device. The human ovum measures 120 µm in diameter.
Protist and plant ova
In protists, fungi and many plants, such as bryophytes, ferns, and gymnosperms, ova are produced inside archegonia. Since the archegonium is a haploid structure, egg cells are produced via mitosis. The typical bryophyte archegonium consists of a long neck with a wider base containing the egg cell. Upon maturation, the neck opens to allow sperm cells to swim into the archegonium and fertilize the egg. The resulting zygote then gives rise to an embryo, which will grow out of the archegonium as a young sporophyte.
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