A polar body, sometimes referred to as a polar cell, is a cell found within the ovum of mammals as well as plants. It is the byproduct of the natural process of cell division during oogenesis and meiosis. Oogenesis is the process by which ova are created, and meiosis is the division of the cells at the time of ovulation, when the ovum is released into the fallopian tubes, and an additional division just after fertilization. In mammals, it is not a functional reproductive cell, and it disintegrates after a period of time, but in plants, the polar body performs a separate function in the development of the resulting offspring.
During meiosis, the number of chromosomes in germ cells are split in half to produce reproductive cells, or gametes. When the female reproductive cell, or ovum, joins with the male reproductive cell, or sperm, the correct full number of chromosomes is regained. The polar body is the other half of the developing female cell, also containing half the required number of chromosomes. However, it does not contain sufficient cytoplasm to function as a full-fledged reproductive cell, because during meiosis, the cytoplasm is distributed into the ovum during a process called cytokinesis. In mitosis, the process that leads to two separate body cells with a full complement of chromosomes, cytokinesis divides the cytoplasm evenly.
In plants, the fertilization process involves both the ovum and the polar bodies. When the ovum is fertilized by the plant's male gamete, the polar bodies also are fertilized by a second male cell. They then develop not into a plant but into endosperm, a cell structure that functions to produce nutrients for the growing plant cell. Endosperm not only helps nourish the growing plant, it also makes seeds and grains highly nutritious for consumption by other animals, including humans.
It has been theorized that something similar might occur within mammals, if two sperm fertilized both the egg and a polar body. This theoretical occurrence is called polar body twinning. Some scientists believe viable twins could be produced through this process, but others believe that the polar body, with its lack of sufficient cytoplasm, would not be able to develop properly. If twinning were possible, the resulting offspring would not be identical twins, because they would share the genes of the mother but would theoretically carry genetic material from two different sperm.