During embryogenesis, a single fertilized oocyte gives rise to a multicellular organism whose cells and tissues have adopted differentiated characteristics or fates to perform the specified functions of each organ of the body. As embryos develop, cells that have acquired their particular fate proliferate, enabling tissues and organs to grow. Even after an animal is fully grown, however, many... » More

During embryogenesis, a single fertilized oocyte gives rise to a multicellular organism whose cells and tissues have adopted differentiated characteristics or fates to perform the specified functions of each organ of the body. As embryos develop, cells that have acquired their particular fate proliferate, enabling tissues and organs to grow. Even after an animal is fully grown, however, many tissues and organs maintain a process known as homeostasis, where as cells die, either by natural death or by injury, they are replenished. This remarkable feature has ancient origins, dating back to the most primitive animals, such as sponges and hydrozoans. The fabulous ability of an embryo to diversify and of certain adult tissues to regenerate throughout life is a direct result of stem cells, nature's gift to multicellular organisms.

For further information see: http://www.ascb.org/ibioseminars/Fuchs/Fuchs1.cfm « Hide