Physiological Changes: Stem Cells

The process of aging is controlled by a variety of cellular and molecular mechanisms. This process is a continuum of events that are initiated during embryonic development, are genetically determined, and continue throughout all of life. There is a genetic program that controls how and where arms and legs form in the embryo, how much these appendages grow after birth and how much bone will be lost as a person becomes older. Interestingly, old in this context means "after skeletal maturity," which occurs in a person's early thirties. Assuming this genetically controlled continuum between embryology and the aging process, it is important to understand the process of tissue formation, the process of tissue expansion, and how tissue is maintained throughout life. Considered here is a class of tissues that form from the middle layer of the three-layered embryo, the mesodermal layer; this class of tissue is summarily referred to as mesenchyme, and includes bone, cartilage, muscle, tendon, ligament, fat, and other connective tissues.