Authors: Henry E. Young, Mark O. Speight and Asa C. Black Jr.
Development of a multicellular organism is accomplished through a series of events that are preprogrammed in the genome. These events encompass cellular proliferation, lineage commitment, lineage progression, lineage expression, cellular inhibition, and regulated apoptosis. The sequential progression of cells through these events results in the formation of the differentiated cells, tissues, and organs that constitute an individual [1]. Although most cells progress through this sequence during development, a few cells leave the developmental continuum to become reserve precursor cells. The reserve precursor cells are involved in the continual maintenance and healing of the tissues and organs throughout the life span of the individual. Until recently it was generally assumed that the precursor cells in postnatal individuals were limited to lineage-committed unipotent progenitor cells specific for various cells, i.e., neuroblasts for neurons, myoblasts for muscle, hepatoblasts for hepatocytes. However, recent studies [1-3] demonstrated the presence of two categories of precursor cells that reside within the organs and tissues of postnatal animals. These two categories of precursor cells are maintenance cells and healing cells. These reserve precursor cells provide for the continual maintenance and repair of the organism after birth.
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