Revisiting Terminology and Characteristics of Stem Cells.

The majority of populations of differentiated cells are subject to renewal. Progenitor cells (cells-predecessors) and differentiated cells, having started the differentiation pathway or having finished it, can divide a limited number of times and are not capable of ensuring tissue regeneration during the whole period of ontogenesis. Renewal of tissues over such a long period is impossible without the participation of a specialized system which is responsible for regeneration. The given system is represented by pluripotent stem cells. Pluripotent stem cells, which conduct regeneration of all body tissues during ontogeny, are formed during implementation of the development program of the fertilized ovum, along with the formation of tissues and organs of the new organism. Being a separate direction of differentiation of embryonic cells, pluripotent stem cells are not embryonic cells. The immune system takes part in the formation of tissue-specific receptors among pluripotent stem cells. The latter remain pluripotent until the formation of tissue-specific receptors in them, and become committed (tissue-specific stem cells) after the formation of such receptors. Committed stem cells (tissue-specific stem cells) that migrate through the extracellular matrix replenish the progenitor cells of all tissues. Mesenchymal stem cells are the precursors of fibroblasts, and they only create conditions in the extracellular matrix for the migration and differentiation of committed (tissue-specific) stem cells in the places of cell death.