Differentiation and transdifferentiation of mast cells; a unique member of the hematopoietic cell family.

Information about the differentiation of mast cells has increased remarkably in the past ten years. This progress has resulted from the introduction of techniques which developed in other fields of experimental hematology. Once mast cells were recognized as a progeny of multipotential hematopoietic stem cells, their unique differentiation processes were clarified. Although most of the progeny of stem cells leave the hematopoietic tissue after maturation, undifferentiated precursors of mast cells leave the hematopoietic tissue. Morphologically, unidentifiable precursors migrate in the bloodstream, invade the connective tissues or the mucosa of the alimentary canal, proliferate, and differentiate into mast cells. Even after their morphological differentiation, some mast cells retain an extensive proliferative potential. There are at least two subpopulations of mast cells: a connective-tissue type and a mucosal type. Connective tissue-type and mucosal mast cells can be distinguished by histochemical, electron microscopical, biochemical and immunological criteria; however, these two types can interchange, and their phenotypes are determined by the anatomical microenvironment in which their final differentiation occurs. Although biochemical natures of the anatomical microenvironment are unknown, molecules that support proliferation and differentiation of mast cells in vitro have been characterized, i.e., interleukin 3 and interleukin 4. In the next ten years, increased information about the differentiation processes will probably induce further understanding of mast cell functions.