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the embryonic connective tissue in the majority of multicellular animals and man.
In the early stages of embryonic development, mesenchyme consists of motile dendriform cells. Most of these cells later lose their motility and unite by their processes into a network (syncytium) or form local aggregates. The mesenchyme originates from cells that have migrated from various germ layers. For example, the chief source of the mesenchyme in coelenterates, worms, and mollusks is the ectoderm (often also the endoderm), and in chordates and echinoderms, the mesoderm. Mesenchyme that originates from endoderm and mesoderm is called endomesenchyme, and mesenchyme originating from ectoderm (the material of the neural columns), ectomesenchyme. In vertebrates and man, endomesenchyme gives rise to various forms of connective tissue in the adult, formed elements of the blood, blood vessels, and smooth muscles. Almost the entire visceral skeleton (the auditory ossicles, gill arches), the pigment cells, and part of the derma are formed from ectomesenchyme.
The poorly differentiated connective-tissue cells (fibroblasts and reticular cells) found in animals and man during the postembryonic period are sometimes called mesenchyme.
REFERENCETokin, B. P. Obshchaia embriologiia, 2nd ed. Leningrad, 1970.
T. A. DETLAF