Mesenchyme is a tissue found in organisms during development. It consists of many loosely packed, nonspecialized, mobile cells. Mesenchyme is derived primarily from the mesoderm , although there are also mesenchymal cells known as the neural crest cells, which derive from ectoderm. Mesenchyme gives rise to diverse structures of the developing organism, including connective tissue , bone, cartilage , teeth, blood and plasma cells, the endothelial lining of the vessels of the circulatory and lymphatic systems, and smooth muscle .
Mesenchymal cells are star-shaped in appearance, with an oval-shaped nucleus and comparatively little cytoplasm . They are widely spaced, with considerable extracellular space between cells. This space is filled with a dense intercellular matrix . An important characteristic of mesenchymal cells is that they are mobile, and move with a crawling, amoeboid motion.
Mesenchymal cells are undifferentiated and are therefore pluripotent — that is, they have the capacity to differentiate into any number of tissue types. A group of mesenchymal cells that will differentiate into another tissue type is called a blastema.
Mesenchymal cells are contrasted with the other major embryonic cell type: epithelial cells . Unlike mesenchymal cells, epithelial cells are not mobile. Epithelial cells form continuous sheets, with little extracellular space between cells. All epithelial cells have two definite ends, the basal end and the apical end. Epithelial cells are attached to a structure known as basement membrane by their basal end.
Many important developmental events take place as a result of interactions between mesenchymal and epithelial cells. Often, epithelial cells are induced by adjacent mesenchymal cells, that is, they change in form or shape in response to signals from the mesenchyme. Induction occurs either via mechanical processes, in which the migrating mesenchymal cells cause changes in the arrangement of epithelial cells, or by molecular agents released by mesenchymal cells.
Epithelial-mesenchymal transitions, in which cells change from epithelial to mesenchymal morphology, are also frequent during development. These transitions take place through the loosening of the cell adhesion molecules that keep epithelial cells organized in tight sheets. The reverse transition (mesenchymal to epithelial) occurs during developmental processes as well.
Although mesenchymal cells are technically found only in embryonic tissue, some cells do remain undifferentiated in adults. These serve as stem cells, which retain the ability to differentiate into diverse types of connective tissue as they are needed by the body for regeneration or repair.
Mesenchyme initially gives rise to three types of cells—fibroblasts, which generate collagen; myoblasts, which form muscle cells; and scleroblasts , which form connective tissue. Scleroblasts later differentiate into osteoblasts , which generate bone; chondroblasts, which generate cartilage; odontoblasts, which generate dentin in teeth; and ameloblasts , which generate tooth enamel.
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