Mesenchyme, or mesenchymal connective tissue, is a type of undifferentiated loose connective tissue that is derived mostly from mesoderm, although some is derived from other germ layers; e.g. neural crest cells and thus originates from the ectoderm. Most embryologists use the term mesenchyme only for those cells that develop from the mesoderm.
The term mesenchyme essentially refers to the morphology of embryonic cells, however, they do persist as stem cells into adulthood. Mesenchymal cells are able to develop into the tissues of the lymphatic and circulatory systems, as well as connective tissues throughout the body, such as bone and cartilage. A sarcoma is a cancer of mesenchymal cells.
Mesenchyme is characterized morphologically by a prominent ground substance matrix containing a loose aggregate of reticular fibrils and unspecialized cells. Mesenchymal cells can migrate easily, in contrast to epithelial cells, which lack mobility and are organized into closely adherent sheets, are polygonal in shape, and are polarized in an apical-basal orientation.