Type of animal embryonic connective tissue Mesenchyme ([1]) is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone.[2][3] The interactions between mesenchyme and epithelium help to form nearly every organ in the developing embryo.[4] Mesenchyme is characterized morphologically by a prominent ground substance matrix containing a loose aggregate of reticular fibers and unspecialized mesenchymal stem cells.[5] Mesenchymal cells can migrate easily (in contrast to epithelial cells, which lack mobility), are organized into closely adherent sheets, and are polarized in an apical-basal orientation. The mesenchyme originates from the mesoderm.[6] From the mesoderm, the mesenchyme appears as an embryologically primitive "soup". This "soup" exists as a combination of the mesenchymal cells plus serous fluid plus the many different tissue proteins. Serous fluid is typically stocked with the many serous elements, such as sodium and chloride