In the human embryo, the mesoderm—one of the three germinal layers, is the source of all connective tissues. Connective tissues perform diverse functions, from connecting and supporting other tissues to transporting solutes, including nutrients and gases. All connective tissues have two components— the non-living extracellular matrix, composed of a gel-like ground substance and protein fibers, and the cells. In contrast to epithelial tissues, the extracellular matrix forms the bulk of connective tissues. Additionally, while epithelial tissues are avascular, most connective tissues, except the cartilage, are highly vascular. Connective tissues, such as in the dermis, also contain sensory receptors to detect stimuli such as pain, pressure, and temperature. Depending on the properties of the matrix and the cell types present, connective tissues can have different physical characteristics. For instance, bone is a hard connective tissue, while blood is fluid.