Connective tissue
The connective tissues originate from the mesenchyme, an embryonic tissue formed by elongated cells, the mesenchymal cells. The mesenchyme develops from the middle layer of the embryo, the mesoderm.
Connective tissue is formed by cells and extracellular matrix. Extracellular matrix consist of protein fibers (collagen, elastic, and reticular) and a fluid known as the ground substance.
Connective tissues are found throughout the body but are never exposed to the environment outside the body. Many connective tissues are highly vascular and contain sensory receptors.