Connective Tissue

Connective tissue is one of the four basic cell types in animal cells, along with muscle tissue, nervous tissue and epithelial tissue. It is found between all other cell types and often surrounds organs. Connective tissue, also called extracellular matrix, is composed of two types of large biomolecules: proteoglycans and fibrous proteins such as collagen, laminin, fibronectin and elastin. The extracellular matrix helps regulate a number of cellular functions including adhesion, migration, proliferation and differentiation. The extracellular matrix proteins are secreted by cells and the composition of the matrix determines the function of the connective tissue.

For example, osteocytes (a type of bone cell with a star-shape) secrete proteins that are combined with inorganic minerals that give bones their strength. In the muscular and nervous systems, there are specific layers of connective tissue that surround muscles and nerves. Epithelial cells often sit on top of a basement membrane composed of connective tissue that helps differentiate the apical and basal sides of the epithelium. The connective tissue surrounding cells can also provide a very specific microenvironment for that type of tissue that helps maintain its function, such as the stem cell niche.