CoachConnective tissue is made of scattered cells in an abundant extracellular matrix that they secrete: protein fibres — collagen (tensile strength; a quarter of all the protein in a mammal), elastin (recoil) — embedded in a hydrated ground substance of polysaccharides. The proportions define its kinds: loose connective tissue under epithelia and around organs (fibroblasts, few fibres, much ground substance); dense connective tissue of tendons and ligaments (parallel collagen); adipose tissue (fat-storing cells); cartilage (a firm gel of collagen and polysaccharides, no vessels); bone (collagen mineralised with calcium phosphate, living cells in cavities, vessels); and blood, whose matrix is the plasma. It binds, supports, stores, transports and defends.
示例
Example 4.12 (Where the tissues come from)
Epithelia derive from all three germ layers (skin from ectoderm, gut lining from endoderm, the lining of vessels and of the coelom from mesoderm); connective tissue, muscle and blood from mesoderm; nervous tissue from ectoderm. The frog’s skin is an ectodermal epithelium on mesodermal connective tissue; its gut, an endodermal epithelium wrapped in mesodermal muscle; its brain, ectoderm folded inward.