Connective tissue develops from the embryonic mesoderm and is the most widely distributed tissue in the body, providing binding and support, protection, insulation and transportation of substances. Every connective tissue is built from the same three components — fibres (collagen, elastic and reticular), a ground substance, and cells — whose relative proportions distinguish one connective-tissue type from another.

Connective tissues fall into three classes. Loose connective tissues have cells and fibres loosely arranged in a semi-fluid ground substance: areolar tissue (a support framework beneath the skin, also a fluid/salt reservoir; contains fibroblasts, macrophages, mast cells), adipose tissue (fat-storing adipocytes make up ~90% of its mass; comes as energy-storing 'white fat' and heat-generating, mitochondria-rich 'brown fat', the latter driving non-shivering thermogenesis in neonates), and reticular tissue (a reticular-cell framework/stroma supporting the lymph nodes, spleen and bone marrow). Dense connective tissues pack fibres and fibroblasts tightly: dense regular (parallel collagen bundles; tendons and ligaments, resists one-directional pulling force), dense irregular (irregular thick collagen bundles; skin dermis and organ capsules, resists multi-directional tension) and elastic (rich in elastic fibres; art …