Connective Tissue

Connective tissue develops from the embryonic mesoderm and, true to its name, is the most widely distributed tissue in the body — it is what physically connects and supports every other tissue type. Its major jobs are binding and support, protection, insulation, and the transportation of substances around the body.

Every connective tissue, whatever its specific form, is built from the same three ingredients: fibres, a ground substance, and cells. Three kinds of fibre can appear in the matrix — collagen fibres (providing tensile strength), elastic fibres (providing stretch and recoil) and reticular fibres (forming delicate supporting networks) — and the proportion of each, together with the nature of the ground substance, is what gives each connective-tissue type its distinct texture and function.

Connective tissues are organised into three broad classes. Loose connective tissues have their cells and fibres arranged loosely within a semi-fluid ground substance. Areolar tissue, found beneath the skin, forms a support framework for epithelium and doubles as a reservoir of water and salts for surrounding tissues (hence it is sometimes called 'tissue fluid'); it contains fibroblasts, macrophages and mast cells. Adipose tissue, structurally similar to areolar tissue, is dominated (about 90% of its mass) by fat-storing adipocytes; it is richly vascularised, reflecting high metabolic activity, and during fasting it releases its stored fat as fuel. Adipose tissue comes in two forms — ordinary 'white fat', which stores nutrients, and mitochondria-rich 'brown fat', which instead burns fat to generate heat directly (non-shivering thermogenesis), a mechanism especially important for temperature regulation in neonates. Reticular connective tissue resembles areolar tissue but its matrix is filled with fibroblast-derived reticular cells that form the internal supporting framework (stroma) of the lymph nodes, spleen and bone marrow.

Dense connective tissues pack their fibres and fibroblasts far more tightly. Dense regular tissue has collagen fibres running in parallel rows (with a few elastic fibres) and a fibroblast-dominant cell population; it forms tendons (muscle-to-bone) and ligaments (bone-to-bone) and withstands strong pulling force applied in one direction. Dense irregular tissue has thick collagen bundles arranged irregularly, again with fibroblasts as the main cell type; it forms the leathery dermis of the skin and the fibrous capsules of organs such as the kidneys, bones, cartilages, muscles, nerves and joints, and can withstand tension from many directions at once. Elastic connective tissue is dominated by elastic fibres, letting tissue recoil after stretching; it is found in the walls of large arteries (maintaining pulsatile blood flow), in ligaments of the vertebral column, and in the walls of the bronchial tubes (aiding the passive recoil of the lungs after inspiration). …