Connective Tissue

Connective tissue is, by both volume and distribution, the most abundant tissue type in the vertebrate body, and it is defined by a feature exactly opposite to that of epithelium: rather than cells packed tightly together, connective tissue consists of relatively few, widely spaced cells embedded within an abundant extracellular matrix that those very cells secrete. The nature of that matrix — whether it is semi-fluid, densely fibrous, firm and calcified, or entirely liquid — is what distinguishes the different sub-types of connective tissue from one another, and each sub-type's mechanical properties follow directly from its matrix.

Loose connective tissue has cells and fibres arranged loosely within a semi-fluid matrix. Its most widespread representative is areolar tissue, found beneath the skin and around most internal organs and blood vessels, where it attaches the skin to the underlying muscles and fills the spaces between organs, acting as a packing and binding material. Areolar tissue contains fibroblasts, the cells that secrete the structural fibres (mainly collagen and elastin) embedded in the matrix, together with wandering macrophages (for defence) and mast cells (which release histamine). A specialised form of loose connective tissue, adipose tissue, has cells (adipocytes) so swollen with stored fat globules that the nucleus is pushed to one edge of the cell; adipose tissue serves as an energy reserve and also cushions and insulates the body.

Dense connective tissue has a matrix packed with closely spaced collagen fibres and relatively few cells. When those fibres run in one parallel direction, the tissue is called dense regular connective tissue, and its high tensile strength along that one axis makes it ideal for tendons (which join muscle to bone) and ligaments (which join bone to bone). When the fibres instead run in many different directions, giving strength against stress from multiple directions at once, the tissue is dense irregular connective tissue, found for example in the dermis layer of the skin.

Skeletal connective tissue provides the body's structural framework and exists in two forms. Cartilage has a firm yet pliable matrix rich in chondroitin sulphate, with its cells, chondrocytes, sitting in small spaces called lacunae; because cartilage can bear weight while still flexing slightly, it forms the smooth articular surfaces at the ends of long bones inside joints, as well as the tip of the nose, the outer ear (pinna) and the rings supporting the trachea. Bone has a much harder matrix, made rigid by deposits of calcium salts around a collagen framework, with its cells, osteocytes, likewise housed in lacunae; bone forms the rigid internal skeleton that supports the body, protects vital organs and serves as the site of attachment for skeletal muscles. …