Cutaneous membranes are typically dry and cover the outside of the body, functioning primarily in support, but also in secretion and some absorption. A connective tissue is a supporting tissue composed of living cells embedded in a nonliving matrix. The three categories are defensive, structural, and sequestering. A ligament is connective tissue consisting of tough layers of collagen laid down in parallel to form a strong connection between two bones. Cartilage and bone differ in that cartilage is avascular, and the collagen in bone is impregnated with calcium salts, making it harder and less resilient than cartilage.
Muscle is derived from mesoderm.
Actin and myosin are abundant in muscle. The three categories of muscle cells are smooth, skeletal, and cardiac, the last two of which are striated. Skeletal muscle cells develop as a consequence of several cells fusing end to end, hence the increased number of nuclei. Intercalated discs in cardiac muscle tissue represent areas of gap junctions between cells, which permits the myocardium to contract as a unit. Homeostasis is the maintenance of constant extracellular conditions within the body.