Cartilage entraps a great deal of water, which makes it an excellent shock absorber. In compact bone, osteocytes form osteons. The central channel for blood supply is an osteonic canal. Canaliculi connect lacunaes that house bone cells and the osteonic canal. Bone cells extend through canaliculi and pass materials from cell to cell.
Bone continually degenerates and builds up.
Smooth muscle cells are spindle-shaped and involuntary, and they line organs, push food through the digestive tract, and regulate blood flow and pressure. Cardiac muscle cells, in the heart, are striated and involuntary and are joined by intercalated disks into a branching pattern. Skeletal muscle cells are striated, voluntary, and move bones. Sliding protein filaments in muscles provide movement in diverse organisms. Tendons attach an intact whole skeletal muscle to bones.