Bones are rigid connective tissues whose functions include protecting and supporting parts of the body. Bone tissue is classified into compact or cortical bone, which is hard and dense, and trabecular or spongy bone, which is porous. Compact bone forms the outer layer and consists of a matrix arranged in concentric rings called lamellae. Spaces between lamellae, called lacunae, contain osteocytes. Spongy bone, located in the interior of the bone, contains trabeculae, thin branching structures made up of lamellae. Spaces between trabeculae contain red or yellow bone marrow. The bone matrix is rich in collagen fibrils that offer flexibility and mineral salts that add stiffness. Bone matrix is secreted by cells called osteoblasts, which mature into osteocytes. Osteoclasts are large, multinucleate cells involved in bone resorption. Osteocytes regulate the cycle of bone loss and formation, known as bone remodeling. During this process, osteoclasts remove the mineral deposits of old bone tissue, and osteoblasts deposit the circulating minerals to create new bone.