Bone tissue is made up mostly of calcium and collagen, which is a protein. The outer layer of bone is hard and dense and called compact or cortical bone. The inner layer is called trabecular or cancellous bone. It is lighter than the compact bone and porous.
Compact bone gives the bone its toughness and makes up the great majority of adult bone tissue. Its collagen helps the bone remain somewhat elastic and resist easy fracturing.
Bone is also made up of osteoblasts, which create and mineralize bone, and osteoclasts, which absorb and digest damaged or old bone cells. Osteoclasts have a different origin than osteoblasts as they come from the same stem cells that create white blood cells. Osteocytes are largely osteoblasts that are no longer active but reside in the bone matrix. Bone also contains osteoid, which is a mix of collagen and other protein and mineral salts made of calcium and phosphate.
Tiny columnar cells called osteons make up compact bone. Osteons in turn are made up of osteoblasts and osteocytes. These are arranged around Haverisan canals.
Bone marrow can be found in the trabecular bone, which is separated from the compact bone by a layer called the endosteum. It is here that the solid components of the blood are created.