After the bone is formed, cartilage may remain at the ends to act as a cushion between bones. Cells in the body then lay calcium down onto the cartilage to form bone. Most bones start out as a softer, more flexible form of tissue called cartilage.
The tissue lining the medullary cavity is called endosteum. Some bones have a space inside called the medullary cavity, which contains the soft, spongy tissue called bone marrow (discussed below). The outside of the bone is covered with fibrous tissue called periosteum. The hard, outer layer of bones is made of compact (cortical) bone, which covers the lighter spongy (trabecular) bone inside. Bone is the supporting framework for your body.
