|There are five main parts of a bone. They are marrow, calcified bone, periosteum, spongy bone, and cartilage. Bone marrow is a soft, crumbly, and gooshy part of the bone, which is in the middle or center. There are two types of marrow, red and yellow. The red marrow produces red blood cells. When you get older, your marrow turns more yellow due to fat cell accumulation. The yellow marrow acts like an emergency reserve in case you get sick when it will turn into blood cell producing marrow.
Calcified bones are hard but not really solid. Spongy bone is hard with tiny holes in it. Periosteum is thin but very strong. The other name for it is outer membrane, it helps to rebuild bone cells. The last is cartilage. It is strong material that holds two bones together and allows them to glide over one another.