Bone cells, which are found within the bone tissue, are responsible for the make-up of the skeleton of vertebrates. There are different types of bone tissue in a bone, made up of various types of bone cells: there is the cortical bone, which is the compact tissue of the hard outer part (cortex) of the bone; the cancellous bone, which is the spongy tissue inside of the compact cortex; and the subchondral tissue, which is the smooth tissue at the end of each bone. Bone cells all work together inside of the bones to help keep up the skeletal system. The bone cells do many things for the skeletal system, such as the development of new bones and continual bone remodeling (the maintenance of bones and the homeostatic regulation of minerals in the body). Types of bone cell include osteoclasts, which break down bone tissue; osteoblasts, which build new bone tissue; osteocytes, which hold the bone together; and lining cells, which protect the bone.