Spongy bone. Light micrograph of a section through spongy (cancellous) bone. Spongy bone is made of a network of bone matrix (yellow) and hollow spaces (white) containing marrow and blood vessels. The bone matrix contains cells called osteocytes (black). These have delicate, branching processes that exchange calcium and phosphorous ions between the bone and its blood supply. The edges of the hollow spaces are lined with osteoblasts (cells that form new bone) and osteoclasts (cells that break down bone) in a layer called the endosteum. Cancellous bone is found at the ends of long bones. Magnification: x185 when printed 10cm wide.