Osteoporosis. Computer artwork showing the anatomy of normal (left) and osteoporotic (right) bone. A bone shaft consists of an outer layer of compact bone made up of cylindrical units called osteons. This layer surrounds the honey-combed interior of spongy (cancellous) bone. In osteoporosis, it is the loss of the structural strength of spongy bone that causes brittle bones to snap. Across top, the sequence shows an osteoclast (green, a type of bone cell) destroying one of the linking supports of spongy bone tissue. Excess bone resorption can lead to osteoporosis. The osteoclast secretes acid granules (red, upper right) to dissolve the bone.