Bone: macroscopic appearance

Grossly, the tissue bone has two different appearances: compact and spongy. Compact bone, appearing as a solid mass to the naked eye, comprises the shaft of long bones and forms a peripheral covering around all bones. Spongy bone looks like a sponge to the naked eye, with bony struts alternating with marrow or loose connective tissue. 1x, 10x

Spongy bone looks like a sponge with bony struts, called spicules or trabeculae, alternating with non-bone tissue, such as loose connective tissue or red marrow. If a bone image shows as much non-bone tissue as it does bony tissue, it is considered spongy bone. Spongy bone is not as strong as compact bone and is found in the interior of bones.

Red marrow (green arrows) fills the spaces between spicules of spongy bone in the epiphyses of adults. Red marrow is the production site of platelets and blood cells. Yellow marrow (blue arrows), rich in adipose cells, fills the shafts of adult long bones, although none is demonstrated in these images. Yellow marrow can be replaced by red marrow under certain conditions.