Spongy bone, also called cancellous or trabecular bone, is one of the two types of calcium tissue that make up bones in the human body. Spongy bone is lighter, softer, and weaker than compact or cortical bone, the other type of calcium tissue, but it has a greater surface area and is much more vascular, or supplied with blood vessels. Spongy bone is found on the inside of some bones, and it is surrounded by the stronger, more protective compact bone. Cancellous bone tissue is found at the end of long bones, at joints, and in the vertebrae, the bones of the spinal column. Cancellous bone makes up a larger portion of the bone than the external compact bone tissue.
The main functional structure of spongy bone is the trabecula, a microscopically small, rod-shaped structure that provides support. Trabeculae are found in many different parts of the body, but they are most often made of collagen. Spongy bone is the only tissue to feature trabeculae made of bone. The large surface area and high vascularity of cancellous bone make it ideal for metabolic activity, such as ion and nutrient exchange.
Spongy bone often contains red bone marrow, the site of production of blood cells. The bone marrow fills up the open spaces between the trabeculae. While the spongy bone tissue itself does not contain blood vessels, the surrounding marrow is full of capillaries, and helps transfer nutrients and other metabolic products from the blood to the bone tissue.
At birth, all of the bone marrow in the human body is red. As a person ages, the bone marrow at the center of long bones is slowly converted to yellow bone marrow, made mostly of fat cells, which does not synthesize blood cells. The bone marrow of a typical adult is half red and half yellow. However, yellow bone marrow can be converted back to red bone marrow if necessary, in the case of severe blood loss.