The bones of the skeleton are structural organs that form the support of vertebrates. Their connecting joints allow movement, and their form allows them to often protect other organs. Bone comes in all forms, but it is not an uniform material. It has a compact outer layer called the compact or cortical bone, and a less dense internal matrix called the spongy, cancellous or trabecular bone. The compact layer is more heavy, and therefore the mass of the spongy bone only accounts for 20% of the total mass of the adult skeleton, although it is more voluminous and has about ten times the surface area of compact bone.
The spongy bone is the soft part of the bone, which contains blood vessels, active cells responsible for the bone metabolism, bone marrow and immune cells. The more porous structure and vascularisation of the mineralised part of spongy bone allows for more metabolic activity of living cells, as well as increased ion and nutrient exchange.
Blood cells are produced in the red bone marrow of the spongy bone, which is vascularized by small capillary vessels. The porous internal structures called trabeculae, are mostly made out of collagen and are dynamic structures providing internal support. They are constantly being remodeled by the active cells of the bone metabolism, osteoblasts and osteoclasts. If the strain on the trabecular bone is altered, the modular trabeculae will change forms to protect the bone structure against this strain. Weakening of the trabecular bone is responsible for most bone fractures as well as bone pathologies (like osteoporosis).