Strictly speaking, there are no “cancellous bones.” Cancellous bone is a type of tissue, whereas nearly bones (the organs) are made of both cancellous (spongy) and compact bone. Cancellous bone makes up 20% of the human skeleton, the remainder is cancellous bone.It is highly vascularized.
Spongy bone is usually located at the ends of the long bones (the epiphysis), with the harder compact bone surrounding it. it is also found inside the vertebrae, in the ribs, in the skull and in the bones of joints.
Spongy bone is softer and weaker than compact bone, but is also more flexible. It is characterized by a lattice-like matrix network called trabeculae (Latin for little beam) that gives it its spongy or honeycombed appearance. The trabeculae may appear to be a random network, but each trabeculae forms along lines of stress to provide strength to the bone.
Cancellous bone is also very porous, and provides spaces for the red bone marrow. Further, cancellous bone has the most surface area exposed to the action of osteoblasts and osteoclasts, so it is the principle site of calcium and phosphate exchange with the tissue fluid.
STRUCTURE OF SPONGY BONE
- Spongy bone is composed of cells called osteocytes that sit in small cavities known as lacunae.
- The lacunae and their accompanying osteocytes are housed in the trabeculae matrix of the bone along with the bone marrow.
- Blood vessels travel through the harder compact bone to the spongy bone, supplying it with the materials necessary to create blood cells.
- Osteocytes positioned close to a blood vessels can take on nutrients and expel waste products through tiny interconnecting channels on the surface of the trabeculae called canaliculi.
- Spongy bone can be converted to compact bone by the action of osteoblasts, bone cells that secrete the material that creates the compact bone matrix.
- It is through this process that the long bones in a human embryo develop.
FUNCTIONS OF SPONGY BONE
- Storage of Bone Marrow- Spongy bone contains red bone marrow that is used in erythropoiesis.
- Site of Erythropoiesis- Inside spongy bone, red blood cells are produced in the red bone marrow at a rate of about 2 million per second. This process also occurs in the liver and spleen.
- Reduces the Weight of the Skeleton- The light weight and low density of spongy bone balances out the heavier and denser compact bone to reduce the overall weight of the skeleton. This makes it easier for muscles to move the limbs.
- Adds Strength and Flexibility to Bones- The trabeculae of spongy bone tend to form along lines of stress, giving the bone strength and flexibility in that area. Spongy bone is also present in the joints of the body and acts as a shock absorber when we walk, run and jump.
- Mineral Storage- The human skeleton stores 99% of the body’s calcium and 85% of the phosphorous.