Parts of the Skeletal System


Most people think of bones as simply a framework that holds up their body, but bones are highly active organs and play a productive role in your life. Bones have three main layers.


Your bones are covered with a thin layer of connective tissue called periosteum.  The periosteum is where your immature bone cells come from.  Like other types of connective tissue, the periosteum layer is made in part by a protein called collagen, which gives them a little bit of flexibility.

Compact bone

Compact bone lies just below the periosteum. Your compact bone is hard, white and smooth. This part of the bone gets its strength from calcium hydroxyapatite, the mineral form of calcium in bones. Compact bone is used primarily as structural support and protection for your body, as well as for calcium storage.

Spongy bone

Spongy bone is the inner core layer of your bones. This is where bone marrow is found.  Bone marrow makes new red blood cells and white blood cells. Spongy bone is not nearly as strong as compact bone. 

Bone cells

Your bones are made up of several types of cells that all contribute to healthy bones. The cell types have similar names, all starting with “osteo-”, which is Greek for “bone”. Bone is continually broken down and rebuilt, but an adult’s bones do not change shape. The cells involved in this process are osteoblasts and osteoclasts.


Osteoblasts secrete proteins that solidify into new bone. Found on the surfaces of bone, osteoblasts control the growth of your bones and the storage of calcium.


Osteoclasts are cells that break down bone. Osteoclasts essentially digest away mineralized bone to help make bones smaller. This is a normal process that is needed to release calcium from your bones for other body functions and to repair the everyday wear and tear of your bones. 

The Spaces Between Bones 


The place where two bones meet is called a joint. Joints are filled with a fluid, called synovial fluid, which lubricates the bone. Synovial fluid allows for smooth, pain-free movement at your joints. 


Cartilage is a connective tissue that is less rigid than bone, but not as flexible as muscle. Cartilage cells secrete the protein collagen. Collagen, along with other secreted proteins, forms a dense web that gives cartilage its structure.  It is extremely strong, yet elastic. 

Cartilage is found between the bones of your ribs and spine, in the joints of your arms and legs, and in your ears and nose. It protects your bones from shock and from rubbing against each other in the joints.  Cartilage is the only tissue in your body that does not have blood vessels. Because of its lack of blood, cartilage is one of the slowest tissues to grow and heal.

Connective Tissues

Ligaments (bone-to-bone)

Ligaments are connective tissues that connect bones to one another. They are located at your joints. One ligament you may have heard of is the anterior cruciate ligament, or ACL. Your ACL connects your thigh bone to your shin bone. Tearing the ACL is a common injury among athletes.

Tendons (bone-to-muscle)

Tendons connect your muscles to your bones. Tendons act as springs during heavy exercise or lifting because they can stretch to help muscles generate more force. An important tendon, the Achilles tendon, attaches your calf muscle to your heel bone.