The defining characteristic of connective tissue is that it comprises an extra-cellular matrix with a few cells. That means it is found within the body, but outside of the cells that form organs. The few cells that are found within the matrix are mobile – mostly of them can move to wherever they are needed to produce more matrix. This matrix forms a variety of substances that provide a framework and support structure for other body tissues and organs. And that framework includes the skeletal system.
Connective tissue is generally thought of as either a soft tissue that surrounds and separates organs or cells, and fills spaces, or as a tough fibre such as ligaments and the tendons that attach muscles to bone. However, there are also specialised forms of connective tissue and these include cartilage, blood, and bone.
Connective tissue is composed of a variety of proteins, including collagens (building blocks), proteoglycans (to maintain tissue stiffness) and glycoproteins (another mixture of protein and sugar). The specialised connective tissues contain other substances, for example bone contains calcium phosphate, which provides it with rigidity and hardness.
Bone starts as flexible cartilage, which is composed of closely packed collagenous fibres and a rubbery gelatinous substance called chondrin. This forms the skeleton of all vertebrate embryos. As the animal grows, the cartilage hardens as calcium phosphate is added. For most vertebrates, the only cartilage left by adulthood is at the joints where it provides protection for the ends of the bones as they move. The human nose, windpipe and ears remain flexible cartilage throughout life. Members of the shark family retain their cartilaginous skeleton until death.
The human skeleton has 206 bones which give the body its shape and framework, and which protect the vital organs. The bones need to be held together to form the skeleton, and this is done by ligaments which are strong bands of fibrous connective tissue composed mainly of collagen. Ligaments have high tensile strength and some elasticity, thus they can restrict the amount and direction of joint movement but pull the joint back into place afterwards and so prevent dislocations.
Where joints between bones have to move, muscles need to be attached. The tendons which provide this attachment are also connective tissue, basically the same structure as ligaments but without the elasticity. They run deep into the muscles, between and around the bundles of muscle fibres which contract to make the joints bend.
Within the skeletal system is another type of connective tissue, bone marrow. Yellow marrow contains fatty connective tissue which the body can use during periods of starvation. Red marrow produces both red and white blood cells, and platelets. Bone also stores various minerals, such as calcium, which are released back into the body when required, transported by blood.
Blood itself is liquid connective tissue that transports various chemicals, including oxygen, hormones, nutrients and waste products around the body. It is as vital to the functioning of other connective tissue as it is to the individual cells of the body and to the body as whole (for example in temperature regulation).
Connective tissue therefore has an important role in forming the skeletal system, holding it together, and connecting it to other parts of the body both directly as it connects to muscles and indirectly via the flow of blood.