Bones and tendons work closely together to help the muscles contract and allow the whole body to move. Knowing the difference in anatomy and function of each of these tissues will help distinguish them.
Anatomy
Bone is a living tissue. It is difficult to think of bone as a tissue sometimes because bone is a dense and hard material. In actuality, bone has a hard outer coat but a soft spongy material inside. Bone is made up of mainly calcium, phosphorous, sodium and a few other minerals along with a protein called collagen. The hard outer part of the bone is called the periosteum and is white and slightly porous in appearance, while the inside is called the marrow (made up of yellow and red marrow), which is spongy and dark red in appearance. As we age some of the red marrow becomes fattier and turns yellow. An adult marrow is half red and half yellow.
Tendon is not as firm as bone but is made up of collagen as well as elastin, which are both white in appearance. The elastin allows for the tendon to stretch while the collagen allows for the tendon to be strong. The collagen is grouped into small bunches of parallel fibers surrounded by a tendon sheath to decrease friction. The whole tendon is made up of multiple mini-groups of collagen bundles surrounded by one final larger tendon sheath. This grouping allows the tendon to handle rapid changes in direction and forces of up to 17 times the body weight.
Blood Supply
The pores in the periosteum of the bone allow for blood channels to flow through into the middle of the bone. The red marrow of the bone is red and spongy inside because of the very rich blood supply. There is a constant movement of nutrients in and out of bone. Tendons, on the other hand, have little to no blood supply at all. The only way nutrients move in and waste moves out is through the pumping action of the tendons and muscles. Due to the poor blood supply, tendons can be slow to heal.
Function
Bone is the foundation for the body. Without it we could not sit or stand upright. Bones are the levers for which the muscles use to move. Additionally, bone is a shock absorber to some degree, translating the forces of movement up and down the long bones of the body. Bone plays a large roll in the immune system as well. The red marrow of the bone is where red blood cells, platelets and white blood cells are made.
Tendons are attached to bone on one side and muscle fibers on the other. The tendons withstand the strength of the muscle contractions on one side and the rigidness of the bone on the other side and act as shock absorbers for the muscles during rapid movement. Without them, the muscles could not effectively contract or move the bone.
References
- NCBI: Structure of the Tendon Connective Tissue
- Kids Health: Bones and What They Do
- "Tendon Injuries;" Maffulli N., et. al.; 2005
