The Osteon

The Osteon

The osteon is usually called a functional unit of bone. This is because an osteon, whether in compact boneDense, strong bone tissue forming the outer layer of bones. or spongy boneA porous bone tissue found inside bones, providing lightweight support., is the smallest functional structure. It still represents all of the abilities of the skeletal systemThe organ system providing structure and support. as a whole.  Think of an osteon like the columns or the pillars that support the front of a house. These columns or pillars can support the roof of the house or the roof of a porch. If a car were to run into one of these pillars, it would crumble horrendously.

That’s because the pillar, just like the osteonsThe structural unit of compact bone, consisting of concentric lamellae around a central canal. in your compact bone, can only take stress in one direction. The random orientation of the trabeculae in spongy bone allows it to take stress in all directions. Osteons consist of sheets of bone. These sheets surround a central canalA hollow canal in an osteon containing blood vessels and nerves.. Within this canal, an artery, a vein, and a nerve fiber are found. This means that bone tissue is vascularizedTo invade a tissue with blood capillaries. and it is also innervated. This is why when you break a bone it hurts.

Let’s look at the layers of concentric lamellaRings of bone matrix surrounding the central canal in an osteon. of an osteon.  Each concentric lamella has collagenA structural protein in the dermis that provides strength and elasticity. fibers running in a different direction than the sheets surrounding it. This is what gives an osteon its incredible ability to take stress in one direction. It also allows an osteon to take a twisting form of stress. What happens is the concentric lamella make sacrificial bonds Molecular bonds in bone collagen that break to absorb impact and reduce fracture risk. between them. When the bone is subjected to a twisting force, these bonds break. The concentric lamella twist in alternative directions. They lock into place and refuse any more twisting motion. This is what is sometimes referred to as a twister resistorA term describing bone’s ability to resist twisting forces..

How do Osteocytes Get Nutrients?

The histological picture on the right is a top down picture. We’ve cut a bone in a transverse section A cut or slice of the body or an organ for study.. We are looking top down onto the compact bone tissue.  The darkened middle of each of these bulls eyes is the central canal. This canal carries the artery, vein, and nerves. you can even see the concentric lamella that make up each osteon. Between the concentric lamella are osteocytes that have been trapped in holes we call lacunae or lacunaSmall spaces in bone tissue where osteocytes reside. singular.

Because these are living cellsThe basic structural and functional units of life., they need to receive glucoseA simple sugar that is the main source of energy for cells. and oxygen. The artery brings these to the bone tissue. They also need to utilize the veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge to carry away by products of cellular respirationThe process of gas exchange, including ventilation, external and internal respiration. like carbon dioxide. Tiny little canals run through each of these concentric lamella. They look like little spikes here in the picture. These little tiny canals are called canaliculiTiny channels that connect osteocytes, allowing communication and nutrient exchange. which literally means little canal. These little canals form an interconnecting highway. Fluids and other substances flow from the arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr to each osteocyteMature bone cells that maintain bone tissue.. They can also flow from the osteocyte to the vein. 

Spongy Bone

Your skeleton is comprised mostly of spongy bone Not compact bone tissue. If your skeleton were composed of compact bone, it would weigh too much. Your muscles would not be strong enough to move the bones.  Spongy bones serves to lighten the skeleton while still providing strength and support.  Much like the supports of a bridge spanning a river, spongy bone has structures called trabeculae. These structures are very similar to the structural trusses that support bridges. TrabeculaeThe lattice-like structures in spongy bone that support stress distribution. are curving structures of sheets or layers of bone called lamellae or lamella singular.  Because these trabeculae are found in spongy bone in almost infinite orientations, spongy bone can take stress from all directions. This ability makes it quite superiorAbove or toward the upper part of the body. to compact bone. Compact bone is only able to withstand force in one, or few, direction.