Index
It is the strongest and densest form of bone in the body.
However, it also contains many small passageways for blood vessels and nerves and houses the cells that repair and maintain bones.
Cortical bone, also known as compact bone, forms the hard outer layer of all bones.
Compact bone is the heaviest and hardest type of bone. It has to be very strong as it supports your body and muscles while you walk, run and move during the day.
About 80% of the bone in your body is compact. It makes up the outer layer of bone and also helps protect the more fragile layers on the inside.
If you were to look at a piece of compact bone without the aid of a microscope, it would appear completely solid throughout the entire process. However, if you looked at it under a microscope, you would see that it is actually filled with many very small passageways or channels for nerves and blood vessels.
Compact bone is made of special cells called osteocytes . These cells are lined up in rings around the channels. Together, a canal and the surrounding osteocytes are called osteons.
Osteons are like thick tubes that go in the same direction inside the bone, similar to a bundle of straws with blood vessels, veins, and nerves in the center.
When looking at the osteons in the bone under a microscope, tube-shaped osteons made up of osteocytes are seen. These bone cells have long branching arms that allow them to communicate with other cells.
Anatomy
Cortical bone is one of two types of bone found in the human body. The other type of bone, cancellous bone is always found deep in the cortical bone and serves as an internal support structure.
The cortical bone is covered by the periosteum and is lined by the endosteum at the diaphyses of the long bones, where it borders the medullary canal.
Cortical bone is made of bone tissue, a type of connective tissue specific to bones. Bone tissue consists of osteocytes, or bone cells, surrounded by a solid intercellular matrix of mineral salts and protein fibers.
The bone tissue matrix is particularly rich in the mineral hydroxyapatite which contains calcium and phosphorus.
Hydroxyapatite is very hard, but itself brittle. Collagen protein fibers interspersed within hydroxyapatite make the tissue flexible and more resistant to stress than pure hydroxyapatite.
Structurally, cortical bone is made of many microscopic cylinders called osteons, which surround small central canals.
Each osteon is formed during fetal development when small blood vessels and nerve fibers pass through mesenchymal tissue that contains osteogenic cells.
These osteogenic cells begin to produce bone matrix and form a ring of bone matrix, known as a lamella, around blood vessels and nerves.
Once the lamella has formed, these osteogenic cells are known as osteocytes and remain trapped in the bone tissue in small cavities known as lacunae.
Osteogenic cells farthest from the central canal form another lamella around the anterior layer and become another group of osteocytes trapped in the lacunae.
These cells receive their nutrients and oxygen from the blood vessels through small channels known as canaliculi, which connect the lagoon and allow all osteocytes to remain in contact with each other.
This process is repeated several more times, resulting in several concentric layers of lamellae around the central channel. As nutrients and oxygen pass from one osteocyte to another, eventually the cells cannot support more lamella layers.
Over time, additional blood vessels develop near the existing osteon to form a new osteon and a fuse to form a continuous layer of compact bone.
Physiology
Cortical bone is the hardest and densest tissue in the human body. It is used to support and protect the soft tissues of the body and to shape the body.
As stress is applied to specific regions of the body through weight-bearing activities, the thickness of the cortical bone can vary over time.
A layer of osteoblast cells surrounds the entire cortical bone along the inner layer of the periosteum. When stresses are applied to the bone, the body responds by activating the osteoblasts to produce a mineral matrix and form additional layers of cortical bone.
When stress on the bone decreases, osteoclast cells break down the mineral matrix to release mineral ions into the blood and reduce bone mass. These processes help control the strength and mass of the bones as dictated by the level of activity in the body.
Related Biology Terms
Spongy bone : the softer and less dense tissue that forms the ends of bones and creates blood cells.
Osteons : functional units of compact bone, created by a network of bone cells and blood vessels.
Osteocyte : a cell that works to maintain and repair bone tissue.
Osteoblasts : cells that establish new bone tissue, adding the matrix.
