Compact bone also called cortical bone, is dense in texture like ivory, but is extremely porous, because it contains many tiny passages for blood vessels and nerves and houses the cell, that repair and maintain bones.
Cortical bone makes up 80% of the human skeleton, the remainder is cancellous bone.
Whereas compact bone tissue forms the outer layer of all bones, spongy bone or cancellous bone forms the inner layer of all bones.
Compact bone is dense so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in weight distribution.
The microscopic structural unit of compact bone is called an osteon or haversian system. The ‘haversian system’ gets its name from an English physician named clopton havers (1657-1702). Only compact bones have osteon as a basic structural unit; spongy bones don’t have osteons.
Osteons in compact bone tissue are aligned in the same direction along lines of stress and help the bone resist bending or fracturing. therefore, compact bone tissue is prominent in areas of bone at which stress are applied in only a few directions.
COMPONENTS OF HAVERSIAN SYSTEM
- located at the center.
- Small blood vessels that are present in the central canal perform the function of supplying blood to the osteocytes.
- The central canal also contains nerve filaments.
- These canals are connected to each other by Volkmann’s canals, which are transverse vessels that run perpendicular to the long axis.
- Volkmann’s canals also connect the blood vessels that are present in the central Haversian canals to the periosteum (dense tissue covering the outer surface of the bone).
- Volkmann’s canals are not surrounded by concentric lamellae.
- concentric rings of a strong bone matrix.
- Mineral salts and collagen fibers lend strength to the matrix.
- The collagen fibers in a lamella in an osteon run parallel to one another, but the collagen fibers run in different directions in the lamella that lies next to it.
- This arrangement also strengthens the osteon.
- It is the presence of mineral salts such as calcium and phosphates in the matrix that makes the bone hard, whereas collagen fibers strengthen the matrix.
✦ Between the lamellae are small spaces called lacunae. Osteocytes are located within these small spaces.
✦ Tiny channels called canaliculi connect the lacunae. Nutrients are supplied to the osteocytes via these channels. Waste products also leave the osteocytes through these channels.
✦ The remnants of previous concentric lamellae can be found between the osteons. These are referred to as interstitial lamellae.
On a concluding note, the Haversian system or the osteon plays a vital role, as it facilitates the supply of oxygenated blood and nutrients to the bones. Moreover, the network of canaliculi also facilitates the passage of substances between the blood vessels and the mature bone cells.
BONE MARROW IN COMPACT BONE
Bone marrow is the tissue comprising the center of large bones. It is the place where new blood cells are produced.
There are two types of bone marrow: red marrow (also known as myeloid tissue) and yellow marrow. Red blood cells, platelets and most white blood cells arise in red marrow; some white blood cells develop in yellow marrow.
The color of yellow marrow is due to the much higher number of fat cells.
Yellow bone marrow present in compact bone, which stores fat after puberty, it is red before puberty.