Department of Anatomy &Histology: Dr.Rajaa Ali
Bone is a rigid form of scleral connective tissue in which the extracellular matrix is impregnated with inorganic salts, mainly calcium phosphate and carbonate, providing hardness.
Unlike ordinary Connective tissue, bone is rigid and hard because the matrix is infi ltrated with inorganic salts.
_ Bone gives attachment to muscles and serves as a lever for muscular action.
_ It bears body weight.
_ It protects vital organs like brain, heart and lungs.
_ Bone stores calcium, phosphate and other ions.
_ It contains bone marrow, which is a haemopoietic tissue.
TYPES OF BONE
Morphologically, bone consists of:
1. Externally, a solid shell of cortical bone called compact bone (found in shell of short bones, shaft of long bones and tables of flat bones).
2. Internally, a framework of trabeculae separated by marrow spaces called spongy or cancellous bone (found in short bones [Fig. ], ends of long bones and diploë of fl at bones).
Microscopically, compact bone consists of:
1. Primary/immature/woven bone—newly formed bone during growth and repair with low mineral content, e.g. callus.
2. Secondary/mature/lamellar bone—definite adult type after remodelling.
The external and internal surfaces of _ bones are covered by membranes called periosteum and endosteum respectively.
_ They have osteogenic potential and are essential for growth and repair.
_ It is a dense connective tissue membrane covering the external surface of bone, except on articular surfaces, sesamoid
bones and at the attachments of tendons and ligaments.
_ Periosteum sends perforating fi bres perpendicular to the bone surface to nail or anchor it to the cortical bone. These
fi bres are called Sharpey’s fi bres.
_ Periosteum also has two layers like perichondrium:
(a) Outer vascular fi brous layer.
(b) Inner cellular osteogenic layer.
_ It has rich nerve supply and is very sensitive.
_ It is involved in bone growth and repair and, therefore, care should be taken to preserve it during bone surgery.
_ It is a thin membrane of vascular loose connective tissue lining the medullary cavity in the long bones and marrow
spaces in the cancellous bones. It extends as a lining into the canal system of a compact bone.
_ Cells of endosteum have osteogenic potential, i.e. they can differentiate into bone forming cells (osteoblasts) during
repair and growth. So this layer should also be preserved during bone surgery.
_ Like any other connective tissue, bone is made of cells, fi bres and ground substance. In addition, its extracellular matrix is
infi ltrated with inorganic salts like calcium phosphate and calcium carbonate to provide hardness and rigidity.
_ The mineral salts (calcium and phosphorus) form needle-like crystals of hydroxyapatite [Ca
10(PO4)6 (OH)2], which are 20–40
nm in length. The needles are arranged parallel to collagen fi bres and partly within them.
_ The organic components (collagen fi bres and ground substance) give plasticity to bone, allowing it to remodel according
to the functional demands placed upon it (orthodontic tooth movement).
_ These are of the following types:
(a) Osteoprogenitor cells: They are young pluripotential cells derived from mesenchymal cells. They differentiate
into osteoblasts and found along the blood vessels in the periosteal buds and in the endosteum.
(b) Osteoblasts (c) Osteocytes _ See Table 6.1.
2. Fibres (95%)
_ These are constituted of collagen fi brils which are composed of type I collagen.
3. Ground substance (5%)
_ This is made of chondroitin sulphate, dermatan sulphate and a specifi c glycoprotein.
_ The fi bres and ground substance form the organic component of bone, which give elasticity and resilience.
4. Inorganic components (Bone salts/Hydroxyapatite)
_ These include:
(a) Calcium phosphate (85%).
(b) Calcium carbonate (10%).
(c) Other salts (5%).
_ These inorganic components are deposited on and around collagen fibrils, which give hardness and rigidity to the bone.
It is the process of removal of inorganic components. As a result the bone becomes soft and flexible.
_ It is the process of removal of organic components. As a result the bone becomes brittle and fragile.
STRUCTURE OF COMPACT BONE
_ Compact bone consists of three systems (sets) of bony lamellae arranged in an orderly manner:
1. Circumferential system
– Outer (periosteal)
– Inner (endosteal)
2. Haversian system or osteon
3. Interstitial system
Circumferential System (Fig. )
_ Outer circumferential system consists of circular lamellae of bony matrix that lie immediately beneath the periosteum.
_ Inner circumferential system also consists of circular lamellae of bony matrix that lie adjacent to the endosteum.
_ Osteocytes are found between the lamellae in the lacunae.
_ Both circumferential systems have the marrow cavity as the centre.
_ The outer system has more lamellae than the inner system.
_ Between the two circumferential systems are numerous Haversian and interstitial systems.