Structure of bone:

· Chemically bones have organic and inorganic matter.

Organic matter

Includes bone cells, collagen fibers and matrix or intercellular substance.

Fibrous proteins like collagen, ossein and chondroitin sulphate

Contribute to flexibility of bone.

Inorganic matter

Mainly calcium phosphate (85%), calcium carbonate (10%), magnesium phosphate, sodium carbonate and sodium chloride.

Responsible for the rigidity and hardness of bone.

· Microscopically

Bone is a connective tissue having osteocytes (bone cells).

Parallel rows of fine collagen fibers embedded in amorphous ground substance.

Ground substance or matrix contains regularly arranged crystals of calcium salts.

Mineral matter makes the intercellular substance substance hard and permeable, forming thin plates or lamellae.

Bone is arranged in the form of concentric system called the Haversian system or osteone.

· Grossly bone can be compact bone and cancellated or spongy bone.

Compact bone

Dense, white, hard and forms the outer shell of a bone.

Found aggregated in portions where more strain on bone is there.

Haversian system is present in compact bone.

Cancellated or spongy bone

Made up of delicate, intercrossing plates which form a meshwork with spaces containing marrow.

Found in the epiphysis of long bones and is always covered by a layer of compact bone.

Haversian system is absent.

· In general bones have external shell of dense compact substance.

· Compact substance encloses more loosely arranged spongy substance.

· Long bones have medullary cavity in their shaft.

· Thickness of compact substance varies with the stress and strain beared by the bone.

· Spongy substance consists of delicate bony plates and spicules running in various directions.

· Spaces between the bony plates are called marrow spaces.

· Some bones have air spaces instead of spongy substance are called pneumatic bones.

· Such air filled cavities are called sinuses and are lined by mucous membrane.

· Flat bone of skull are composed of

Lamina externa: An outer layer of ordinary compact bone.

Lamina interna: Inner layer of very dense bone.

Diapole: Spongy layer in between the above two.

· Periosteum: Membrane covering the outer surface of bone except at the articular cartilage. Consist of

Outer protective fibrous layer.

Inner cellular osteogenic layer.

· Endosteum: Thin fibrous membrane lining medullary cavity and larger Haversian canals.

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