Orthopaedic Surgery/Histology of Bone

	Histology of Bone
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Histology of Bone

In simple terms bone is a type of specialised connective tissue. Though bone comes in several shapes and sizes the structure and composition of bone is the same in all.

Structure

Gross Structure

Fig 1:Diagram depicting the Structure of bone

The human eye can discern only two types of bone. For a surgeon this distinction is what he encounters in the operating room. A cross section of any bone will demonstrate these two types of bones. 'Compact or cortical bone is usually thick dense bone that forms the outer shell covering almost all bones in the body. The inner aspect consists of bone that is not as dense as cortical bone and has a honeycomb appearance. This is cancellous bone. In tubular bones, long or short, cancellous bone is usually present at either ends. In irregular or flat bones they make up the entire inner body of the bone. Screws used for fracture fixation differ in structure depending on whether they are used to fix cancellous (less dense) or cortical (more dense) areas.

Microscopic Structure

Fig 3:Transverse section of decalcified bone x250

Cross sections of the bone when studied under the microscope reveal quite a different picture. By and large they could be either mineralised or unmineralised (Osteoid). Osteoid is normally found in children with a growing skeleton. In the transition from cartilage to bone at the growing end of a bone, bone first formed is unmineralised and then calcium salts are laid down giving them the strength of normal bone. Bone formed at the site of fracture healing is also osteoid, which eventually gets minerailised. When mineralisation does not occur adequately as in rickets or Osteomalacia the osteoid content of bone increases. Osteoid is not as strong as normal mineralised bone is and either fractures easily (osteomalacia) or deforms (rickets).
Mineralised bone could be either Woven Bone (Immature) or Lamellar Bone (mature). The basic microscopic unit of bone is an Osteon. Haversian canals run through the entire length of the bone carrying blood vessels. They are interlinked to each other through the Volkmans canals. On a transverse section of the bone each of these haversian canls is surrounded by a groupd of lacunae, which lodge an Osteocyte. (Fig 3)The entire group of osteocytes link to each other and to the centrally located haversian canal through cytoplasmic extensions that run throug tiny channels called canaliculi. The osteocytes derive their nutrition through their cytoplasmic extensions from the vessels in the haversian canal. In mature bone the osteons are arranged in layers (Lamellar bone) while in developing bone they are arranged randomly (woven bone). Lamellar bone makes up the compact or cortical bone in the skeleton. When the same lamellar bone is loosely arranged it makes up trabecular bone. Its called trabecular bone because of the trabecular pattern that shows up on an x-ray. Woven bone is found on the growing ends of an immature skeleton or at the site of fracture healing.

Compostion

Fig 2:Diagram depicting the Composition of bone

Bone is composed of protein,minerals and cells (see Fig 2)

Matrix

Matrix is the ground substance in which the cells of bone exist. The organic components of matrix would be collagen, mucopolysaccharides and a large non-homogenous group of non-collagenous proteins. The other important constituents of the matrix are the inorganic minerals calcium, phosphorus and to a lesser extent magnesium. The matrix forms the extracellular material that binds all the cellular constituents of bone together.

Cells

Cells in the human body that are specific to bone are the Osteoprogenitor cells, Osteoblasts, Ostecytes and the Osteoclasts.