Bone Structure and Function

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BONE ULTRASTRUCTURE

Bone is  is a specialised form of dense connective tissue.

Bone function :

  1. Mechanical – gives the skeleton the necessary rigidity to function as attachment and lever for muscles and supports the body against gravity.
  2. Chemical – Calcium homeostasis & metabolism
  3. Hematological

Three types of bone can be distinguished macroscopically:

  1. Trabecular bone – (also called cancellous or spongy bone) consists of delicate bars and sheets of bone, trabeculae, which branch and intersect to form a sponge like network. The ends of long bones (orepiphyses) consist mainly of spongy bone.
  2. Compact bone – does not have any spaces or hollows in the bone matrix that are visible to the eye.
  3. Woven bone – immature, disorganised bone.

Compact Bone (Cortical)compact bone

  • Compact bone consists almost entirely of extracellular substance – the matrix.
  • Osteoblasts deposit the matrix in the form of thin sheets which are called lamellae. Lamellae are microscopic structures.
  • Collagen fibres within each lamella run parallel to each other. Collagen fibres which belong to adjacent lamellae run at oblique angles to each other. Fibre density seems lower at the border between adjacent lamellae, which gives rise to the lamellar appearance of the tissue.
  • Bone which is composed of lamellae when viewed under the microscope is also called lamellar bone.
  • In the process of the deposition of the matrix, osteoblasts become encased in small hollows within the matrix, lacunae & become osteocytes.
  • In mature compact bone most of the individual lamellae form concentric rings (Concentric lamellae) around larger longitudinal canals (approx. 50 µm in diameter) within the bone tissue. These canal are calledHaversian canals.
  • Haversian canals typically run parallel to the surface and along the long axis of the bone.
  • The canals and the surrounding lamellae are called a Haversian system or an osteon.
  • Cement lines surround the outer border of each osteon; – histologically they are seen as the demarcation between the point that bone resorption ceased, and bone formation began; – collagen fibers and canaliculi do not cross cement lines; – these lines do not bind adjacent lamellae but rather separate them.
  • A Haversian canal generally contains one or two capillaries and some nerve fibres.
  • Immediately beneath the periosteum and endosteum a few lamella are found which run parallel to the inner and outer surfaces of the bone. They are the circumferential lamellae and endosteal lamellae.(vs. concentric lamellae of the Haversion systems)
  • A second system of canals, called Volkmann's canals, penetrates the bone more or less perpendicular to its surface. These canals establish connections with the inner and outer surfaces of the bone. Vessels in Volkmann's canals communicate with vessels in the Haversian canals.

Trabecular Bone (Cancellous)cortical bone

  • Lamellae in trabecular bone do not form Haversian systems
  • Lamellae of trabecular bone are deposited on preexisting trabeculae depending on the mechanical forces on the bone (Wolff's Law).[QTL-Question id=1]
  • Osteocytes, lacunae and canaliculi in trabecular bone resemble those in compact bone.
  • Lamellar bone forms both trabecular bone and compact bone, which are the two macroscopically recognizable bone forms.

Woven Bone (primary bone)

  • forms embryonic skeleton, and is largely absent from normal bone after age 4 yrs
  • is seen in fracture callus in both children and adults
  • does not contain lamellae, and rather has a relatively disorganized array of collagen and irregular mineralization pattern;

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