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Bone is a specialized connective tissue characterized by mineralized extracellular matrix. Bones are the organs of skeletal system; bone tissue is the structural component of bones.


  • Consists of cells  and extracellular matrix.
  • Bones are covered by periosteum
  • Bone cavities are lined by endosteum
  • Bone cavities contain red bone marrow
  • Bones are the organs of skeletal system; bone tissue is the structural component of bones.

Bone Matrix

  • If mineral is removed, bone is too bendable
  • If collagen is removed, bone is too brittle

90% Collagen fibers are found, which are divided into:

Type 1, Type 3, Type 5, Type11, Type13

10% are the non collagenous proteins including:

1. Proteoglycans

2. Multiadhesive glycopoteins

3. Bone specific vitamin k dependent proteins

4. Growth factors and cytokines

Multiadhesive Glycoproteins

These are responsible for  attachment of bone cells and collagen fibers to the mineralized ground substance


  • Osteonectin
  • Osteopontin
  • Sialoprotein I&II

Bone Specific Vitamin K Dependent Proteins

  • Osteocalcin
  • Protein S

Bone Cells

1. Osteocytes


3. Bone-lining cells

4 .Osteoclasts

5.Osteoprogenitor cell

Osteoprogenitor cells

These are located in the periosteum and endosteum. They are very difficult to distinguish from the surrounding connective tissue cells.


Osteoblasts may form a low columnar “epitheloid layer” at sites of bone deposition. They contain plenty of rough endoplasmatic reticulum (collagen synthesis) and a large Golgi apparatus.PAS +ve granules are present in the cytoplasm


Osteocytes are the mature bone cells. These are:

  • Stellate.
  • Surrounded by matrix,
  • Can make small amounts of matrix to maintain it.

Lacunae: spaces occupied by osteocyte cell body

Canaliculi: canals occupied by osteocyte cell processes

Nutrients diffuse through tiny amount of liquid surrounding cell and filling lacunae and canaliculi. Then can transfer nutrients from one cell to the next through gap junctions.

States of osteocytes

Quiescent osteocytes

Paucity of rER & decrease golgi apparatus

Formative osteocytes

More abundant  rER& golgi apparatus

Resorptive osteocytes

Abundant RER, golgi apparatus and lysosomes

Bone lining cells

Those present on external surface are called periosteal cells

Those present on internal surface are called endosteal cells

They are involved in mantainance and nutritional support of osteocytes


Large multi-nucleated bone-resorbing cells.. Osteoclasts attach themselves to the bone matrix (howships lacunae) and form a tight seal at the rim of the attachment site.

Osteoclasts are stimulated by parathyroid hormone and inhibited by calcitonin

The cell membrane opposite the matrix has deep invaginations forming a ruffled border. Osteoclasts empty the contents of lysosomes into the extracellular space between the ruffled border and the bone matrix. The released enzymes break down the collagen fibres of the matrix.


Bone is, again like cartilage, surrounded by a layer of dense connective tissue, the periosteum. A thin layer of cell-rich connective tissue, the endosteum, lines the surface of the bone facing the marrow cavity. Both the periosteum and the endosteum possess osteogenic potency.

Classification of bones

  • Compact Bone
  • Cancellous (Spongy) Bone

Immature bone

  • Doesn’t  display  an organized lamellated appearance.
  • More cells.
  • Cells are random.
  • Matrix has more ground substance.
  • Takes haematoxylin stain.

Compact Bone

Central or Haversian canals: parallel to long axis

Lamellae: concentric, circumferential, interstitial

Circumferential lamellae on the periphery of a bone

Interstitial lamellae between osteons

Osteon or Haversian system: central canal, contents, associated concentric lamellae and osteocytes

Osteons (Haversian systems)

¨     Blood vessel-filled central canal (Haversian canal)

¨     Concentric lamellae of bone surround central canal

¨     Lacunae and canaliculi contain osteocytes and fluid

Perforating or Volkmann’s canal

Present perpendicular to long axis. Both perforating and central canals contain blood vessels. Direct flow of nutrients from vessels through cell processes of osteoblasts and from one cell to the next.

Circulation in Bone

Blood vessels from periosteum penetrate bone

Vessels of the central canal

Nutrients and wastes travel to and from osteocytes via

a.     Interstitial fluid of lacunae and canaliculi

b.     From osteocyte to osteocyte by gap junctions

See histology slides here.

Cancellous (Spongy) Bone

Trabeculae: interconnecting rods or plates of bone.  Like scaffolding.

¨     Spaces filled with marrow.

¨     Covered with endosteum.

¨     Oriented along stress lines

See histological slides here.

Want a clearer concept, also see
Lecture on Bone
Images of Compact bone
Images of Spongy bone

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