howMed Know Yourself
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.
Characteristics
- 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
Example
- Osteonectin
- Osteopontin
- Sialoprotein I&II
Bone Specific Vitamin K Dependent Proteins
- Osteocalcin
- Protein S
Bone Cells
1. Osteocytes
2.Osteoblasts
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
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
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
Osteoclasts
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.
Periosteum
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.
