Bone l General Histology & Biology MCQ for dental students

Bone

• Bone is a hard calcified Connective tissue formed of bone cells in a solid extracellular matrix (formed of CT fibers "collagen type I" and calcified ground substance). It is highly vascular, covered by periosteum and lined by endosteum.

Function of bone:

1. Support of the body (by skeleton).
2. Protection of essential organs (as CNS, lung, BM...).
3. Reservoir of calcium.

Anatomical types of bone:

• Long - short - flat - irregular.

Histological types of bone:

• Compact bone as shaft of long bones – spongy bone as ribs.

Bone cells: there are four types of bone cells: 

1. Osteogenic cells (osteoprogenator stem cells):

• Arise from UDM cells or pericytes.

Site:

• Inner layer of periosteum and endosteum.

Light microscope :

• Spindle shaped pale cells with flat nuclei.

Electron microscope :

• Features of mitotically active cells (centrioles and many ribosomes).

Function:

• They can divide and differentiate to:

1. Osteoblasts (with ­ vasculature).
2. Chondroblasts (with ¯ vasculature).

 

2. Osteoblasts (bone forming cells):

• Arise from osteogenic cells.

Site:

• Inner layer of periosteum and endosteum.

Light microscope :

• Oval basophilic cells with eccentric nuclei and few Processes.

Electron microscope :

• Features of protein-forming cells (­ mitochondria, rough endoplasmic reticulum and unstained Golgi).

Function:

• They form organic component of matrix while its Alkaline phosphatase deposit calcium in matrix then change to osteocytes When trapped inside lacunae.

 

3. Osteocytes (mature bone cells): arise from osteoblasts.

Site:

• Between bone lamellae (inside lacunae that Communicate by canaliculi).

Light microscope :

• Small oval basophilic cells present singly inside lacunae.

Electron microscope :

• They have thin processes connected by gap junction Inside canaliculi.

Function:

• They maintain the bone matrix.

 

4. Osteoclasts (bone resorbing cells):

• Formed by fusion of 6-20 monocytes.

Site:

• At surface (in Howship’s lacunae) and at endosteum.

Light microscope :

• Foamy acidophilic multinucleated giant cells (120 μm) with irregular ruffled border (microvilli).

Electron microscope :

• mitochondria, rough endoplasmic reticulum , Golgi and lysosomes.

Function:

• Remodeling of bone during growth and after fractures (by HCO3 and lysosomal enzymes).

Note Book :

• Alkaline phosphatase of osteoblasts ® bone calcification.
• Acid phosphatase of osteoclasts ® bone decalcification.

 

Bone matrix:

• A solid calcified media formed of:

1. Inorganic substance (calcium): form 50% of bone.
2. Organic substance (glycoprotein): form 25% of bone.
3. Water: form 25% of bone.

Compact bone (ivory bone): it is formed of:

1. Haversion system (osteons):

• Each is formed of longitudinal Haversion canal surrounded by 5-20 concentric interstitial lamellae.

2. Volkmann’s canals:

• Transverse canals connecting periosteum, Haversion canals and endosteum.

3. Bone lamellae:

• Layers of calcified collagenous bundles between which osteocytes are present in their lacunae. They are of 3 types:

1. External circumferential lamellae: regular lamellae close to periosteum.
2. Interstitial lamellae: irregular lamellae present between Haversion systems.
3. Internal circumferential lamellae: regular lamellae close to endosteum.

4. Covering periosteum:

• Vascular CT membrane formed of 2 layers:

1. Outer fibrous layer (formed of collagenous bundles "type I" and fibroblasts): for muscle attachment.
2. Inner osteogenic layer (formed of osteogenic cells): for nourishment, repair of bone and peripheral appositional growth (increase of bone in width by

• Formation of new osteoblasts and their transformation to osteocytes).

5. Lining endosteum:

• Vascular CT membrane surrounding bone marrow cavities. It is similar to inner osteogenic layer of periosteum.

Spongy bone (concellous bone): it is formed of:

Bone trabiculae:

• Irregular bone plates containing osteocytes and multiple bone marrow cavities.

Multiple bone marrow cavities:

• Lined by endosteum and containing red active bone marrow with blood sinusoids.

Covering periosteum:

• As that of compact bone

Differences between bone and cartilage:

Cartilage 

1. firm flexible Connective tissue formed of 2 types of cartilage cells in a rubbery extracellular matrix
2. non-vascular Connective tissue  and receive nourishment from inner chondrogenic layer of perichondrium (no extra-cellular canaliculi)
3. 3 types: hyaline cartilage, elastic cartilage and fibro-cartilage
4. chondrocytes are single or grouped cells , can divide , not communicate
5. matrix: is formed of Connective tissue  fibers "collagen type II" and flexible ground substance
6. covered by perichondrium

Bone

1. hard calcified Connective tissue  formed of 4 types of bone cells in a solid extracellular matrix
2. highly vascular Connective tissue  as blood vessels are present in Haversion canals and Volkmann's canals while lacunae and extra-cellular canaliculi contain tissue fluid
3. 2 types: compact bone and spongy bone
4. osteocytes are single cells – cannot divide - their processes communicate inside extra-cellular canaliculi
5. matrix: is formed of Connective tissue fibers "collagen type I" and calcified ground substance
6. covered by periosteum and lined by endosteum