Haversian System: Understand the Structure, Explain the Function, Apply Your Knowledge

 HAVERSIAN SYSTEM

Definition:

The Haversian system (osteon) is the structural and functional unit of compact bone but absent in spongy bone.

Structure:

1. Haversian canal: A central canal containing blood vessels (artery & vein) and nerves.
2. Concentric lamellae: they are rings shape calcified bone matrix arranged around the Haversian canal.
3. Lacunae: Small depressed spaces between the lamellae containing osteocytes.
4. Canaliculi: Tiny channels within lamellae connecting lacunae with one another and with the Haversian canal.
5. Cement line: The outer boundary of the Haversian system.
6. It contain Volkmann’s canal.                                                                                                                 

   

   Haversian system 

   
   

Volkmann’s Canal

Definition:

Volkmann’s canals are transverse or oblique channels in compact bone (they are absent in spongy bone) that connect adjacent Haversian canals with one another and convey blood vessels from the periosteum and medullary cavity.

Contents:

1. Blood vessels
2. Nerve fibres

Important features:

• They run transversely or obliquely to the Haversian canals.
• They are lined by endosteum, the lining of medullary cavity.
• They do not have concentric lamellae surrounding them like Haversian canal.

 

Functions:

·         They connect Haversian canals with one another and with blood vessels from the periosteal and medullary regions.

• They are connected with canaliculi, so they help to supply nutrients and oxygen to osteocytes.
• Provides strength and support to compact bone.
• Facilitates the removal of metabolic waste from osteocytes.

Important point: Nutrients diffuse from blood vessels in the Haversian canal through the canaliculi to reach osteocytes located far from the blood supply.

Blood Supply of Haversian

Haversian canal (HC): Blood vessels are derived mainly from

1.       Nutrient artery

2.      periosteal arteries

3.       Metaphyseal arteries

4.      Epiphyseal arteries

 

UNDERSTANDING QUESTIONS FROM THE HAVERSIAN SYSTEM

Question:

Why is the Haversian system structurally adapted for the passage of nutrients to osteocytes located far away from the Haversian canal?

Answer:

The Haversian system contains a central Haversian canal carrying blood vessels. Lacunae are small spaces that contain osteocytes.

Canaliculi connect the lacunae with one another and with the central canal.

Haversian canal → Canaliculi → Lacunae → Canaliculi → Lacunae

Thus, nutrients and oxygen reach nearby osteocytes through the canaliculi. In the same way, nutrients and oxygen pass through the canaliculi from one lacuna to another, allowing distant osteocytes to receive nutrients and oxygen.

Question:

Why is the Haversian system present in compact bone but not in spongy bone?

Answer:

Compact bone has a dense matrix, so vascular canals and canaliculi help supply its osteocytes.

Spongy bone has thin trabeculae exposed to marrow spaces, so osteocytes receive nutrients from the blood vessels in the bone marrow by diffusion through the canaliculi, without requiring a Haversian canal.

Cell biology and epithelial tissue slide identification : OSPE

 Cell biology

TOPIC / SLIDE 1:

HUMAN CELL

True/False OSPE Questions

Instruction: Write T (True) or F (False) for each statement.

1. The plasma membrane of a typical human cell is composed of a phospholipid bilayer.
2. The nucleus contains most of the genetic material of a typical human cell.
3. The nucleolus is surrounded by a double membrane.
4. Mitochondria are the major sites of ATP production in the cell.
5. Ribosomes are responsible for protein synthesis.
6. The rough endoplasmic reticulum is involved in lipid synthesis.
7. The Golgi apparatus modifies, sorts, and packages proteins.
8. Lysosomes contain hydrolytic enzymes for intracellular digestion.
9. Centrioles are surrounded by a double membrane.
10. All human cells contain a nucleus.

Answer Key: 1. T, 2. T, 3. F, 4. T, 5. T, 6. F, 7. T, 8. T, 9. F, 10. F.

Important explanations for students: 3. False, the nucleolus is a non-membrane-bound structure. 6. False, the smooth endoplasmic reticulum is mainly involved in lipid synthesis. 9. False, centrioles are non-membranous structures composed of microtubules. 10. False, mature red blood cells lack a nucleus.

 

TOPIC/ SLIDE 2:

EPITHELIUM: SIMPLE SQUAMOUS EPITHELIUM

 

Histological features:

 

1.      A single layer of flat, scale-like cells rests on a basement membrane.

2.      Each cell contains a single, centrally placed, flattened oval nucleus.

Distribution:

 

Endothelium: Lines blood vessels.

Endocardium: Lines the inside of the heart.

Mesothelium: Lines the pleura, pericardium & peritoneum. Types 1 pneumocyste at alveoli of the lungs.

 

 

 

TOPIC/ SLIDE 3: SIMPLE CUBOIDAL EPITHELIUM

 

Histological features:

1.      A single layer of cube-shaped cells rests on a basement membrane.

2.      Each cell contains a single, centrally placed, round nucleus.

Distribution: Thyroid follicles. Kidney tubules.

 

 

 

 

TOPIC/ SLIDE 4: SIMPLE COLUMNAR EPITHELIUM

 

Histological features:

 

1. A single layer of tall, column-shaped cells rests on a basement membrane.
2. Each cell contains a single, oval nucleus, usually located near the base.
3. The free surface may have microvilli or cilia. Goblet cells may also be present in between columnar cells.

Distribution:

• Gastrointestinal tract: Lining epithelium of Stomach, small intestine and large intestine.
• Reproductive system: lining epithelium of uterus and uterine tubes.
• Ducts: Some larger bile ducts and pancreatic ducts.

 

 

 

 

TOPIC/ SLIDE 5 & 22 PSEUDOSTRATIFIED CILIATED COLUMNAR EPITHELIUM

 

Histological features:

 

1. A single layer of tall and short cells rests on the basement membrane.
2. The oval nuclei lie at different levels, giving a false appearance of multiple layers.
3. All cells rest on the basement membrane, but not all cells reach the free surface.
4. Cilia are present on the free surface.

 

Distribution: Trachea and bronchi. Epididymis and vas deferens .

Q. Why is it called pseudostratified?

Ans: Pseudo means false & stratified means arranged in multiple layers. The nuclei lie at different levels, creating the false appearance of multiple layers. However, all cells rest on the basement membrane, although not all cells reach the free surface because some cells are small in height. Therefore, it is called pseudostratified epithelium.

TOPIC/SLIDE 7: STRATIFIED EPITHELIUM: STRATIFIED CUBOIDAL EPITHELIUM

 

Histological features:

 

1.      This stratified epithelium is formed by only 2-3 cells layers.

2.      Name of stratified epithelium are decided according to the shape of the surface cell.  

 

Distribution:   

 

Ducts of sweat glands, salivary glands and mammary glands

Conjunctiva

TOPIC/SLIDE 6: STRATIFIED COLUMNAR EPITHELIUM

Histological features:

1.       This stratified epithelium is formed by 2-3 cells layers.

2.      Basal cells are cuboidal to low columnar in shape 

3.      Apical group of cells are columnar in shape.

 

Distribution:  

 

Large duct of salivary gland, palpebral conjunctiva.

 

 

TOPIC/SLIDE: 8 KERATINIZED STRATIFIED SQUAMOUS EPITHELIUM

Histological features:

 

1. It consists of many layers of cells.
2. Basal cells are cuboidal or columnar and rest on basement membrane.
3. Cells become gradually flattened from the basal to the superficial layers.
4. The superficial cells are dead, flattened and lack nuclei. They contain keratin.
5. It is found on dry surfaces and provides protection against friction and water loss.

 

Distribution: Epidermis of the skin.

Non keratinized stratified squamous epithelium

Histological features:

 

1. It consists of many layers of cells.
2. Basal cells are cuboidal or columnar and rest on basement membrane.
3. Cells become gradually flattened from the basal to the superficial layers.

4.      Apical /Surface cells are  alive, nucleated and squamous in shape.

5.      It found in wet surface.   

 

Distribution: Lining epithelium of esophagus and vagina, oral cavity.

 

 

TOPIC/SLIDE 9: TRANSITIONAL EPITHELIUM

 

Histological features

 

1.      Surface cells are enlarging and known as dome shape cell or umbrella cells.

2.      In empty urinary bladder, this epithelium are 5-6 layers, but during distended bladder, they look like 3-4 layers.

Distribution:

 Ureter, urinary bladder, part of urethra

 

Empty Urinary bladder         Distended urinary bladder

TOPIC/SLIDE 10: CARDIAC MUSCLE

Histological features:

1.      Shape of cell: short, cylindrical

2.      Centrally placed single nucleus

3.      Branching present

4.      Cross striation present

5.      Intercalated disc is present

 

Longitudinal section            

Cross section

TOPIC/ SLIDE 11: SPINAL CORD

Histological Features

1. H shape Gray matter is present in the center & is surrounded by white matter.
2. Gray matter contains:
• Nerve cell bodies (neurons)
• Neuroglia
• Blood vessels
• Initial portions of nerve cell processes
3. White matter contains mainly myelinated nerve fibers and neuroglia.
4. Central canal is lined by ependymal cells & centrally placed..
5. The gray matter is divided into anterior, posterior and lateral horns (the lateral horns are prominent in certain spinal cord regions).

 

SLIDE 11: PERIPHERAL NERVE

Histological features

1.      Nerve fibre: Consists of an axon surrounded by a myelin sheath (in myelinated nerve fibres) & endoneurium.

2.      Nerve bundle (fascicle): A collection of nerve fibres surrounded by perineurium.

3.      Nerve trunk: A collection of nerve bundles surrounded by epineurium.