cytology 2 l General Histology & biology revision for dental students

cytology 2  l General Histology & biology revision for dental students

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Cytology (Cell biology)

• Cells are the structural and functional units of human where cells ® tissues ® organs ® systems ® human.

Shape of cells:

• may be round, oval, flat, cubical or columnar.

Size of cells (4-150 μm):

• may be small (as cerebellum), medium (as liver) or large (as ovum).

Function of cells:

1. absorption
2. digestion and excretion
3. respiration
4. Ion transport and secretion
5. sensation
6. conduction and contraction.

Structure of cells:

• each cell is composed of cytoplasm and nucleus.

Cytoplasm is formed of:

1. Cell organelles: permanent components which perform certain function essential for life.
2. Cell inclusions: temporary materials produced by cell activity and may disappear.
3. Cytoskeleton: a supportive network of microtubules, microfilaments and intermediate filaments.
4. Matrix: a colloidal medium for cellular contents. It is formed of proteins, fats, carbohydrates, minerals and ions.

 

Nucleus is formed of:

1. Chromatin material: the DNA and histone protein that forms the 46 chromosomes.
2. Nuclear membrane.
3. Nucleolus: a site of ribosome synthesis.
4. Matrix: a colloidal medium for nuclear contents. It is formed of proteins, fats, carbohydrates, minerals and ions.

 

Cell organelles

• Cell organelles are permanent components which perform certain function essential for life. They may be:

Membranous (contain enzymes):

• as cell membrane, mitochondria, endoplasmic reticulum (rough endoplasmic reticulum and smooth endoplasmic reticulum ) , Golgi apparatus, lysosomes .

Non-membranous (do not contain enzymes):

• as ribosomes, centrioles .

 

1. Cell membrane = plasma membrane = plasma lemma:

• Membranous cytoplasmic organelles concerned with cell covering. Its thickness is 7.5-10 nm , cell membrane structure seen in light microscope and electron microscope :  

light microscope:

• not seen (except with silver or PAS).

electron microscope: 

• a tri-lamellar membrane (3 layers): dark - light - dark.

The cell membrane structure is :

1. Proteins (60%): present as:

• Peripheral proteins: small molecules (at outer and inner surface).
• Integral (trans-membrane) proteins: small and large molecules (at the middle).

2. Lipids (30%): present as:

•  Phospholipid bilayer.

1. each phospholipid molecule has:
2. Hydrophilic polar (charged) heads (at outer and inner surface).
3. Hydrophobic non-polar (noncharged) tails (at the middle).

•  Cholesterol molecules: at the inner cytoplasmic surface.

3. Carbohydrates (10%):

• which form the cell coat (glyco-calyx) by:

1. Linking to outer protein ® glycoproteins.
2. Linking to outer lipid ® glycolipids.

v Function of cell membrane:

1. Phagocytosis:

• engulfing solid materials (called phagosomes) in.

2. Pinocytosis:

• engulfing fluid materials (called pinocytic vesicles) in.

3. Exocytosis:

• expulsion of residual bodies out.

4. Passive diffusion (with concentration gradient):

• for gases, ions .

5. Active transport

• against concentration gradient by using energy of ATP : for ions, amino acids , NB: Sodium-potassium pump (® k+ in and Na+ out) is an example of active transport.

6. Selective transport

• through binding material with receptors present on glyco-calyx :  for viruses, drugs, hormones .

7. Modification to:

• Microvilli: many projections (each contains actin microfilaments) – as in intestine, liver .
• Cilia: many projections (each contains 20 microtubules "9×2 + 2") – as in trachea, oviducts.
• Flagellum: one projection containing 20 microtubules "9×2 + 2" - as in sperm.
• Basal enfolding for mitochondria: to facilitate active transport of ions - as in kidney.

8. Role of glycocalyx:

• Receiving chemical massages and neuro-transmitted signals (through its receptors).
• Antibodies on its surface ® immunity or allergy.
• Formation of cellular junction ® adhesion of adjacent cells.
• Formation of basal lamina (of basement membrane) with type IV collagen of CT.

2. Mitochondria:

Definition :  

• Membranous cytoplasmic organelles concerned with respiration and energy production.
• Mitochondria are contractile, motile and can divide as bacteria (every 10 days).

light microscope

mitochondria appear as granules, rods or filaments (blue with iron hematoxylin and green with Janus green).

electron microscope

• mitochondria are oval with 2 membranes and 2 spaces:

1.  Outer smooth membrane.
2.  Inner folded membrane: forming folds (cristae).
3.  Inter-membranous space: containing enzymes of oxidative decarboxylation.
4.  Inter-cristal space: containing enzymes of Kreb’s cycle.

Function of mitochondria:

1. Regulation of calcium and magnesium metabolism.
2. Mitochondria are the power house of the cell as they supply energy to all cellular activities as muscle contraction, active transport, protein synthesis.
3. Mitochondria are the site of oxidative decarboxylation and Kreb’s cycle (so each hepatocyte contains ± 2000 mitochondria).

 

3. Endoplasmic reticulum:

• Membranous cytoplasmic organelles concerned with protein and lipid synthesis.
• Types: there are two types of endoplasmic reticulum:

Rough endoplasmic reticulum (rER): covered with ribosomes.

light microscope

•  a basophilic network beside nucleus (its basophilia is due to ribosomes).

EM: communicating wide tubules (cisternae) more demonstrated in protein-forming cells (as fibroblasts, plasma cells, pancreatic cells …).

Function of Rough endoplasmic reticulum :

Protein synthesis (with the aid of ribosomes).

Protein storage then delivery to Golgi apparatus as transfer vesicles.

 

Smooth endoplasmic reticulum (sER): not covered by ribosomes.

LM: not seen (due to absence of ribosomes).

EM: communicating narrow tubules more demonstrated in steroid-secreting cells (as endocrine cells …).

Function of Smooth endoplasmic reticulum:

Lipid synthesis and glycogen storage.

Detoxification of harmful substances (as drugs …).

Regulation of mineral metabolism as:

Calcium in muscle cells ® muscle contraction.

Chloride in gastric cells ® Hcl formation.

4. Golgi apparatus (secretory apparatus):

Membranous cytoplasmic organelles concerned with packaging and secretion of protein.

LM:

Golgi apparatus appears as a network at the apex of secretory cells and around nucleus of nerve cells. Golgi apparatus is stained brown by silver - not stained by hematoxylin and eosin (giving negative Golgi image).

electron microscope : 

It is formed of three types of vesicles:

• Transfer vesicles (small and rounded): they contain protein particles coming from Rough endoplasmic reticulum.
• Flatted vesicles (Golgi sacules): they are stack of vesicles with immature convex (cis) face and mature concave (trans) face.
• Secretory vesicles (large and rounded): they contain modified condensed and packed proteins (that comes out by exocytosis).

v Function of Golgi apparatus:

1. Modification, condensation and packaging of secretory products of the cell (by transferase and sulfatase enzymes).
2. Shares in cell membrane formation and repair after exocytosis.
3. Shares in lysosome formation (with the aid of ribosomes and Rough endoplasmic reticulum )