cytology 3  l General Histology & biology revision for dental students

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Cell organelles

 

Lysosomes:

• Membranous cytoplasmic organelles concerned with intracellular digestion. Its thickness is 0.2-0.4 μm.

Light microscope :

• lysosomes appear as dark granules (by acid phosphatase stain) in phagocytic cells (as macrophages).

Electron microscope : lysosomes appear in two forms:

Primary homogeneous lysosomes:

• They contain only hydrolytic enzymes as lipase, proteases, acid phosphatase ... (formed by ribosomes and modified by Golgi apparatus).

Secondary heterogeneous lysosomes:

• They contain hydrolytic enzymes and phagosomes (phago-lysosomes).

Function of lysosomes:

1. Intra-cellular digestion of nutritive phagosomes and old mitochondria, so they are named "digestive apparatus" , NB: after digestion the remaining parts (residual bodies) are exocytosed.
2. Killing and destruction of phagocytized bacteria and viruses.
3. Hydrolysis of thyroid colloid (in thyroid follicles) to activate its hormone.
4. Hydrolysis of kidney proteins (in glomerular filtrate) to help their reabsorption.
5. Facilitate penetration of sperm head to ovum (during fertilization).
6. Post-mortem autolysis, so they are named also "suicidal bags".

 

Ribosomes

• Non-membranous cytoplasmic organelles concerned with protein synthesis.

Light microscope :

• ribosomes are oval basophilic structures (rRNA) formed in nucleolus.
• They are stained blue by hematoxylin and appear in the following forms:

1. Attached ribosomes (on rER): for synthesis of secreted proteins as hormones, enzymes.
2. Free ribosomes: for synthesis of cytoplasmic proteins.
3. Isolated bodies: as Nissl granules of nerve cells.
4. Spiral structures: named "polysomes".
5. Fragmented structures: named "microsomes".

Electron microscope : each ribosome is formed of:

• Small ribosomal subunit.
• Large ribosomal subunit (formed of two parts between which there is a groove for the polypeptide chain).

Function of ribosomes:

• Protein synthesis (with the aid of t-RNA) through translation of mRNA nucleotides to a sequence of amino acids that bound together to create a polypeptide chain (newly formed protein).

Protein synthesis occurs through the following steps:

1. Transcription: formation of a complementary mRNA in the nucleus from genetic DNA code in presence of RNA polymerase enzyme.
2. Translation: formation of a polypeptide chain of amino acids (newly formed protein) in the cytoplasm by the aid of ribosomes, mRNA and t-RNA.
3. Secretion: the newly formed protein leaves ribosome to enter rough endoplasmic reticulum then enters Golgi apparatus as a transfer vesicle and finally exocytosed as a secretory vesicle.

NB:

• Features of "protein-forming cells": euchromatic nucleus with prominent one or more nucleolus - many mitochondria, many rER, well-developed Golgi apparatus and many secretory vesicles , Examples: CT fibroblasts …

• Features of "steroid-forming cells": euchromatic nucleus with prominent one or more nucleolus - many mitochondria, many sER, well-developed Golgi apparatus and many lipid droplets. Examples: suprarenal spongiocytes .

 

Centrioles:

• Non-membranous cytoplasmic organelles concerned with cell division
• centrioles are not present in nerve cells and erythrocytes because these cells cannot divide.

Light microscope :

• two tubular structures at right angle with each other (near the nucleus). They can be stained with hematoxylin.

Electron microscope :

• each centriole is a hollow cylinder formed of 27 microtubules (arranged as 9 radiating triplets) embedded in a protein matrix giving cartwheel appearance in cross section.

Function of centrioles:

1. Cell division by their duplication and movement of each pair towards one pol to form the mitotic spindle.
2. Formation of cilia and flagella.

• Cilia: are motile hair-like processes extending from the cell membrane of epithelial cells in respiratory system, ♂ and ♀ genital system …
• Flagella: are similar to cilia but single, longer and present only in sperms for their movement.

 

Each cilium or flagellum is formed of:

• Rootlet: 7 microtubules (embedded in the cytoplasm).
• Basal body: a migrated single centriole formed of 27 microtubules (arranged as 9 radiating triplets).
• Shaft: 20 microtubules (arranged as 9 radiating doublets + 2 central singlets).

 

Nucleus

• Nucleus is present in all cells except RBCs and platelets.

Shape of nucleus:

• may be flat - round - oval - rod-shaped - kidney-shaped - bilobed - segmented - lobulated.

Site of nucleus:

• may be central - eccentric - apical - basal.

Size of nucleus:

• may be small, medium or large.

Number of nuclei:

• may be one - two (as hepatocytes, superficial cells of transitional epithelium …)
• many (as osteoclasts, skeletal muscle cells …).

Function of nucleus:

1. carrying of genetic information (by DNA)
2. control of cellular activities (by RNA)
3. control of cell division.

Structure:

• the nucleus is formed of chromatin material, nuclear membrane, nucleolus and matrix , NB: nuclear membrane, nucleolus and matrix disappear during cell division.

Chromatin material:

• It is the site of genetic information (the 46 chromosomes).

Light microscope :

• basophilic granules formed of nucleo-proteins (nucleo-somes) which is DNA and histone protein.

Electron microscope :

• it is formed of peripheral chromatin, nucleolus associated chromatin and chromatin islands.

Types of chromatin material: there are two types of chromatin:

Eu-chromatin

Light microscope :

• Light staining (so cannot be seen by LM)

Electron microscope :

• Extended threads (less coiled DNA)

Activity

• More active (as in hepatocytes)

Hetero-chromatin

Light microscope :

• Dark staining (so can be seen by LM)

Electron microscope :

• Condensed particles (more coiled DNA)

Activity

• Less active (as in lymphocytes)

Nuclear membrane:

Light microscope :

• It appears as a single basophilic membrane (due to basophilic peripheral chromatin).

Electron microscope :

• it appears as an envelope formed of two membranes (separated by peri-nuclear space):

1. Inner fibrillar membrane with peripheral chromatin on its inner surface.
2. Outer granular membrane (continuous with rough endoplasmic reticulum) with ribosomes on its outer surface.
3. Nuclear pores are present in nuclear membrane and covered by diaphragms to regulate RNA passage.

Nucleolus

• It is the site of rRNA (ribosome) synthesis. It is well developed in protein forming cells and may be numerous.

Light microscope :

• one or two rounded basophilic masses (due to basophilic nucleolus associated chromatin).

Electron microscope :  it is formed of two areas:

• Light area.
• Dark area:

1. Pars amorpha (DNA cod of rRNA).
2. Pars fibrosa (early rRNA).
3. Pars granulosa (mature rRNA).

Matrix = nuclear sap = nucleoplasm = karyolymph:

• A clear colloidal medium for nuclear contents and RNA movement , It is formed of proteins, fats, carbohydrates, enzymes, minerals and ions, According to amount of matrix the nucleus may be:

1. Open-face nucleus: with great amount of matrix (as hepatocytes …).
2. Condensed nucleus: with less amount of matrix (as lymphocytes …).

Nucleic acids (DNA and RNA)

• There are two types of nucleic acids:

RNA (Ribo Nucleic Acid):

• RNA is present in nucleus and cytoplasm.
• The RNA molecule consists of one chain (single helix) formed of ribose sugar and phosphate.

Types of RNA:

1. Messenger RNA (mRNA): arise from DNA and act as code carrier.
2. Transfer RNA (t-RNA): arise from DNA and act as amino acid transporter.
3. Ribosomal RNA (r-RNA): arise from nucleolus and act as protein factories.

DNA (Deoxy-ribo Nucleic Acid):

• DNA is present in nucleus and mitochondria.
• The DNA molecule consists of two chains (double helix) formed of deoxyribose sugar and phosphate.
• Catalyzed by DNA polymerase

 

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