Cytology /lec
Dr.sarab
Endomembranous system :The eukaryotic cells of mammals are characterized by presence of highly organized
endomembranous system . It includes all cell organelles except the ribosomes, mitochondria
and centriole.
It is formed of:
1. Plasma membrane.
2. Outer nuclear envelope
3. Endoplasmic reticulum
4. Golgi- complex
5. Lysosomes
6. Vesicles
The members of the endomembranous system are
connected together. Smooth ER, rough ER, and outer
nuclear membrane are directly connected together by
membranous structures, while the other membranes
connected together indirectly by transport vesicles .
Transport vesicles arise from RER and fuse in Golgi
complex, while secretory vesicles arise from Golgi
complex and fuses with the apical plasma membrane.
Endoplasmic Reticulum (ER)
ER is one of the cytoplasmic organelles concerned with protein synthesis. It appears as
basophilic material filling the basal cytoplasm of pancreatic acinar cells under light microscope
and called ergastoplasm. In neuron it appears as basophilic granules around the nucleus called
Nissl’s granules.
Under EM it appears as a network composed of interconnected membranous tubules, parallel
lamellae, cisternae or dilated sacs
ER network is connected with the outer nuclear membrane, and contains different sets of enzymes in the lumen, and on cytosolic and
luminal surfaces. There are two types of ER:
a. Rough ER
b. Smooth ER
Rough ER
It is called ergastoplasm in pancreatic acinar cell and Nissl’s
granules in neurons and formed of a network of lamellae,
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cisternae, or saccules and sacs according to the rate of synthesis and transport of proteins from
it to Golgi complex. Their cytosolic surface is covered by ribosomes . They form the Nissl’s
granules in nerve cells .
How does RER synthesize proteins?
Synthesis of proteins
The role of RER in protein synthesis is summarized in the following steps:
Lamellae of RER
Nucleus
Ribosomes
Cisternae of RER
1. Ribosomes synthesize the polypeptide chain by translation of the genetic language of mRNA
into amino acid language of protein.
2. Proteins pass through pores (translocons) and accumulate within the lumen of RER-cisternae
or lamellae.
3. Proteins are modified by adding carbohydrates (glycosylation of proteins) or lipids (lipidation
of proteins).
4. Proteins are folded and condensed by enzymes called chaperons.
5. Then proteins are stored in transport vesicles or microvesicles that separate by budding and
fuse in Golgi-cisternae
Rough ER is well developed in cells producing large amount of proteins such as pancreas,
plasma cells and glandular epithelial cells.
Smooth ER
It is composed of a network of smooth tubules; free of ribosomes . Smooth ER develops from
rough ER and is connected directly to it.
Functions of smooth ER:
1- It acts as a site for synthesis and metabolism of lipids (phospholipids, steroids and fatty
acids).
2- It contains enzymes for detoxification of chemicals including drugs and pesticides.
3- Smooth ER is well developed in liver, muscles and steroid hormone producing endocrine
cells.
Ribosomes
There are three types of cytoplasmic RNA:
a. Messenger RNA (mRNA)
b. Transfer RNA (tRNA)
c. Ribosomal RNA (rRNA)
Ribosomes appear as basophilic granules under light microscope and as electron dense
granules consisting of two subunits (small and large subunits) under electron microscope.
Ribosome is the machine of protein synthesis .
Prior to protein synthesis, the ribosomal subunits are present as separate entities, which are
inactive.
In bacterial ribosome, the large subunit is consisted of three molecules of rRNA in
combination with forty five different proteins, while the small subunit is composed of one
molecule of rRNA and thirty three different proteins.
Ribosomal subunits bind together forming active or functioning ribosome when the two
subunits attach to mRNA.
In protein synthesis many active ribosomes bind to mRNA forming polyribosomes. Free
polyribosomes or polysomes attaching mRNA synthesize proteins to be used by the cell itself.
Ribosomes attaching RER synthesize protein to be secreted outside or used by the cell.
Golgi complex
It is a station for modifying, sorting and packaging of the synthesized protein to be exported
outside the cell or to be used inside the cell. It is also called Golgi –body or Golgi –apparatus
and appears as dark brown structures by light microscope. It made up of membranes similar to
those of smooth ER .The membranes form the walls of a number of flattened sacs are stacked
over one another .Towards their margins the sacs are continuous with small rounded vesicles
.Their lumen is not communication with that of ER. Material from ER reaches the Golgi
complex through vesicles. By EM it consists of 3 structures:
a. Cis-Golgi is the region nearest the nucleus .
They cover the convex face of or the immature face of Golgi complex. They receive the
newly synthesized protein from RER by microvesicles or transport vesicles (They are smaller in
size and carry immature protein synthesized in RER. They arise from RER and fuse in
CGN.)They are consisted of a network of interconnected cisternae called Cis-Golgi Network
(CGN). They are site of protein phosphorylation.
b. Trans-cisternae is the region nearest the cell membrane. They form the concave face or
mature surface of Golgi. From it arise the secretory vesicles which carry the modified or mature
proteins. They consist of interconnected network called Trans-Golgi Network (TGN). They are
the site of performs the following functions:-
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1-proteolysis of some proteins converts them from inactive to active forms.
2-add sugar residues to proteins.
3-the various substances are stored out and packed in appropriate vesicles .The latter may be
secretory vesicles , lysosomes transport of membrane to the cell surface.
c. Medial Golgi is the intermediate part.
They are located between Cis – and trans-Golgi cisternae. They are connected with them by
transport vesicles. Here sugar residues are added to proteins to form protein-carbohydrate
complex.
1. Microvesicles or transport vesicles.
They are smaller in size and carry immature pro-tein synthesized in RER. They arise from RER and fuse in CGN.
2. Secretory vesicles.
They are larger in size than transport vesicle and carry mature proteins that have been modified in Golgi cisternae.
They arise from TGN and fuse with plasma membrane to release their content outside the cell in exocrine or
endocrine glands.
Secretory vesicles that contain hormones remain in the cytoplasm until the cell receives a signal that determines the
amount of secretion.
Secretory vesicles containing hydrolytic enzymes form primary lysosomes to digest the intracellular foreign or
useless components surrounded by unit membranes.
Functions of Golgi apparatus:
1. Modification of Proteins:
Golgi-complex contains different sets of enzymes that are able to modify the received newly
synthesized immature protein.
The processes of protein modification include:
Glycosylation: It is the process of addition of carbohydrates and takes place in middle and
trans-cisternae. It results in formation of glycoprotein.
Phosphorylation: It is the process of addition of phosphate group and occurs in Cis-cisternae.
Sulfation: It is the process of addition of sulfate group into amino acids of protein.
Lipidation: it is the process of formation lipoprotein.
2. Processing and Packaging of Proteins: In the cisternae of Golgi-apparatus the processing of
received immature proteins from RER occurs. After modification and condensation proteins
become mature, and then proteins are stored within the secretory vesicles.
3. Sorting of proteins.
In Golgi apparatus proteins are sorted according to which target they are synthesized.
Golgi adds a sorting signal (carbohydrate) to direct the protein toward specific organelle.
4-G.A.plays an important role in keeping the cell membrane and cell coat in a good condition .
The covering envelopes of the secretory products close perforated areas of the cell membrane
after excretion of these products.
5-G.A . and the E.R. share in the
formation of lysosomes.
Lysosomes :
They form the digestive system of the cell. They break down all non-functional or harmful
complex macromolecules into simple molecules which can be reused by the cell .
Composition: They are unit membrane bounded digestive spherical organelles. Lysosomes contain concentrated hydrolytic enzymes include (protease, lipase, carbohydrases, and acidic
phosphatase); about forty types of hydrolytic enzymes. They are active in acidic environment at
pH 5.0. They function to break down of proteins, nucleic acids, oligosaccharides and
phospholipids. They are present in all cells except mature erythrocytes.
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Types of lysosomes:
1. Primary lysosome
It is a small vesicle that arises from Trans - cisternae of Golgi-complex and contains homogenous material. It carries concentrated hydrolytic enzymes, but still not engaged in the digestive
process.
Its fusion with membrane bounded endosome, phagosome or autophagosome containing material to be digested results in formation of secondary lysosome.
2. Secondary lysosome:
It is a larger than primary lysosome and composed of membrane bounded vesicle containing
heterogenous material . It results from fusion of primary lysosomes and membrane
bounded vesicle or endosome that contains material to be digested (ex. Bacterial cell.).
The intracellular functions of lysosomes:
1- Digestion of complex molecules.
2- Digestion of organelles that lost their ability to work in the
cell.
3- Break down of ingested foreign materials.
4- Lysosomes are responsible for cellular death.
Apoptosis is a normal process of programmed cell death due to
release of the lysosomal enzymes into the cytosol.
Removal of cells forming webs between the fingers in
developing hand and removal of the upper layer of skin and
intestinal wall are examples for apoptosis.
Hypoxia – oxygen deficiency increases the cellular acidity that
causes releasing of lysosomal enzyme and digestion of cell
contents and death.
Lysosomal Storage Disease - Hereditary deficiency of
lysosomal enzymes leads to accumulation of lipid materials of cellular membranes in nerve
cells which results in mental retardation and death. This disease is called Tay-Sach,s disease.
And Lysosomal glycogen storage disease in which there is abnormal accumulation of
glycogen.
Sources of the Lysosomal Digested
Materials
The followings are the different sources for materials that are digested by lysosomes. Materials
may be taken from outside the cell or found inside the cell :
1. Phagocytosis.
Engulfing of large foreign solid particles such as bacterial cells results in formation of
membrane encapsulated phagosome containing the foreign materials. Fusion of primary
lysosome with the phagosome results in formation of secondary lysosome or digestive
vacuole.
2. Pinocytosis.
It is the process of internalization of fluid by membrane–bounded vesicles – pinocytotic vesicle
or endosome. Fusion of the endosome with primary lysosome results in formation of
multivesicular body or secondary lysosome or digestive vacuole.
4. Receptor –Mediated Endocytosis.
It is the process of internalization of specific legends into the cell within Clathrin –coated
vesicles. Then the cytoplasmic coated vesicles lose their coats and transform to endosomes.
Fusion of the primary lysosome with the endosome results in formation of secondary lysosome
or digestive vacuole.
5. Cytosolic Proteins.
They are malformed protein or inactive enzymes are selectively taken up by proteasomes.
The intracellular degradation of materials starts generally by fusion of endosome, phagosome or
autophagosome with primary lysosome that results in:
1. Formation of digestive vacuole or secondary lysosomes.
2. The useful break down product leaves the digestive vacuole into the cytoplasm while the
undigested material condenses within the vacuole to form residual body.
3. Reduction of plasma membrane during endocytosis is neutralized by increase of plasma
membrane during exocytosis.
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Fig. show Mechanism of internalization of specific ligand by receptor mediated endocytosis through formation of Clathrincoated vesicles that loss their coat and transform into uncoated endosome. Endosome receives hydrolytic enzymes from
primary lysosomes to digest the internalized materials. Notice that the receptors and clathrin molecules recycle again into the
cell membrane.
Peroxisomes or Microbodies
They have the following characters:
1- Small spherical to ovoid membrane –bound vesicles.
2- Self- replicating organelles; they increase in size and undergo fission to form new peroxisomes.
3- Present in almost all animal cells. (they are most prominent in cells of liver and renal tubules)
4- Peroxisome contains more than forty oxidative enzymes.
Functions of peroxisomes
1- They catablize the long fatty acid chains into acetyl Co-enzyme A, and Hydrogen ions that
combines with oxygen ions to form hydrogen peroxide (H2 O2).
2- Acetyl Co-enzyme A is used for cellular metabolism, while hydrogen peroxide is able to
detoxify various noxious (e.g: ethanol), and kill microorganisms. Peroxisomes are extensive in
liver and kidney .
Defect in enzymes of peroxisomes can result in metabolic disorder associated with storage of
abnormal lipids in some cells(brain , adrenal).
Other peroxisomes contain the enzyme (catalase) which destroys hydrogen peroxide, thus
preventing the H2O2 accumulating in the cell.
Proteasomes
They are small organelles composed of protein complexes. They are responsible for the
following mechanisms for cytosolic proteolysis of specific types of proteins (fig. 51).
Proteasomes have functions like:
1. Degradation of proteins that act in metabolic regulation
2. Elimination of denatured, damaged, or malformed proteins
3. Cleavage of antigenic proteins into small polypeptide fragments (epitopes) in antigen
presenting cells (APCs).
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Fig.: Lipid globules are surrounded by smooth endoplasmic reticulum, SER. It is formed of network free of ribosomes.
B
A
A
Fig.: Electron micrographs showing different forms of RER according to its activity and the rate of protein synthesis
that accumulates inside its lumen and the released protein.
A. Flattened parallel lamellae covered by ribosomes.
B. Network of dilated sacs or cisternae filled by the synthesized proteins.
Fig.: Diagram illustrating the sources of the materials digested by lysosomes; from outside or inside the cells.
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