Classification of Cytoplasm (With Diagram) | Term Paper | Cell | Biology

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Term Paper on the Classification of Cytoplasm

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The cytoplasm is the protoplasm which surrounds the nucleus and is bounded peripherally by the cell membrane. It may be homogeneous, vacuolated, granular, reticular or fibrillar. The ground substance of the protoplasm, hyaloplasm, contains a number of bodies and structures, vacuoles, and so on and also a number of very tiny particles which undergo active movement—Brownian movement.

The cytoplasm is capable of performing different kinds of work directed by the nucleus. Specialisations, of the cytoplasm for special functions, the appearance as well as the protoplasmic constituents are also changed from cell to cell. Hence certain groups of cells will be identified by their nuclei and also by the appearance and the amount of cytoplasm.

In the light microscope the cytoplasm can be classified into two groups as [Flow chart 1.1]:

i. Cytoplasmic organelles.

ii. Cytoplasmic inclusions.

1. Cytoplasmic Organelles:

I. Membranous Organelles:

A. Plasma Membrane:

The plasma membrane or plasmalemma or cell membrane is the outer covering of the cell and is a flexible, responsive and dynamic structure. This membrane isolates the individual cell from its neighbours and takes part in the maintenance of the internal environment by active transport of ions and nutrients.

Under the light microscope, the membrane is thin and invisible, and sometimes the limits of a cell may be distinguished because the cell membrane is folded to form a cuticular or brush border or because mucoprotein or other cellular secretion is present to coat the membrane. By the electron microscope it is claimed to be about 80Å (or 80A.U.) thick and shows the membrane to be a trilaminar (triple-layered) structure.

This basic trilaminar structure of all cell membrane is generally described as unit membrane. At high magnification with electron microscope, the cell membrane consists of double (bimolecular) layer of lipid molecules (light – stained), which are sandwitched within the two densely stained protein layers. It is also suggested that the thickness of lipid layer is about 30Å and that of the each protein layer is about 25Å. Total thickness of the cell membrane then becomes 80Å.

Lipid layer is mostly phospholipid of which the head end contains the water- soluble and positively charged phosphate group (polar or hydrophilic) while the tail end contains the water insoluble and negatively charged lipid group (non-polar or hydrophobic). Phospholipid molecules of the lipid layer are arranged in two rows where the hydrophobic ends line up side by side in same row but abutting on the hydrophobic ends of the other row (Fig. 1.3). Thus the nonpolar groups of lipid molecules face each other but the protein molecules that form the inner and outer layers of the unit membrane are adsorbed on the polar groups.

Functions of Plasma Membrane:

It facilitates the transport of materials across it. The process may be direct, possibly controlled by enzymes, as in the case of sodium or potassium. Small vesicles, by a process of pinocytosis (drinking by cells), encircle and carry fluid within it across the membrane. By the pinocytic process fluid of smaller molecules (0.01-2.0µ) can be gulfed in (Fig. 1.4).

The inducer can increase greatly the process of pinocytosis. Reverse pinocytosis or emeiocytosis (vomiting by cell) is a process under which the granule or vacuole surface membrane fuses with the cell membrane and the fused portion then ruptures out within the pericapiliary space leaving the cell membrane intact.

Rhopheocytosis is the process by which larger molecule of fluid are taken in. Cytopemphis or podocytosis is the process by which the molecules are engulfed into the cell but the vacuole containing those moves across the cell and discharges its contents onto another surface of the same cell.

This process is commonly seen in kidney tubular cells and intestinal epithelial cells. The phago­cytosis (eating by cells) is similar to engulfing of solid materials by the amoeba. When various end products of digestion diffuse out of the vacuole into the cytoplasm, the indigestible residue is eliminated by reverse phago­cytosis.

In addition to transport, the cell membrane:

(a) Helps in the protection of cell,

(b) Receives stimuli from the outside,

(c) Takes in food, and

(d) Excretes waste products.

When broken, it is quickly regenerated from the cytoplasm possibly with the help of surface tension.

B. Endoplasmic Reticulum (Ergastoplasm):

These consist of network of canals (tubules) and vesicles (cisteranae). These are three dimensional and bounded by membrane of about 80Å in thickness. The elements of the endoplasmic reticulum may connect intermittently with the plasma membrane at one hand and on the other hand with the outer nuclear membrane.

Two types of endoplasmic reticulum have been recognized:

i. Rough-Surfaced Endoplasmic Reticulum:

This reticulum is studded with osmeophilic granules — the ribosomes lying in rows in contact with the membranes of the endoplasmic reticulum (Fig. 1.5). The roughness of the membrane is due to the presence of these granules—Palade granules.

ii. Smooth-Surfaced Endoplasmic Reticulum:

This type of endoplasmic reticulum does not possess osmeophilic granules – the ribosomes at the outer border of the membrane. This is why it is smooth.

Functions of Endoplasmic Reticulum:

As the smooth-surfaced endoplasmic reticulum is very abundant in the interstitial (Leydig) cells of the testis and in cells of the corpus-luteum, this reticulum is concerned with the synthesis of steroid hormones.

i. In the parietal cells of the gastric mucosa, it is concerned with secretion of hydrochloric acid.

ii. In the skeletal muscle, it (sarcoplasmic reticulum) is concerned in some way with binding of the Ca⁺⁺ ions and also plays in conducting impulses in the substances of muscle cells.

iii. In the liver cells both types of reticula are concerned with the synthesis of protein and carbohydrate.

Microsome and Microsomal Fractions:

Different fractions of cells can be obtained by centrifugation. After removing the nuclei and mitochondria, one such cell fraction obtained by centrifugation at about 20,000 to 100,000g was known as microsomal fractions and was erroneously known to be a separate type of cell organelle. But in electron microscopic studies of this microsomal fraction reveal that this is nothing but a chief composition of rough-surfaced and smooth-surfaced membranous vesicles of the endoplasmic reticulum.

In the intact fractions the ribosomes are adherent to the rough-surfaced vesicles. Smooth-surfaced vesicles are also present in some portions of microsomal fractions. So there is nothing to believe that the microsome or microsomal fractions have got a discrete and independent structure in the cytoplasm.

C. Golgi Apparatus (Golgi Complex):

The structure (Fig. 1.6) looks like a network of fine threads (Golgi network) or irregular granular material. It is usually located near the nucleus, and in the gland cells found between the nucleus and apex of the cell.

Following main structures can be observed in Golgi apparatus under electron microscope:

1. Flattened vesicles.

2. Secretory vesicles.

3. Micro vesicles.

1. Flattened (Distended) Vesicles:

These are the most prominent vesicles in the Golgi apparatus. In a longitudinal section it looks like tubules arranged in stack. The peripheral portion of each vesicle becomes distended with its content and then the distended portion becomes separated from the vesicle to constitute the ovoid-to- round-shaped secretory vesicles.

2. Secretory Vesicles:

Under electron microscope the se­cretory vesicles are not normally visible until these are budded off from the distended peripheral end of flat­tened vesicles. These vesicles become electron dense only after being distended with their contents – the pro­tein material. Recently it is known that protein material after-being synthesised in the rough surface (granular) of the endoplasmic reticulum (vide Ribosomes) is stored in the flattened vesicle and the secretory vesicle as well.

The contents of the secretory vesicles are ultimately discharged at its cell surface as zymogen granules. Pro­tein materials, being synthesised in the ribosome of the endoplasmic reticulum, are not transferred to the flat­tened vesicles directly but through a via medium—the microvesicles of the Golgi apparatus.

3. Microvesicles:

These are small in size and their diameters are about 40 mµ. Microvesicles have been considered to be the carrier systems of the protein molecules synthesised in the rough surface of the endoplasmic reticulum to the flattened vesicles. From autoradiograhic studies Caro and Palade have described that protein molecules, after being synthesised, first appear in the microvesicles and then in the flattened vesicles or the secretory vesicles. In certain cells these are numerous in between the region of the rough-surfaced endoplasmic reticulum and the flattened vesicle.

Functions:

It is probably concerned with synthetic process of the cell, specially – secretions. The secretory substance, being synthesised by the endoplasmic reticulum, passes to the Golgi apparatus which possibly modifies the products of synthesis by concentrating and chemically altering it to some extent. In addition, Golgi apparatus independently synthesises polysaccharide part of glucoprotein secretion.

D. Mitochondria:

These are relatively solid bodies, granular, rod-shaped or filamentous in form and remain scattered throughout the cytoplasm (dimensions varying from 0.5 to 5.0 microns). They are surrounded by a trilaminar double membrane, the inner one of which remains folded and forms a number of partitions, the cristae mitochondriales.

These cristae may be complete, septate or incomplete. In the living cell they are found to move about. They are more numerous and longer in the young and active cells. The sites where Krebs cycle enzymes (cyclophorase system) are present in high concentration, like liver, kidney (Fig. 1.7) and heart possess mitochondria in a larger amount.

They are comparatively few in skeletal muscle fibres where the enzymes are also less. Numerous projecting particles known as elementary particles are present on the inner mitochondrial membrane and cristae, the fluid of the intramitochondrial space is called matrix. The matrix may contain small dense granules. It has been postulated that most of the enzymes of the mitochondria are present on the elementary particles, the coenzymes in the matrix, and inorganic ions like calcium and magnesium in the granules (Fig. 1.8).

The number and size of mitochondria give an indication of the energy requirements of a particular cell. Mitochondria are comparatively rare in cancer cells which derive their energy from anaerobic glycolysis. For light microscopy, mitochondria can be stained supravitally with Janus Green B. Sometimes they remain grouped together at one pole of the cell. Chemically they contain about 40 percent of fat (neutral fat, phospholipids and cholesterol) and two different kinds of proteins. Ribonucleic acid (RNA) remains in combination with protein as ribonucleoprotein (RNP).

Functions:

All the enzymes of the citric acid cycle with the exceptions of certain dehydrogenase are present in the fluid content of the mitochondria. The acetic acid – the breakdown products of pyruvic acids, fatty acids and also of amino acids are fed into the mill of Krebs cycle enzymes of mitochondria. In presence of oxygen, the Krebs cycle runs within the mitochondria with the catalysing help of another set of enzymes – respiratory enzymes.

These are flavoprotein enzymes and cytochrome, and present in the inner membrane of the mitochon­dria. These respiratory enzymes use certain products of Krebs cycle as substrate. These enzymes present in the mitochondria help in oxidative phosphorylation and are the site for formation of adenosine triphos­phate (ATP) which is the high energy-producing substance in the cell.

The mitochondria supply 95 percent of cell’s energy and are called power-house or pow­er-plant of the cell. Recently it has been studied that the mitochondria also possess some amount of deoxyribonucleic (desoxyribonucleic) acid (DNA). There is also some indication that RNA is also synthesised in association with DNA and this RNA helps in synthesising certain amount of pro­tein in the mitochondria.

E. Lysosomes:

The lysosome has been discovered and recognised as a separate cytoplasmic organelle. Its size varies from 0.25µ to 0.50µ. These are membranous ves­icles having a spherical and bag-like structure and are filled with hydrolytic enzymes capable of demolishing large molecules (protein, carbo­hydrate, lipids and nucleic acids) into fragments which may then be oxidised by the mitochondria.

The lysozymes are present in all animal cells except in the erythrocytes. Certain leucocytes contain a specific type of granules which are considered to be lysosomes. The enzymes of lysosomes are potent enough to digest its own cellular contents in which it inhabits but in ordinary conditions it is not so happened.

Under certain conditions it may digest its own cellular content and for this reason it is sometimes described dramatically as suicide bag. Lysosomal enzymes, like other proteins, are synthesised by the ribosomes of granular endo­plasmic reticulum of the same cell and are transported to the Golgi apparatus in the form of microvesicles for storage. The stored enzymes are ultimately budded off from the stack of Golgi saccules and developed into primary or inactive lysosomes.

Primary lysosome develops into active or secondary lysosome (autolysosome) only during intracellular digestion. The fusion of primary lysosome with the particle brought into the surface of the cell (phagosome) or the intracellular material gives rise to secondary or active lysosome. Lysosomes that digest the degenerated mitochondria or other intracellular structures are specifically described as cytolysosome.

Functions:

i. The general function of the lysosome is the intracellular digestion and for this reason it is sometimes described as digestive apparatus of the cell (Fig. 1.9). When a particulate or food substance comes in contact with cell surface, the substance is engulfed by the cell membrane forming a membranous vesicle with engulfed particle.

The lysosome thus meets with the particle and is fused. Hydrolysing enzymes of the lysosome thus digest the food particle. However this is happened in some kinds of cells, because in such cells the particulate is not utilised by the cell without the help of lysosomes.

ii. Another function is cell necrosis or autolysis. When the cell is damaged, the lysosomal digestive enzymes are released and digest off cellular elements. In case of degeneration of its own cellular organelles, such as mitochondria and ribosomes during acute anoxia, the lysosomes then digest them only to maintain the energy requirement of the cell.

iii. Phagocytosis is also a remarkable function of lysosome. Leucocytes contain lysosome like granules which destroy the ingested bacteria. During this process, the blood cells themselves are destroyed along with the bacteria. For this phagocytic (phagocytotic) functions, certain cells contain (macrophages and polymorpho-nuclear leucocytes) recognisable lysosomes. In damaged cells the numbers of lysosomes are increased greatly. Thus it is clear that lysosome stores safely a quite good number of destructive enzymes within the cells only to remove the foreign material through phagocytosis.

iv. It has been postulated that the lysosome brings about death of cells only to make space for the new cells. Systemic growths of cells are made through the planned death of cells through the lysosomal activity.

v. The rupture of lysosome is the stimulus for cell division and alteration of the behaviour of lysosome may be one of the causes of cancerous growth.

vi. Furthermore acrosomes of the spermatozoa appear to be a large lysosome.

II. Ribosomes or Claude’s Particles:

They are ribonucleoprotein in nature and are found scattered throughout the cytoplasm either singly or in groups (polyribosomes or polysomes) and range in size from 100 to 150A in diameter. They are so rich in RNA that they may contain as much as 60% of total RNA in the entire cell. These ribonucleoproteins are concerned with protein synthesis and their presence gives the membrane a strong basophilia. Cells responsible for the secretion of proteins have an abundance of granular reticulum.

Function:

Being attached to the rough-surfaced endoplasmic reticulum ribosomes (Fig. 1.5) synthesise protein and the canals of the reticulum work as passageways through which proteins move on way to Golgi apparatus. So ribosomes are protein factories.

III. Centrosome (Cylocentrum or Cell Centre):

It consists of another specialised part of clear cytoplasm, the centrosphere, containing in its interior two or more deeply staining particles – the centriole (generally arranged in pairs, i.e., diplosome) lying close to the nucleus in the resting cell. Electron microsco­py has revealed the centriole to be an empty cylinder which is 3 to 5µ long, the compact walls of which are made of thin parallel nine tubular structures longitudinally arranged. Each tubule consists, in turn, of three subunits or triplets. The pericentriolar bodies or satellites which are round in shape are attached to centriolar tubules by a filament of chromatid (Fig. 1.10).

Cenlriole is closely related to spindle formation during mitosis (normal cell division) and also in sustaining other fibrillar structure like cilia and flagella. At the beginning of cell division, the centriole and centrosome divide. A system of radiating lines, made up of microtubules, then grows out from each of the two newly formed centrioles and the whole structure, due to its star-like shape, is called aster. The two asters grow in size and repel each other till they occupy the opposite poles of the elongated cell. The diverging fibres from the two masters meet at the equator of the cell and form the achromatic spindle.

Along the fibres of this spindle half the numbers of chromosomes – formed by the breaking up of the nucleus in the mean-time are drawn towards each aster and then the cell divides. Each daughter cell thus carries one aster and half the number of chromosomes. After division the radiating lines (microtubules) of the astral system disappear leaving the centri­oles and the centrosome only. Nerve cells have got no centriole and are incapable of reproduction.

Functions:

Centrioles control polarisation of spindle fibres and play some part in their formation.

Plasmosin:

It is a constant and characteristic constituent of cytoplasm. It consists of elongated protein particles rich in deoxyribonucleoprotein. They join up lengthwise and form the so-called intracellular fibrils, e.g., tonofibrils in the epithelial cells, myofibrils in the muscles, and neurofibrils in the nerves.

Vacuoles:

They are demonstrated by placing living cells in a dilute neutral red dye solution. Some amounts of lipid material are often found around the vacuoles.

Nissl Bodies (Granules):

They are found in nerve cells.

2. Cytoplasmic Inclusions:

The cytoplasmic inclusions are not the living metabolic machinery of the body but are certain structures present in the cytoplasm of the cells.

These are:

1. Stored foods.

2. Secretion granules.

3. Pigments.

4. Crystals.

I. Stored Foods:

A healthy subject can withstand starvation: for weeks, only because of his stored foods in the cytoplasm. Every cell for its metabolic functions requires fuel and this fuel is always supplied from external source. When this external source fails to supply, the internal food source—stored foods in the cytoplasm thus maintain it for a certain period. These stored foods are the protein, carbohydrate and fat which are present as inclusions in certain cells.

(i) Carbohydrate:

It is absorbed from the intestine in the form of monosaccharide and stored in the cytoplasm of animal cells as macromolecules- glycogen. It is stored as such particularly in the liver cells and also in other cells too. In ordinary H and E stain, glycogen in the cells cannot be demonstrated because the glycogen is not stained with H and E. But in such preparation glycogen is dissolved and removed, keeping a characteristic pattern of irregular space with ragged border.

The cells, having completed devoid of glycogen, give a smooth appearance with H and E stain. Glycogen can be stained well with periodic acid-Schiff (PAS) which gives a brilliant red colour with glycogen. Like fat in the cell, carbohydrate does not tend to push nucleus side-wards of the cell. In electron microscope glycogen of the cytoplasm in unstained cell appears as pale amorphous areas. It has got a particulate appearance of granular material having 150Å to 400Å in diameters.

(ii) Fat:

It is mostly stored in connective tissue fat cells and it may also accumulate in the liver cells under certain conditions of dietary deficiency. In ordinary preparation, fat is generally dissolved out keeping a clear smooth and circular outline in the cytoplasm.

Because the clearing agent which is used has the property of dissolving the fat. Fat in the cell is demonstrated convincingly osmium tetroxide fixative which does not allow fat to be dissolved off by clearing agents. In the above fixative the fat looks like a dark brown or black mass.

Fat at first accumulates in the cell as droplets and then fuse to form large droplet. In a cut section these large droplets like a signet ring. A large amount of droplets tend to displace the nucleus towards the periphery of the cell. The spaces left over by the dissolved fats are not irregular but it is spherical and smooth. As already noted these differ from the spaces left by dissolved glycogen.

(iii) Protein:

It is rarely stored as cytoplasmic inclusions and cells at a certain stage consume their own cytoplasm as a stored food. Reserve of protein mainly exists in the matrix.

II. Secretion Granules:

Digestive enzymes and other fluid materials are synthesised from raw materials brought in the cytoplasm through the blood and tissue fluid. These materials remain in the cytoplasm as small globules or droplets of fluid and usually precipitated in the form of granules during fixation. These granules can be stained success­fully with special histochemical techniques.

III. Pigments:

There are certain pigments in the cytoplasm. In order to be visible these pigments do not require any dye for staining. These pigments are present as cytoplasmic inclusion in the cell.

They may be classified into two groups:

a. Endogenous type.

b. Exogenous type.

a. Endogenous Pigments:

These pigments are those which are synthesized within the body.

These are as follows:

i. Haemoglobin and Its Derivatives:

Haemoglobin is the iron containing pigment of the red blood corpuscles (R.B.C.). The red colour of the R.B.C. and the blood as well is due to the practice of the haemoglobin. In normal condition the life span of red cells are only a few months as they are destroyed by phagocytosis. The haemoglobin of these cells is broken down into haemosiderin (iron containing pigments) and haematoidin (non-iron containing pigments).

Haemosiderin is disposed in the cytoplasm of phagocytes as granules or an irregular mass. Normally it is also present in certain amount in phagocytes of spleen, liver, bone marrow, and the quantity is increased during rapid destruction of R.B.C. in diseased state. A green pigment, biliverdin, is the breakdown product of haemoglobin.

On reduction biliverdin gives rise to a yellow-brown pigment, bilirubin. Haematoidin is a breakdown product of haemoglobin during destruction of R.B.C. and is identical to bilirubin. Bilirubin does not contain iron and is very soluble. For this reason it is dissolved in the blood and is not stored in the cells and thus is continuously removed from the liver cells into the bile.

ii. Melanin:

This is a brown-to-black pigment present in the skin, eye, etc. This pigment is absent in the albino. The dark colour of the Negros is due to the presence of this pigment in a large amount. This pigment is present in the cytoplasm of cells as granules or cluster of granules. It is tyrosine derivative and its concentration in the cells alters during derangement of tyrosine metabolism.

b. Exogenous Pigments:

These are the yellowish pigments:

i. Carotene.

ii. Lipochrome.

Lipochrome is present in the vegetables. Besides these, certain dusts like carbons and minerals like lead and silver are also present. Carotene pigments sometimes accumulate in the cells when excess carrot is consumed. Sometimes it may look like jaundice as the carotene pigment gives a yellow colour of the skin and the body fluid. Lipochrome is also a yellow pigment present in certain cells particularly in older ones. Its quantity mostly depends upon the wear and tear of cells.

Furthermore the quantity of lipochrome in the cells is mostly dependent upon the activity of cells. It is present in the liver, muscle and cardiac cells. Recently it is described that lipochrome is present in the lysosome and is sometimes referred to as lipofusin.

It is soluble in fat solvent and present in several types of cells already described:

i. Dust:

Coal dust may be deposited in the body through inspired air. Pigmentation may occur in the system which may not be so harmful.

ii. Minerals:

Certain minerals like silver or lead taken through mouth may produce pigmentation in the body. Gray pigmentation of the body may occur due to taking excessive silver as medicine. Similarly excessive lead may produce lime in the gums.

IV. Crystals:

Certain cells such as Sertoli and interstitial cells of the testes contain certain protein aceous crystalline materials. In significance is quite unknown.