Mrs. Ofelia Solano Saludar

Department of Natural SciencesUniversity of St. La Salle

Bacolod City

CYTOLOGY: ORGANELLES

A mesh of interconnected membranes that function in protein synthesis and transport.

Irregular network of branching and anastomosing tubules maybe expanded into broad flat saccules called cisternae.

Cisternae become associated to form lamellar systems of flat cavities; there may also be isolated vesicles.

Basic areas of cytoplasm under LM, called chromidial substance or ergatoplasm correspond to aggregation of ER under EM.

ROUGH ENDOPLASMIC RETICULUM (rER) or

ERGATOPLASM

The degree of development of the reticulum & the relative proportions of its tubular, cisternal & vesicular elements vary greatly in different cell types & in different phases of the physiological activity of the same cell type. The amount of granular reticulum in a cell is related to protein synthesis; in pancreatic acinar cells which produce enzymes, it is very extensive.

These membranes may connect with the nuclear envelope, surface plasma membrane, & with smooth or agranular ER.

It is totally absent in mature erythrocytes and some small lymphocytes.

Besides its role in protein synthesis, the ER is associated with synthesis of ascorbic acid, and contains detoxifying enzymes in a liver cell.

The basophilia of these structures reside in small particles of ribonucleo-protein called

ribosomes, adhering to the outer surface of the limiting membrane of the reticulum.

rER with ribosomes

Whether attached or occurring free in the cytoplasm, ribosomes are usually associated in clusters called polysomes or polyribosomes.

When cells are disrupted and subjected to differential centrifugation, the rER is broken up to form microsomes, small, membrane-bound bodies with their external surfaces studded with ribosomes

Ribosomes & Polysomes, liver cell

Are slightly larger in eukaryotes. Consists of a small and larger subunits, composed of 4 types of rRNA

synthesized in the nucleoli, and almost 80 different kinds of proteins.

RIBOSOMES

What happens to proteins synthesized

by the ribosomes?

Proteins can move from one compartment to

another by gated transport (red),

transmembrane transport (blue), or vesicular

transport (green). In principle, a signal could be required for either

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Unassociated with ribosomes, its membranes consisting only of interconnected tubular and vesicular elements.

The sER may connect with rER, the plasmalemma and the Golgi apparatus.

It is usually a close-meshed tridimensional network of tubules and seldom takes the form of cisternae.

SMOOTH or AGRANULAR ENDOPLASMIC RETICULUM

(sER)

FUNCTIONS: Synthesis of membrane phospholipids, cholesterol, and

ceramide. Release and recapture of Ca+2 and the cycle of

contraction & relaxation in striated muscle (sarcoplasmic reticulum) .

Biosynthesis of steroid hormones in endocrine glands: adrenal cortex, interstitial cells of Leydig, and corpus luteum.

Synthesis of neutral fats in intestine epithelial cells Formation of HCl in parietal cells of the stomach . Cholesterol & lipid metabolism and detoxification

processes in the liver cells. Drug detoxification using cyt P450 which is a family of

heme proteins that participate in hydroxylation of barbiturates, phenytoin or benzopyrene (a carcinogen found in cigarette smoke), makes them soluble in water, and allows excretion into the urine (activation of cyt P450 by one agent enhances the detoxification of another = has clinical implications)

Function as sites of energy release and ATP formation; powerhouse of the cell.

Appear as slender rods or filaments about 0.4-0.8 m in diameter & 4-9 m long by phase microscopy.

They are located in regions of the cell where energy is utilized to produce metabolic work.

MITOCHONDRIA

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They lie in relation to contractile elements in muscle cells; at the base of cilia to produce movement in the sperm middle piece; between the basal infoldings of the proximal tubule of the kidney for active transport.

Abundant in cells with very high metabolic rate (parietal cells of the stomach, kidney tubule cells and the adrenal cortex). In the parenchyma liver cells, there maybe 2000-3000 mitochondria.

Has 2 compartments: the intermembrane space or outer compartment; & a large intercristal space or inner compartment comprising all the area within the inner membrane occupied by a moderately dense mitochondrial matrix.

Are closely packed membranous plates in skeletal muscle & kidney tubule cells; tubular or vesicular in steroid-hormone synthesis.

Tubular mitochondria:

the inner membrane

projections are relatively broad

tubes. Occurence: steroid hormone synthesizing cells (adrenal cortex, Leydig cells in

testis)

Sacculus-type mitochondria:

the inner membrane tubuli are

decorated with pearl-like

broadenings (adrenal cortex)

Mitochondria with prisms: some of the

cristae form triangulartubelike structures

(astrocytes)

They contain their own bacteria-like DNA (codes for respiratory enzymes complexes; nuclear DNA codes for membrane and matrix proteins, replication and transcription enzymes); synthesize their own mRNA, rRNA, tRNA, ribosomes

There are several maternally-inherited or infection/ environmen- tal toxicity-related mitochondrial deficiency diseases.

Most of these are characterized by muscular dysfunction, or senility.

Replicates through binary fission.

Mitochondria and endosymbiosis According to the theory of endosymbiosis, a larger

prokaryote engulfed a smaller prokaryote 1.5 billion to 700 million years ago.

The larger organism gained benefited from the metabolism of “protomitochondria/ protochloroplast,” while providing a stable environment and the raw materials the endosymbionts required.

Eukaryotic cells cannot survive without mitochondria & endosymbionts cannot survive outside their hosts.

Mitochondrial division is remarkably similar to the prokaryotic methods.

This is not ordinarily visible and stained in routine histological sections. After silver impregnation or exposure to osmium tetroxide, it is selectively blackened, and becomes visible as a reticular network of canals and vacuoles.

A site of accumulation and concentration of the secretory product in glandular cells.

It controls traffic in small vesicles involved in membrane recycling between organelles and from cytoplasm to surface.

GOLGI APPARATUS & DICTYOSOMES

Synthesis of the specific protein takes place on the ribosomes in protein-secreting cells.

In the lumen of the ER, the protein accumulates and transported to the Golgi region.

It is processed in compartments and finally released from the Golgi complex in the form of secretory granules.

Concentrates and packages some hydrolytic enzymes which remain inside lysosomes.

Highly fenestrated regions called trans-Golgi network (formerly GERL), are believed to be potential sites of lysosome biogenesis.

Consists of several membrane-bounded, flattened cisternae arranged in parallel arrays about 300A apart.

The stack of saccules are disc-like and slightly curved (dictyosomes).

Has an outer convex surface (cis, forming or immature face), and an inner concave surface (trans, secreting or mature face).

Precipitated proteins, membrane-limited spherical condensing vacuoles containing secretory products, & mature secretory granules with a dense homogenous content are associated with the concave surface.

Numerous small vesicles 300-800A in diameter are associated with the convex outer surface of each stack of flattened saccules.

Majority are smooth surface vesicles but few are limited by a membrane with a coating of fine short filaments or bristles, the coated vesicles.

These coats are utilized in vesicular traffic. Different coat proteins select different cargo, and shape the transport vesicles that mediate the various steps in the biosynthetic- secretory and endocytic pathways.

Organelle is dynamic: continually changing and maintaining a state of dynamic equilibrium between the rate of membrane

loss at the maturing face, and the rate of new membrane being contributed by the reticulum via the transitional

vesicles.

These are small, dense bodies of varying size and shape; limited by a membrane

Contain a number of hydrolytic enzymes (proteases, glycosidases, nucleases, phosphatases, phospholipases, sulfatases) for intracellular digestion.

These enzymes are secreted in the ER and transported from the trans-Golgi network to developing lysosomes.

They are present in all cells except RBC, but are particularly numerous in macrophages, neutrophil leukocytes, hepatic cells and cells of the proximal tubule of the kidney.

They can be identified by electron cytochemistry using a modified Gomori technique for acid phosphatases.

LYSOSOMES

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Primary lysosomes – containing enzymes that have not yet engaged in digestive activities

Secondary lysosomes – vacuolar structures that are sites of current or past digestive activity.

Current concepts of the functions of lysosomes. Synthesis occurs in the rough endoplasmic reticulum (RER), and the enzymes are packaged in the Golgi complex. Note the heterophagosomes, in which bacteria are being destroyed, and the autophagosomes, with RER and mitochon- dria in the process of digestion. Heterophago- somes and autophago- somes are secondary lysosomes. The result of their digestion can be excreted, but sometimes the secondary lysosome creates a residual body, containing remnants of undigested molecules. In osteoclasts, the enzymes are secreted to the extracellular environment.

Spherical cytoplasmic bodies, about 0.2-1.0 m in diameter surrounded by a single membrane and 1st observed in the cells of the proximal convoluted tubule of the kidney.

Abundant in animal cells, close to ER.

Membranes contain cyt b5 & NADH-cyt b5 reductase, and have similar lipid composition as ER (derived from ER?)

MICROBODIES or PEROXISOMES

They perform iportant oxidative functions. Flavin oxidases use O2 as electron acceptors, producing toxic H2O2. Catalase and other hydrolases convert H2O2 to H2 and O2.

Protect cells from excessive H2O2 production during administration of lipid and cholesterol-lowering drugs.

Energy released during oxidation is not stored as ATP but may contribute to maintenance of body temperature.

Involved in gluconeogenesis (the conversion of fats to carbohydrates).

Have an important role in the synthesis of specialized phospholipids required for nerve cell myelination.

Related diseases: X-linked single enzyme deficiency (adrenoleukodystrophy), several enzymes (Zellweger syndrome).

Like mitochondria and plastids, peroxisomes are thought to be self-replicating organelles.

Because they contain no DNA or ribosomes, however, they have to import their proteins from the cytosol.

A specific sequence of three amino acids near the C- terminus of many of these proteins functions as a peroxisomal import signal.

The mechanism of protein import is distinct from that of mitochondria and chloroplasts, and oligomeric proteins can be transported into peroxisomes without unfolding

Lifeless accumulations of metabolites or cell products needed for proper cell function

1. Pigment granules – have highly organized structure and enzymatic activities: Melanin – dark brown

or black pigment produced in and bound to granules called melanosomes; the latter are considered organelles of epidermal melanocytes

Lipofuscin – yellowish-brown granules, referred to as “wear and tear” pigment

Hemosiderin (ferritin) & bilirubin – breakdown products of hemoglobin

CYTOPLASMIC INCLUSIONS

2.Lipid – appear as round, clear areas in the cytoplasm in ordinary histological sections, because the lipid is extracted by solvents.

By fixation in osmium, lipid can be rendered resistant to extraction, appearing as black, spherical droplets of varying size.

The degree of blackening depends upon the degree of unsaturation of the lipid and the nature of the fixative used.

Lipid droplets do not have limiting membranes.

Adrenal gland showing lipid droplets (L) and anomalous

mitochondria (M).

3.Glycogen – can be stained by the PAS reagent, which imparts a brilliant red color to the glycogen.

They are normally confined in the cytoplasm, but may accumulate in the nucleus in diabetes and glycogen-storage disease.

Two types of glycogen particles are:Alpha particles – rosette-like aggregates of large

size (900-950A) found in the liver. Beta particles – dense, irregular spherical body

about 300A in diameter and may occur individually in some cell types.

4.Crystals – probably protein in nature, occur in a few cell types (crystals of Charcot- Bottcher of Sertoli cells, and interstitial cells of the testes), where the materials occur free in the cytoplasm, not bounded by a membrane.

Crystalloids also occur in granules of eosinophils which are classified as lysosomes, some peroxisomes, and occasionally within mitochondrial cristae.

EM show a regular lattice pattern; within the crystal is a highly ordered pattern of closely spaced, parallel dense lines about 80A wide.

5.Secretory granules – EM of pancreatic acinar cells show zymogen secretory granules accumulating in the apical cytoplasm as large, spherical dense granules.

The limiting membrane cannot be resolved as a separate structure, and recently has shown to be enzymatically active.

6.Vacuoles – these cavities differ from the vesicles by lacking the bounding membrane. The vacuoles of normal cells are storage cavities.

QUESTIONS?