Cytology, General Histology and Embryology - V. K. Napkhanyuk 2002

Cytology
Cytoplasm

The cytoplasm (cytoplasma) is a colloidal system consisting of hyaloplasm, membranous and non-membranous Organelles, as well as inclusions.

The hyaloplasm (from Greek hyalos — transparent, translucent) is the cytoplasmic matrix. Physically and chemically, it is a complex colloidal system containing various polymers (Proteins, Nucleic Acids, Polysaccharides) that can transition between sol and gel states.

The hyaloplasm serves as a medium that integrates all Cellular Structures and facilitates their interaction. It mediates numerous Intracellular Transport processes, including the movement of Amino Acids, Fatty acids, NUCLEOTIDES, and sugars.

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Fig. 7. Classification of inclusions

Cytoplasmic inclusions (inclusiones cytoplasmaticae) are non-essential Cell components that appear and disappear depending on the metabolic state and lack a strictly defined Structure (Fig. 7).

Organelles are permanent microstructures essential to all Cells, responsible for carrying out vital cellular Functions (Fig. 8).

Membranous organelles

Membranous organelles include Mitochondria, Lysosomes, Peroxisomes, The Endoplasmic reticulum, and the Golgi apparatus.

Mitochondria (mitochondriae) are microscopic general-purpose membranous organelles whose primary function is The oxidation of Organic compounds and the synthesis of ATP molecules.

Mitochondria are 1-10 µm in length and 0.5 µm in thickness. They have an oval, elongated shape. Under an Electron microscope, both inner and outer mitochondrial membranes are visible, separated by the intermembrane space.

The outer mitochondrial membrane (membrana mitochondrialis externa) separates the mitochondrion from the hyaloplasm; it has a smooth contour and a sac-like shape.

The inner mitochondrial membrane (membrana mitochondrialis interna) forms inward folds called cristae and encloses the internal content known as the matrix.

The space enclosed by the inner membrane is filled with the mitochondrial matrix, which contains fine filaments 2-3 nm thick and granules 15-20 nm in size. The filaments represent DNA molecules, while the granules are mitochondrial Ribosomes.

Fig. 8. Classification of organelles

Lysosomes (lysosoma) are submicroscopic general-purpose membranous organelles that carry out the intracellular enzymatic breakdown of exogenous substances (taken up by The Cell via endocytosis) as well as endogenous components — removing organelles and inclusions during normal cell turnover or in response to altered functional activity.

Lysosomes are oval bodies measuring 0.2-0.4 µm. They contain over 60 Proteolytic Enzymes enclosed within a bounded membrane sac.

The main forms of lysosomes include:

1. Primary lysosomes.

2. Secondary lysosomes (phagolysosomes, or heterophagosomes, and autophagosomes).

3. Residual bodies.

Peroxisomes (peroxysoma) are submicroscopic general-purpose membranous organelles capable of neutralizing reactive oxygen species, breaking down ethanol and uric acid, and regulating METABOLISM.

Peroxisomes are oval, membrane-bound bodies measuring 0.3-1.5 µm. They contain a granular matrix featuring a central core composed of crystalline structures made of fibrils and tubules.

The endoplasmic reticulum (reticulum endoplasmicum) is a submicroscopic general-purpose membranous organelle that forms a continuous intracellular Circulatory system, ensuring the TRANSPORT AND STORAGE of substances, Protein Synthesis, and lipid and Carbohydrate Metabolism.

There are two types of the endoplasmic reticulum:

— smooth endoplasmic reticulum (agranular, reticulum endoplasmicum nongranulosum), formed by double membranes of vacuoles and tubules with a diameter of 50–100 nm;

— granular endoplasmic reticulum (reticulum endoplasmicum granulosum), formed by a double membrane of sacs, cisternae, and tubules 20–100 nm wide. Ribosomes are attached to these membranes on the hyaloplasm side.

The Golgi apparatus (complexus Golgiensis) is a microscopic, general-purpose membranous organelle that completes The formation of the cell's secretory products.

The Golgi apparatus consists of a set of interconnected flattened sacs and cisternae about 25 nm thick, transport vesicles that deliver protein secretions from the endoplasmic reticulum, and secretory granules through which secretions are released from the cell. All these structures are concentrated in a distinct small zone known as the dictyosome.

Non-membranous organelles

Non-membranous organelles include ribosomes, the centrosome, microfilaments, microtubules, cilia, and flagella.

Ribosomes (ribosomata) are submicroscopic, general-purpose non-membranous organelles that serve as the primary sites for the synthesis of protein and polypeptide molecules found in all cells.

Ribosomes are mushroom-shaped and measure 25x20x20 nm. They are complex ribonucleoproteins composed of proteins and RNA molecules in a 1:1 ratio. A ribosome consists of two subunits: large and small.

The subunits are built from a ribonucleoprotein strand in which rRNA interacts with various proteins to form the body of the ribosome. The subunits are connected at an angle to each other, forming a mushroom-like structure.

The centrosome, or cell center (centrosoma), is a microscopic, general-purpose non-membranous organelle that ensures the Separation of Chromosomes during Cell Division.

The centrosome consists of two centrioles surrounded by a centrosphere. In a cell not preparing for division, it is located near The Nucleus.

Two centrioles lying side by side are called a diplosome. In the diplosome, the centrioles are positioned at an angle to each other. A mother and a daughter centriole are distinguished. The end of the daughter centriole is oriented perpendicularly to The surface of the mother centriole.

Centrioles (centriolum) consist of 9 triplets of microtubules arranged in a circle, forming a hollow cylinder 0.2 µm wide and 0.3–0.5 µm long. The triplets are connected by special arms made of the protein dynein. The microtubule system in the centriole is described by the formula (9x3)+0, which proves the absence of microtubules in its central part.

Surrounding each centriole is a structureless or finely fibrillar matrix, and sometimes a few additional structures associated with the centrioles are present. These are the so-called satellites.

The centrosphere is the organelle-free hyaloplasm surrounding the centrioles, radially penetrated by microtubules.

Microfilaments (microfilamenti) are submicroscopic, general-purpose non-membranous organelles that function as the Cytoskeleton. Depending on their STRUCTURE AND FUNCTIONS, microfilaments are classified as follows:

— true microfilaments, located in the cortical layer of the cytoplasm directly beneath the Plasmalemma. These are thin fibers 5–7 nm in diameter, composed of Actin, Myosin, Tropomyosin, and α-actinin proteins;

— intermediate microfilaments, or microfibrils (microfibrillae), which are thin protein threads arranged in bundles, 10–15 nm in diameter. They are responsible for maintaining cell shape.

Microtubules (microtubuli) are submicroscopic non-membranous organelles whose primary function is to form an elastic yet rigid cytoskeleton necessary for maintaining cell shape.

Microtubules consist of Globular proteins—tubulins, whose molecules can polymerize in a specific way by stacking on top of one another to form rounded subunits measuring 5 nm. The wall of a microtubule consists of tightly packed subunits, with 13 subunits forming a ring with an outer diameter of about 24 nm and an inner lumen width of 15 nm.

Cilia (cilium) are specialized organelles that appear as thin cylindrical cytoplasmic extensions 200 nm wide and 5–10 µm long. A cilium is covered by The Plasma Membrane from base to tip. An axial filament (axoneme) is located inside the extension, and the proximal part of the cilium (the basal body) is embedded in the cytoplasm.

The axoneme (filamentum axiale) consists of 9 doublets of microtubules connected by arms that form the wall of the axoneme cylinder. A pair of central tubules is located in the center of the axoneme. The microtubule System of the axoneme has the formula (9x2)+2.

The basal body (corpusculum basale) consists of 9 triplets of microtubules, which are also interconnected by arms. The microtubule system of the basal body has the formula (9x3)+0, just like in the centriole.

The axoneme and basal body are structurally connected, forming a single functional unit. Two microtubules of the basal body triplets continue as the microtubules of the axoneme doublets.

Flagella are thin, cylindrical cytoplasmic outgrowths structurally similar to cilia. They are about 150 µm long and roughly 200 nm in diameter.

Slides for study

Slide 1. Fat inclusions (Fig. 9).

High magnification. Examine and draw the slide. Polygonal cells with large red nuclei are visible. The pink granular cytoplasm contains black, rounded inclusions of various sizes (fat inclusions).

Label on the drawing: 1) Liver cells: a) lipid inclusions; b) nucleus; 2) capillary with red Blood Cells.

Slide 2. Glycogen inclusions (Fig. 10).

Low magnification. Observe the slide. At this magnification, locate the central part of the section where glycogen is distributed relatively evenly within the cells.

High magnification. In the center of the section, red glycogen clumps are scattered throughout the Cell Cytoplasm alongside purple nuclei. At the periphery of the section, glycogen clumps may coalesce on one side of the cell while the other remains clear. Draw the slide.

Label on the drawing: 1) liver cells; 2) cytoplasm with glycogen inclusions; 3) nucleus; 4) blood capillary.

Slide 3. Golgi apparatus (Fig. 11).

Low magnification. Observe the slide. Locate large cells with a clearly visible network of the Golgi apparatus surrounding the nucleus. The cytoplasm has a greenish tint.

High magnification. Examine the nucleus (it is pale, large, and features a brown nucleolus). The black-stained Golgi apparatus stands out clearly around the nucleus. Draw the slide.

Fig. 9. Fat inclusions in axolotl liver cells. Osmium tetroxide staining. Safranin. x 900:

1 — liver cells (a — lipid inclusions; b — nucleus); 2 — capillary with red blood cells

Label on the drawing: 1) nucleus; 2) Golgi apparatus; 3) cytoplasm.

Examine the micrographs (Figs. 12–14) and answer the Review Questions.

Fig. 10. Cytology/practical/9.html">Glycogen Inclusions in axolotl liver cells. Best's carmine and hematoxylin staining. x 900:

1 — liver cells; 2 — cytoplasm with glycogen inclusions; 3 — nucleus; 4 — blood capillary

Fig. 11. Golgi apparatus. Osmium impregnation. x 400: 1 — nucleus; 2 — Golgi apparatus; 3 — plasmalemma

Fig. 12. Golgi apparatus. Electron micrograph. x 84,000:

1 — smooth-surfaced membranes; 2 — vacuoles; 3 — vesicles (after L. N. Mikhailova)

Fig. 13. Mitochondria. Electron micrograph. х 100,000:

1 — outer mitochondrial membrane; 2 — inner mitochondrial membrane; 3 — mitochondrial cristae; 4 — mitochondrial matrix; 5 — intermembrane space (after Yu. N. Kopaev)

Fig. 14. Cell center (centrosome). Electron micrograph: a — centrioles in oblique and longitudinal section, х 160,000; b — centrioles in oblique and transverse section, х 90,000; 1 — centrioles; 2 — peripheral doublet microfibrils (after N. P. Dmitrieva)

Review Questions

1. History of creation and Main principles of T. Schwann's Cell Theory. Its erroneous postulates.

2. Modern cell theory. Key principles.

3. Definition of the cell as the structural unit of living organisms.

4. Structure, chemical composition, and PHYSICOCHEMICAL PROPERTIES OF the elementary biological membrane.

5. Cell wall (glycocalyx/cell coat). Structure and functions.

6. Types of cell junctions and their characteristics.

7. Transport of substances across the plasmalemma. Phagocytosis. Pinocytosis.

8. Non-cellular structures and their characteristics.

9. Cytoplasm. Hyaloplasm. Characteristics.

10. Definition and classification of inclusions.

11. Definition and classification of organelles.

12. Characteristics of membranous organelles.

13. Characteristics of non-membranous organelles.

14. Structure and Functions of the Golgi apparatus.

15. Structure and functions of the endoplasmic reticulum.

16. Lysosomes, peroxisomes. Structure. Functions.

17. Mitochondria. Structure. Functions.

18. Ribosomes. Structure. Functions.

19. Organelles of special purpose. Structure. Functions.

20. Centrosome. Structure. Functions.

Sample Examination Questions

1. Non-cellular structures. Structure. Characteristics.

2. Cell theory. Main tenets of T. Schwann's cell theory. Modern perspective.

3. Cell membranes. Structure. Functions.

4. Cytoplasm. Hyaloplasm. Characteristics. Functions.

5. Inclusions. Classification. Characteristics of Specific types of inclusions.

6. Organelles. Definition. Classification.

7. Membrane-bound organelles. Structure. Functions.

8. Non-membranous organelles. Structure. Functions.

9. Specialized organelles. Structure. Functions.