Cytology Overview Quiz

Cytology

• The study of cells, tissues, organs, and organ systems.

Cell

• The structural and functional unit of an organism.
• Composed of:
  • Protoplasm
  • Cell membrane
  • Cytoplasm
  • Nucleus

Cytoplasm

• Contains:
  • Cytoplasmic organelles
  • Inclusions
  • Cytoskeleton
  • Cytosol (matrix)

Cell Membrane (Plasmalemma)

• Surrounds all cells.
• Also known as plasma membrane.
• Some organelles are also surrounded by similar membranes.
• Both are called unit membranes.
• Not visible under a light microscope (L.M) due to its thinness (7.5-10 nm).
• Visible under an electron microscope (E.M):
  • Low magnification: Appears as a single electron-dense line.
  • High magnification: Appears as two electron-dense lines separated by an electron-lucent line, creating a trilaminar appearance known as the unit membrane.
• Comprised of:
  • Phospholipids
  • Cholesterol
  • Proteins

Phospholipids

• Form the backbone of the plasma membrane.
• Create a bilayer with hydrophilic phosphate heads directed outwards and hydrophobic fatty acid tails directed inwards.

Cholesterol

• Present in the lipid bilayer.
• Exists in a 1:1 ratio with phospholipids.
• Fills gaps between fatty acid tails.

Proteins

• Located within the lipid bilayer.
• Two types:
  • Integral (intrinsic):
    • Embedded in the lipid bilayer.
    • Can move within the bilayer.
    • Transmembrane proteins extend across the bilayer, protruding from both sides and forming channels for water-soluble molecules.
  • Peripheral (extrinsic):
    • Associated with the cytoplasmic or extracellular surface of the cell membrane.

Cell Coat (Glycocalyx)

• Site: External surface of the cell membrane.
• Structure: Fine filamentous material of varying thickness, as seen under E.M.
• Chemically: Oligosaccharides conjugated with membrane proteins (glycoproteins) and lipids (glycolipids).
• Function:
  • Cell recognition
  • Protection
  • Intercellular adhesions

Functions of the Cell Membrane

• Maintains structural integrity of the cell.
• Controls movement of substances into and out of the cell (selective permeability).
• Recognizes antigens, including foreign cells and altered cells.
• Transports essential substances such as hormones through specific receptors.
• Establishes transport systems for specific molecules:
  • Passive transport: Movement across the membrane without energy expenditure.
  • Active transport: Requires energy expenditure to move substances against their concentration gradient.

Sodium and Potassium Transport

• Certain ions, like sodium and potassium, are actively transported through transmembrane channels.
• Some ions pass passively.
• Lipid-soluble substances like steroid hormones diffuse rapidly through the lipid bilayer.

Mass Transfer

• Involves vesicles or vacuoles that are formed from or fuse with the cell membrane:
  • Endocytosis: Taking substances into the cell.
    • Fluid-phase pinocytosis (cell drinking): Non-specific uptake of substances dissolved in fluid.
    • Phagocytosis (cell eating): Engulfing large particles like bacteria, cell fragments, and cells.
  • Exocytosis: Releasing cell products into the extracellular compartment.
    • Vesicles containing products fuse with the cell membrane, releasing their contents to the exterior.

Cytoplasmic Organelles

• Membranous organelles: Surrounded by a membrane similar to the cell membrane.
  • Mitochondria
  • Rough endoplasmic reticulum (RER)
  • Smooth endoplasmic reticulum (SER)
  • Golgi apparatus
  • Lysosomes
• Non-membranous organelles: Not surrounded by a unit membrane.
  • Free ribosomes
  • Microtubules
  • Centrioles
  • Cilia
  • Flagella
  • Filaments

Mitochondria

• Site: Present in all cells except mature red blood cells.
• Size, number, and shape vary depending on the cell type.
• Number increases in cells with high metabolic activity, such as liver cells.
• L.M: Stainable with supravital stains like Janus green, appearing as green rod, ovoid, or thread-like bodies.
• E.M:
  • Rounded or oval structure enclosed by two trilaminar unit membranes.
  • Outer membrane is smooth.
  • Inner membrane is folded into shelf-like structures called cristae.
  • Intermembrane space: Narrow space between outer and inner membranes.
  • Matrix space: Large space enclosed by the inner membrane.
  • Elementary particles: Closely packed club-shaped particles attached to the inner surface of the inner membrane.
• Self-replicating by fission.
• Function:
  • Energy house of the cell, producing ATP.
  • Calcium regulation in the cytoplasm.
  • Can synthesize proteins due to the presence of DNA, RNA, and ribosomes.

Ribosomes

• Non-membranous organelles present in all cells except mature red blood cells.
• Manufactured in the nucleolus and released into the cytoplasm.
• Two types:
  • Free ribosomes: Scattered in the cytoplasm, synthesize proteins for internal use.
  • Attached ribosomes: Attached to membranes forming the RER, responsible for synthesis and segregation of proteins for secretion.
• Composed of proteins and rRNA.
• Stainable with hematoxylin, toluidine blue, and methylene blue (basic dyes).
• Each ribosome consists of two subunits: small and large.
• L.M: Responsible for cytoplasmic basophilia due to their rRNA content.
  • Free ribosomes: Diffuse basophilia.
  • Attached ribosomes: Localized basophilia.
• Polyribosomes (polysomes): Several ribosomes attached to a thread of mRNA, forming groups or rosettes, either free or attached to the RER.

Smooth Endoplasmic Reticulum (SER)

• L.M: Not visible, but gives acidophilic staining to cytoplasm when present in large amounts.
• E.M: Branching network of tubules in the cytoplasm, not studded with ribosomes.
• Function:
  • Synthesizes steroid hormones in steroid-secreting cells (e.g., adrenal cortex).
  • Synthesizes phospholipids for all cell membranes.
  • Prominent in steroid-secreting cells of the adrenal cortex.
  • Detoxifies drugs in liver cells.
  • Participates in glycogen, cholesterol, and lipid synthesis.
  • Regulates calcium levels in striated and cardiac muscle.

Golgi Apparatus

• Site: Found near the nucleus.
• Structure: Stacked flattened membrane-bound sacs called cisternae.
• Function:
  • Modification of secretory proteins.
  • Concentration and packaging of secretory products into vesicles.
  • Modification, segregation, and packaging of lysosomal enzymes.
  • Synthesis of membrane proteins and membrane recycling.

Lysosomes

• Membranous cell organelles containing hydrolytic enzymes.
• Origin: Synthesized in rER and transported to the Golgi complex.
• Contents: Hydrolases like acid phosphatase, proteases, nucleases, and lipases.
• Optimal activity at pH 5.
• Site: Found in all cells except mature erythrocytes, but are numerous in phagocytic cells.
• Structure:
  • L.M: Visible by specific histochemical reactions for acid phosphatases.
  • E.M: Rounded small membranous vesicles.
• Types:
  • Primary lysosomes: Newly formed, homogenous, not involved in digestion.
  • Secondary lysosomes: Formed after fusion of primary lysosomes with other substances, heterogeneous in appearance.
• Residual body: Indigestible material remaining in secondary lysosomes, released by exocytosis.
• Function:
  • Intracellular digestion of materials from inside and outside the cell.
  • Heterophagy: Break-down of materials from outside the cell, such as bacteria for cellular defense.
  • Autophagy: Break-down of worn-out cell components, providing a source of nutrients and energy.