Cytology Overview by Dr. Esam El-Shwihy

Cytology Overview

• Cytology is the study of cells, which are the fundamental units of structure and function in organisms.
• Cells consist of a cell membrane (plasmalemma) and protoplasm, which includes cytoplasm and nucleus.

Cell Structure

• Cell Membrane Functions:
  • Maintains cell integrity
  • Controls substance exchange (selective permeability)
  • Recognizes antigens and foreign cells
• Protoplasm Components:
  • Cytoplasm: Contains cytosol, organelles, and inclusions
    • Cytosol: Semifluid matrix
    • Organelles: Active structures crucial for cell function
    • Inclusions: Nutrient storage, not essential for life
  • Nucleus Components:
    • Nuclear envelope, nucleolus, chromatin, nuclear matrix

Microscopy Techniques

• Light Microscope:
  • Ordinary stains highlight cell features (e.g., hematoxylin for basophilia, eosin for acidophilia).
• Electron Microscope:
  • Provides greater detail; distinguishes electron-dense (dark) and electron-light (lucent) structures.

Cell Membrane Details

• Molecular Structure:
  • Fluid-mosaic model includes phospholipid bilayer, cholesterol, and proteins
• Phospholipids:
  • Form double layers with polar (hydrophilic) heads and non-polar (hydrophobic) tails.
• Membrane Proteins:
  • Integral Proteins: Embedded in the lipid bilayer
  • Peripheral Proteins: Associate with the bilayer's surface

Membrane Transport Mechanisms

• Passive Transport:
  • Moves substances based on concentration gradient without energy
• Active Transport:
  • Moves substances against concentration gradient; requires energy (e.g., sodium-potassium pump)
• Vesicle-Mediated Transport:
  • Endocytosis:
    • Cell intake of substances (e.g., phagocytosis for solids, pinocytosis for fluids)
  • Exocytosis:
    • Release of substances from the cell

Mitochondria

• Membranous organelles crucial for energy production
• Found in all cells except mature red blood cells
• Composed of:
  • Outer Membrane: Smooth, no selective permeability
  • Inner Membrane: Folded into cristae; site of ATP production via oxidative phosphorylation
  • Matrix: Contains enzymes, ribosomes, RNA, DNA, and Ca²⁺ ions.
• Functions:
  • ATP synthesis
  • Calcium regulation and detoxification

Ribosomes

• Non-membranous organelles key for protein synthesis
• Present in all cells except mature erythrocytes
• Types:
  • Free Ribosomes: Scattered in cytoplasm, synthesize internal proteins
  • Attached Ribosomes: Bound to rough endoplasmic reticulum (rER), synthesize secretory proteins
• Structure contains rRNA and proteins; visible only under EM as small particles.

Endoplasmic Reticulum (ER)

• Two types:
  • Smooth ER (sER): Lacks ribosomes, involved in lipid synthesis, detoxification, and calcium regulation
  • Rough ER (rER): Studded with ribosomes; synthesizes secretory and membrane proteins
• Continuous network essential for cellular functions.

Golgi Apparatus

• Membranous organelle involved in protein processing
• Consists of stacks of flattened cisternae
• Functions include:
  • Protein modification (glycosylation, sulfation)
  • Concentrating and packaging proteins and lysosomes
  • Membrane recycling with the plasma membrane

Lysosomes

• Membranous organelles containing hydrolytic enzymes
• Classified into:
  • Primary Lysosomes: Newly formed, homogeneous contents
  • Secondary Lysosomes: Heterogeneous contents formed from fusion with vesicles (e.g., phagosomes, autophagosomes)
• Act as intracellular digestive systems; release contents via exocytosis.