Histology Basics and Microscopy Techniques

Histology

• The study of the normal microscopic structure of cells (cytology) and tissues in relation to their functions.
• Hierarchy: Cells → Tissues → Organs → Organisms

Staining

• Differentiation of various structures by taking different colors.
• Types:
  • Acidic:
    • Eosin (E): Pink
    • Hematoxylin & Eosin (H&E): Purple
  • Basic:
    • Hematoxylin (H): Blue

Special Stains

• Carbohydrates:
  • Periodic Acid Schiff (PAS): Magenta
• Lipids:
  • Frozen sections are used.
  • Sudan III: Orange
  • Sudan Black: Black
  • Osmic acid: Black

The Microscope

• Magnifies images to reveal details of the object.
• Types:
  • Light microscope: 40, 100, 400, 1000 times magnification
  • Electron microscope: Up to 50,000 times magnification.
    • Transmission EM
    • Scanning EM

Structure of the Cell

• Cell membrane: Surrounds the cell and regulates material passage.
• Cytoplasm: Contains:
  • Cytosol (cytoplasmic matrix)
  • Cell organelles
  • Cell inclusions
• Nucleus

Cell Organelles

• Living components in the cytoplasm
• Types:
  • Membranous:
    • Cell membrane
    • Mitochondria
    • Endoplasmic reticulum (smooth and rough)
    • Golgi apparatus
    • Lysosomes
  • Non-membranous:
    • Ribosomes
    • Cytoskeleton
      • Microtubules (as centriole, cilia)
      • Filaments (Thin, intermediate, & thick)

The Cell Membrane

• The outer limiting membrane that surrounds the cell.
• Light microscope (LM): Too thin to be seen, but its carbohydrate contents can be stained by PAS.
• Electron microscope (EM): Trilaminar structure: two dark (electron-dense) lines separated by a light one (electron-lucent).
  • Called unit membranes

Chemical Structure

• Lipid:
  • Phospholipid
  • Cholesterol
• Protein:
  • Integral (intrinsic protein)
  • Peripheral (extrinsic protein)
• Carbohydrate:
  • Cell coat (Glycocalyx)

Lipid Structure

• Phospholipid double layers:
  • Hydrophilic polar head (outwards)
  • Hydrophobic non-polar tail (center)
• Cholesterol: Located on the cytoplasmic side of the lipid bilayer, giving stability and rigidity to the membrane. It forms channels for lipid-soluble substances.

Integral Protein

• Located within the lipid bilayer and not easily extracted.
• Small particles: Partially embedded in the lipid bilayer.
• Trans-membranous protein: Large in size, completely embedded in the lipid bilayer and crosses it. It acts as channels for water-soluble substances.

Peripheral Protein

• Located on the periphery and loosely attached to the outer surface of the cell membrane.
• Easily extracted.

Carbohydrate Component

• Consists of glycolipids and glycoproteins.
• Projects from the external surface of the membrane, forming the:
  • Cell coat or Glycocalyx:
    • PAS positive
    • EM: Fuzzy appearance
    • Thick in the intestine

Functions of the Cell Membrane

1. Maintains the internal composition of the cell.
2. Cell coat (glycocalyx) functions:
   • Attachment to other cells:
   • Receptors:
   • Cell-to-cell recognition:
   • Immunity:

Bulk Transport

• Exocytosis:
• Endocytosis:
  • Phagocytosis:
  • Pinocytosis:

MCQ

• The cell coat is:
  • Well developed and thick in the intestine.
• How the cell membrane appears by EM:
  • Trilaminar.
• Which substance of the following is responsible for decreasing lipid bilayer movement?
  • Cholesterol molecule.
• Which of the following stains is responsible for staining of the cell membrane?
  • PAS.