Histology and Microscopy Overview

Histology

• The study of the microscopic structure of normal tissue.
• Focuses on understanding the microanatomy of cells, tissues, and organs.
• Connects structure with function.

Microscopy

• Light Microscopy (LM):
  • Resolution power: 0.2 um
  • Maximum magnification: x1000
• Electron Microscopy (EM):
  • Resolution power: 0.2 nm
  • Maximum magnification: x100,000

Cell Stains

• Cells are colorless & require staining for visualization under a light microscope.
• Acidic Stains:
  • Example: Eosin
  • Stains basic structures (acidophilic).
• Basic Stains:
  • Example: Hematoxylin
  • Stains acidic structures (basophilic).
• Neutral Stains:
  • Example: Leishman's stain
  • Combination of acidic & basic stains, used for blood cells.
• Vital Stains:
  • Stain living structures within a living animal.
  • Example: Trypan blue or India ink for phagocytic cells.
• Supravital Stains:
  • Stain living cells outside a living animal.
  • Example: Brilliant Cresyl blue for reticulocytes (immature RBCs).
• Metachromatic Stains:
  • Produce a new color after staining.
  • Example: Toluidine blue stains mast cell granules violet.
  • Metachromasia refers to the change in the original color of the stain.

Cell Membrane

• Composed of a phospholipid bilayer with:
  • Intrinsic Proteins:
    • Integral proteins that span the membrane.
    • Act as pathways for water-soluble substances.
  • Extrinsic Proteins:
    • Small molecules attached to the membrane surface.
    • Form a non-continuous layer.
• Carbohydrate Component:
  • Oligosaccharides attached to proteins (glycoproteins) or lipids (glycolipids).
  • Form the cell coat or glycocalyx.
  • Contains cell receptors for drugs, hormones, and bacteria.

Cell Membrane Functions

• Maintains the internal composition of the cell.
• Cell coat participates in cell adhesion, recognition, protection, and immunity.
• Allows transport of substances through the membrane:
  • Passive diffusion: Gases and water.
  • Facilitated diffusion: Glucose.
  • Active transport: Sodium pumps.
  • Selective permeability: Through receptors.
  • Bulk transport (vesicular transport): Macromolecules enter/leave the cell through vesicles.

Vesicular Transport

• Exocytosis: Substances leave the cell.
• Endocytosis: Substances enter the cell.
• Phagocytosis: Solid substances enter the cell.
• Pinocytosis: Fluid enters the cell.

Mitochondria

• Membranous cell organelle.
• "Powerhouse of the cell."
• Responsible for cell respiration and energy production (ATP).
• Number varies with cell activity (e.g., liver cells contain 1000-2000/cell).
• Present in all cells except RBCs.
• Located at sites of high activity (e.g., apical part of ciliated cells).
• LM: Appear as granules, rods, or filaments.
• EM:
  • Vesicle shaped, oval or rounded.
  • Double membranes, separated by an inter-membranous space.
  • Inner membrane forms folds called cristae.
  • Mitochondrial matrix contains lipids, proteins, carbohydrates, Ca, Mg, DNA, and RNA.
  • Oxidative enzymes are attached to the cristae.

Mitochondria Functions

• Energy production (ATP).
• Can form their own proteins and divide due to the presence of DNA and RNA.

Endoplasmic Reticulum

• Membranous network within the cytoplasm formed of flattened communicating vesicles and tubules.
• Two Types:
  • Rough (granular) ER:
    • Contains ribosomes.
    • Site of protein synthesis.
  • Smooth (agranular) ER:
    • Lacks ribosomes.
    • Involved in lipid synthesis, steroid hormone synthesis, glycogen formation and storage, and muscle contraction (through Ca pumps).

Epithelial Tissue

• Forms sheets that cover and line body surfaces.
• Characterized by:
  • Closely packed cells.
  • Little intercellular substance.
  • Apical surface: Faces free space.
  • Basal surface: Anchored to a basement membrane (B.M.).
• Functions:
  • Protection
  • Absorption
  • Secretion
  • Excretion
  • Filtration
  • Diffusion
  • Sensory reception

Classification of Epithelial Tissue

• Classification is based on the number of cell layers and the shape of the surface cells.
• Simple Epithelium:
  • One cell layer.
• Stratified Epithelium:
  • Multiple cell layers.

Simple Epithelium Types

• Simple Squamous Epithelium:
  • Flattened cells with centrally located nuclei.
  • Function: Diffusion, filtration, and secretion.
  • Sites:
    • Lining of blood vessels (endothelium).
    • Lining of body cavities (mesothelium).
    • Alveoli of lungs.
    • Glomerular capsule of kidneys.
• Simple Cuboidal Epithelium:
  • Cube-shaped cells with centrally located nuclei.
  • Function: Secretion, absorption, and protection.
  • Sites:
    • Kidney tubules.
    • Small ducts of glands.
    • Thyroid follicles.
• Simple Columnar Epithelium:
  • Tall, column-shaped cells with oval nuclei.
  • Function: Absorption, secretion, and protection.
  • Sites:
    • Lining of the stomach and intestines.
    • Gallbladder.
    • Uterine tubes.
• Simple Columnar Ciliated Epithelium:
  • Tall, column-shaped cells with cilia on the apical surface.
  • Function: Movement of substances.
  • Sites:
    • Trachea.
    • Uterine tubes.
• Simple Columnar Non-ciliated Epithelium:
  • Tall, column-shaped cells without cilia.
  • Function: Secretion and absorption.
  • Sites:
    • Stomach.
    • Rectum.
    • Gallbladder.
• Pseudostratified Columnar Epithelium:
  • All cells rest on the B.M., but nuclei are at different levels, giving the illusion of multiple layers.
  • Function: Secretion and movement of substances.
  • Sites:
    • Male reproductive system (vas deferens, epididymis).
    • Respiratory system (nose, larynx, trachea, bronchi).

Stratified Epithelium Types

• Stratified Squamous Epithelium:
  • Multiple layers of cells, with flattened cells on the apical surface.
  • Functions: Protection.
  • Sites:
    • Skin (epidermis).
    • Oral cavity.
    • Oesophagus.
    • External ear.
    • Cornea.
    • Nasal orifices.
• Transitional Epithelium (Stratified Cuboidal):
  • Multiple layers of cells with a capacity for stretching.
  • Function: Protection and accommodation (distensibility).
  • Sites:
    • Urinary bladder.
    • Ureters.
    • Some parts of the urethra.
    • Renal calyces.
    • Renal pelvis.
• Stratified Columnar Epithelium:
  • Few layers, with column-shaped cells on the apical surface.
  • Function: Protection.
  • Sites:
    • Fetal esophagus (ciliated).
    • Recto-anal junction.
    • Large ducts of glands.
    • Male urethra (penile portion).
• Stratified Cubical Epithelium:
  • Rare type, with few layers of cubical cells.
  • Sites:
    • Ducts of sweat glands.

Explore the intricate details of histology, focusing on the microscopic structure of tissues and the tools used for visualization. This quiz covers light and electron microscopy techniques, various types of cell stains, and their applications in understanding cellular structures. Test your knowledge on how these components connect structure with function.