Instructors: Dr. Diana Bochynska, DVM, dipECVP, and previous instructors: Matthew Valentine, Dr. I. Irimescu, C. Fuentealba, M. Smith, M. Zibrin, L. Bogdanovic.
Page 2: Lecture Overview
Definitions, Origin, Functions, Relevance of Epithelium
Characteristics of Epithelium
Classification of Epithelia:
- A. Surface/ Lining Epithelia
- B. Glandular Epithelia
Definition: A tissue composed of closely aggregated cells with very little extracellular substance.
Main Types of Epithelium:
- Lining (Surface) Epithelia:
- Forms sheets covering body surfaces or lining organs and cavities.
- Glandular Epithelia:
- Specialized cells that synthesize, store, and release various products.
- Special Epithelia:
- Contains sensory nerve endings, found in skin, ears, tongue, and modified neurons in the nose and eyes.
Page 6: Origin and Histogenesis of Epithelia
Germ Layer Origins:
- Ectoderm: Epidermis of skin
- Mesoderm: Mesothelium (lining body cavities), endothelium (lining vessels and heart chambers)
- Endoderm: Epithelial lining and glands of gastrointestinal tract, respiratory system, and urinary bladder.
Page 7: Functions of Epithelial Tissues
Protection:
- Protects against mechanical injury, harmful chemicals, microorganisms, and water loss (e.g., epidermis).
Friction Reduction:
- Smooth endothelial cells reduce friction in blood vessels and heart chambers.
Cleaning:
- Ciliated respiratory epithelium removes dust and foreign particles.
Diffusion:
- Endothelium of capillaries promotes diffusion of gases and nutrients.
Absorption:
- Epithelial cells in the intestine absorb nutrients.
Secretion:
- Specialized epithelial tissues secrete enzymes, hormones, and mucus.
Sensation:
- Contains sensory nerve endings that convey sensations from various organs.
Page 8: Relevance of Epithelial Study
Foundational Knowledge: Understanding tissue organization and functioning is essential.
- Applications:
- Systemic histology; understanding diseases related to epithelial components (glandular diseases, skin diseases, tumors/neoplasia classifications and progression).
Page 9: Characteristics of Epithelium
High Cellular Density: Very little intercellular substance.
Connections to Connective Tissue: Closely associated with supporting connective tissue (CT).
Avascular Nature: Lining epithelium has few nerve endings.
Prominent Basement Membrane (BM): Acts as a selective barrier.
Cell Polarity: Epithelial cells are polarized with apical, lateral, and basal surfaces.
Cohesiveness: Epithelial cells exhibit strong adhesion to each other.
Page 10: Contrast with Connective Tissue
Connective Tissue Characteristics:
- Lots of intercellular matrix and few cells.
Histological Stains:
- H&E stain reveals differences in cellular density.
Basement Membrane (BM):
- Acts as a selective barrier; small blood capillaries do not penetrate epithelium.
- Stains with PAS (Periodic acid Schiff) for carbohydrate detection.
Page 13: Cohesion in Epithelial Cells
Intercellular Adhesion:
- Epithelial tissues subject to pressure and traction (e.g., epidermis, urinary bladder) exhibit marked adhesion, strong due to cell junctions like E-cadherins.
Page 14: Types of Intercellular Junctions
Tight Junctions:
- Prevent molecules from passing through the intercellular space (impermeable).
Desmosomes:
- Anchoring junctions that bind adjacent cells and reduce internal tension.
Gap Junctions:
- Allow communication between cells (ions, small molecules).
Page 15: Gap Junctions Explained
Function: Provides cytoplasmic channels connecting adjacent cells; necessary for chemical and electrical communication (e.g., in heart muscle and nervous tissue).
Page 16: Classification of Epithelia
Two Main Types:
- A. Lining Epithelia
- Simple (one layer)
- Stratified
- B. Glandular Epithelia
- C. Specific Types:
- Pigment, sensory, etc.
Page 17: Types of Epithelia Based on Shape
Various Types of Epithelium:
- Simple squamous
- Simple columnar
- Stratified squamous
- Transitional
- Pseudostratified columnar
Page 18: Simple Squamous Epithelium
Structure: Flat, elongated cells with a centrally located nucleus.
Common Locations:
- Mesothelium (lining body cavities)
- Alveolar walls in lungs
- Endothelium (capillaries)
Page 19: Vascular Endothelium Example
Notes on Staining: H&E = Hematoxylin & Eosin - common histological staining method.
Page 20: Simple Cuboidal Epithelium
Characteristics: Cuboidal shape, all sides equal, distinct cell limits.
Examples:
- Lining ducts of glands, choroid plexus, thyroid follicles, lens of the eye.
Page 21: Collecting Tubule Example
Observation: Notice the distinct cell limits and euchromatin.
Page 22: Simple Columnar Epithelium
Structure: Tall, narrow cells; ovoid nucleus near base.
Common Locations:
- Lining stomach, intestines, gall bladder, uterine tubes.
Page 23: Simple Columnar Epithelium in Gall Bladder
Characteristics: Surface cells lose nuclei, consist mainly of keratin.
Page 29: Esophageal Epithelium (Non-keratinized)
Noteworthy Features: Stratification visible at high magnification.
Page 30: Keratinized Cells in Mammary Gland
Observation: Dead keratinized cells visible in tissues of high abrasion areas.
Page 31: Transitional Epithelium (Urothelium)
Function: Lines urinary passages, providing an osmotic barrier.
Key Characteristics: Changes shape from cuboidal to flattened depending on bladder fullness.
Page 32: Bladder Epithelium Changes
Relaxed vs Stretched: Observed changes in epithelium structure based on bladder volume.
Page 33: Epithelial Repair Mechanisms
Dynamics: Epithelial cells are constantly lost and replaced; stem cells have high mitotic potential located in basal layers.
Page 34: Glandular Epithelia Classification
Criteria for Classification:
- Number of cells (unicellular/multicellular)
- Shape of ducts and secretory units (adenomeres)
- Type of product
- Mode of secretion
Page 35: Unicellular Glands
Locations: Found in intestinal lining and respiratory tracts; example includes goblet cells.
Page 36: Mucinogen Granule Staining
Characteristics: Blue staining of mucinogen granules with Alcian blue in goblet cells.
Details on cell structure and cytoplasm appearance in serous glands with histological observations.
Page 51: Mucous Gland Observations
Characteristics: Cytoplasm looks frothy with a basal nucleus.
Page 52: Mixed Glands Overview
Structure: Mucous and serous cells sharing duct systems and how they are arranged.
Page 53: Sebaceous Gland Characteristics
Histological Appearance: Centrally located nuclei and pale, foamy cytoplasm due to lipid processing.
Page 54: Secretion Modes Classification
Three Types of Secretion Methods:
1. Merocrine: Secretion without loss of cytoplasm.
2. Holocrine: Cell disintegration contributes to secretion.
3. Apocrine: Partial loss of cytoplasm during secretion process.
Page 55: Merocrine Secretion Process
Overview: Examples including sweat glands, mechanisms discussed with microscopic examination.
Page 56: Apocrine Secretion Characteristics
Detailed Mechanism: Describes how cells release products with partial cytoplasmic loss, with examples.
Page 57: Holocrine Secretion Mechanics
Explanation: Process of cell disintegration in sebaceous glands as the mode of secretion.