Histology: Tissues – Epithelial Tissue Overview

Histology: Tissues

Introduction to Tissues

Tissues are groups of cells similar in structure and function. The study of tissues is called histology. Understanding tissues is fundamental to anatomy and physiology, as tissues form the basis for all organs and body structures.

• Definition: A tissue is a group of similar cells that perform a common function.

• Types of Tissues:

  • Epithelial tissue: Covers surfaces and lines cavities

  • Connective tissue: Supports and binds other tissues

  • Muscle tissue: Produces movement

  • Nerve tissue: Controls and communicates

Studying Human Tissue: Microscopy

Microscopy is essential for studying tissues, as most are too small to be seen with the naked eye.

• Preparation: Tissues are fixed (preserved), cut into thin sections, and stained to enhance contrast.

• Types of Microscopy:

  • Transmission Electron Microscopy (TEM): Uses metal stains; electrons pass through thin sections to reveal internal structures.

  • Scanning Electron Microscopy (SEM): Provides 3D images of surfaces; electrons scan the surface of the specimen.

  • Light Microscopy: Uses visible light and stains to view tissue sections.

Epithelial Tissue

Overview and Types

Epithelial tissue forms boundaries between different environments, protects, secretes, absorbs, and filters.

• Main Types (by location):

  • Covering and lining epithelium: On external and internal surfaces (e.g., skin, lining of digestive tract)

  • Glandular epithelium: Secretory tissue in glands

Functions of Epithelium

• Protection: e.g., skin

• Absorption: e.g., digestive tract, urinary system

• Filtration: e.g., kidneys

• Excretion: e.g., glands, urinary system

• Secretion: e.g., glands

• Sensory reception: e.g., skin, sensory organs

Five Characteristics of Epithelial Tissues

• Polarity: Cells have an apical (top) and basal (bottom) surface.

• Specialized contacts: Cells are closely joined by tight junctions and desmosomes.

• Supported by connective tissue: All epithelial sheets rest upon and are supported by connective tissue.

• Avascular but innervated: No blood vessels, but supplied by nerve fibers.

• Regeneration: High capacity to regenerate, especially in areas subject to friction.

Basement Membrane

• Structure: Consists of the basal lamina (from epithelium) and reticular lamina (from connective tissue).

• Function: Reinforces epithelial sheet, resists stretching and tearing, defines epithelial boundary.

Classification of Epithelia

Epithelia are classified by the number of cell layers and the shape of the cells.

• Number of Layers:

  • Simple epithelium: Single layer of cells

  • Stratified epithelium: Two or more layers of cells

• Cell Shape:

  • Squamous: Flattened and scale-like

  • Cuboidal: Box-like, as tall as they are wide

  • Columnar: Tall and column-shaped

Type	Layers	Shape	Example Location	Function
Simple Squamous	1	Flat	Alveoli of lungs, lining of heart	Diffusion, filtration
Simple Cuboidal	1	Cube-like	Kidney tubules, glands	Secretion, absorption
Simple Columnar	1	Tall, column	Digestive tract lining	Absorption, secretion
Stratified Squamous	Multiple	Flat (surface)	Skin, mouth, esophagus	Protection
Pseudostratified Columnar	Appears stratified, but is not	Columnar	Trachea, upper respiratory tract	Secretion, movement of mucus

Specialized Epithelia

• Transitional epithelium: Found in urinary bladder; allows stretching.

• Mesothelium: The epithelium of serous membranes in the ventral body cavity.

• Endothelium: The friction-reducing lining of lymphatic vessels, blood vessels, and heart.

Examples and Applications

• Simple squamous epithelium: Allows rapid diffusion; found in alveoli (lungs) and capillaries.

• Stratified squamous epithelium: Protects underlying tissues; found in skin, mouth, esophagus, and vagina.

• Simple cuboidal epithelium: Secretion and absorption; found in kidney tubules and small glands.

• Simple columnar epithelium: Absorption and secretion; lines digestive tract, may have microvilli or cilia.

• Pseudostratified columnar epithelium: Secretes mucus; ciliated type lines trachea.

Summary Table: Epithelial Tissue Types

Type	Key Features	Main Function	Location
Simple Squamous	Single layer, flat cells	Diffusion, filtration	Alveoli, capillaries
Simple Cuboidal	Single layer, cube-shaped	Secretion, absorption	Kidney tubules, glands
Simple Columnar	Single layer, tall cells	Absorption, secretion	Digestive tract
Stratified Squamous	Multiple layers, flat surface cells	Protection	Skin, mouth, esophagus
Pseudostratified Columnar	Single layer, nuclei at different levels	Secretion, movement	Trachea, upper respiratory tract
Transitional	Multiple layers, shape varies	Stretching	Urinary bladder

Key Terms

• Apical surface: The upper free surface exposed to the body exterior or the cavity of an internal organ.

• Basal surface: The lower attached surface.

• Basement membrane: Layer between epithelium and underlying connective tissue.

• Avascular: Lacking blood vessels.

• Innervated: Supplied by nerve fibers.

Additional info:

• Some slides reference figures (e.g., Fig 4.5b, 4.6) and images of tissue sections, which are standard in histology textbooks for visual identification.

• For exam preparation, focus on being able to identify tissue types by structure, function, and location.