Tissue: The Living Fabric – Epithelial Tissue Study Notes

Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells that are similar in structure and perform common or related functions. The study of tissues is known as histology. In multicellular organisms, individual body cells are specialized to carry out specific functions, contributing to the overall function of the tissue.

• Tissue Types: Epithelial, Connective, Muscle, and Nerve tissues

• Histology: The microscopic study of tissues

Types of Primary Tissues

Overview of Tissue Types

• Epithelial tissue: Covers body surfaces and lines cavities

• Connective tissue: Supports, protects, and binds other tissues

• Muscle tissue: Produces movement

• Nerve tissue: Controls and communicates via electrical impulses

Studying Human Tissue: Microscopy

Preparation of Tissue Samples

• Fixation: Preserves tissue structure

• Sectioning: Slicing thin enough for light or electron microscopy

• Staining: Enhances contrast to visualize cellular components

Epithelial Tissue

General Features

Epithelial tissue forms boundaries between different environments and is classified based on location and function. It covers body surfaces, lines cavities, and forms glands.

• Covering and lining epithelium: Outer layer of skin, lining of open cavities

• Glandular epithelium: Forms glands

Functions of Epithelial Tissue

• Protection

• Absorption

• Filtration

• Excretion

• Secretion

• Sensory reception

Five Characteristics of Epithelial Tissues

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

• Specialized contacts: Cells fit closely together via junctions

• Supported by connective tissue: Basal lamina and reticular lamina form basement membrane

• Avascular but innervated: No blood vessels, but supplied by nerves

• Regeneration: High capacity for renewal

Cell Polarity in Epithelial Tissue

• Apical surface: Exposed to exterior or cavity, may have microvilli or cilia

• Basal surface: Attached to underlying connective tissue via basal lamina

Specialized Contacts

• Tight junctions: Prevent leakage of extracellular fluid

• Desmosomes: Anchor cells together

Support by Connective Tissue

• Basement membrane: Consists of basal lamina and reticular lamina

• Provides structural support and acts as a filter

Avascular but Innervated

• No blood vessels in epithelial tissue

• Nutrients diffuse from underlying connective tissue

• Supplied by nerve fibers

Regeneration

• High regenerative capacity

• Stimulated by loss of apical-basal polarity and lateral contacts

• Some exposed to friction or hostile substances

Classification of Epithelia

Criteria for Classification

• Number of cell layers: Simple (one layer) or stratified (multiple layers)

• Cell shape: Squamous (flat), cuboidal (cube-like), columnar (tall)

Table: Classification of Epithelial Tissues

Type	Number of Layers	Cell Shape	Main Functions	Locations
Simple Squamous	1	Flat	Absorption, filtration	Kidney glomeruli, air sacs of lungs, lining of heart, blood vessels
Simple Cuboidal	1	Cube-like	Secretion, absorption	Small glands, kidney tubules
Simple Columnar	1	Tall	Absorption, secretion	Digestive tract, gallbladder, uterine tubes
Pseudostratified Columnar	1 (appears multilayered)	Varied	Secretion, propulsion of mucus	Trachea, upper respiratory tract
Stratified Squamous	2+	Flat (surface), cuboidal/columnar (deeper)	Protection	Skin, mouth, esophagus
Stratified Cuboidal	2+	Cube-like	Protection	Some sweat and mammary glands
Stratified Columnar	2+	Tall	Protection, secretion	Male urethra, large ducts of glands
Transitional	Varies	Varies	Stretching	Urinary bladder, ureters

Cells of Epithelial Tissues

• Squamous cells: Flattened and scale-like

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

• Columnar cells: Tall and column shaped

Simple Epithelia

Simple Squamous Epithelium

• Single layer of flattened cells

• Allows rapid diffusion and filtration

• Found in air sacs of lungs, lining of heart, blood vessels

• Endothelium: Lines blood vessels and heart

• Mesothelium: Lines serous membranes in ventral body cavity

Simple Cuboidal Epithelium

• Single layer of cube-like cells

• Functions in secretion and absorption

• Found in kidney tubules, small glands

Simple Columnar Epithelium

• Single layer of tall, closely packed cells

• Functions in absorption and secretion

• Lines digestive tract, gallbladder, uterine tubes

Pseudostratified Columnar Epithelium

• Cells vary in height, nuclei at different levels

• Functions in secretion and propulsion of mucus

• Lines trachea and most of upper respiratory tract

Stratified Epithelial Tissues

General Features

• Two or more cell layers

• More durable than simple epithelia

• Protection is major role

Stratified Squamous Epithelium

• Most widespread stratified epithelium

• Surface cells are squamous; deeper layers are cuboidal or columnar

• Located for wear and tear (skin, mouth, esophagus)

Stratified Cuboidal Epithelium

• Rare

• Found in some sweat and mammary glands

• Typically two cell layers thick

Stratified Columnar Epithelium

• Limited distribution

• Found in pharynx, male urethra, some glandular ducts

Transitional Epithelium

• Forms lining of hollow urinary organs

• Basal layer cells are cuboidal or columnar

• Ability to change shape with stretch

• Found in urinary bladder, ureters

Glandular Epithelium

Glands

• One or more cells that make and secrete an aqueous fluid called a secretion

• Classified by site of product release (endocrine or exocrine) and number of cells (unicellular or multicellular)

Endocrine Glands

• Ductless glands

• Secrete hormones directly into blood or lymph

• Target organs respond in specific ways

Exocrine Glands

• Secrete products onto body surfaces or into body cavities

• Include mucous, sweat, oil, and salivary glands

• Can be unicellular (e.g., goblet cells) or multicellular

Unicellular Exocrine Glands

• Found in epithelial linings of intestinal and respiratory tracts

• Produce mucin, which forms mucus

Multicellular Exocrine Glands

• Composed of a duct and a secretory unit

• Classified by duct structure (simple or compound) and secretory unit shape (tubular, alveolar, or tubuloalveolar)

Table: Classification of Multicellular Exocrine Glands

Type	Duct Structure	Secretory Unit Shape	Example
Simple tubular	Unbranched	Tubular	Intestinal glands
Simple alveolar	Unbranched	Alveolar	No important example in humans
Compound tubular	Branched	Tubular	Brunner's glands of small intestine
Compound alveolar	Branched	Alveolar	Mammary glands
Compound tubuloalveolar	Branched	Tubular and alveolar	Salivary glands

Types of Secretion

• Merocrine: Products secreted by exocytosis (e.g., sweat glands)

• Holocrine: Products secreted by rupture of gland cells (e.g., sebaceous glands)

• Apocrine: Accumulate products but only apex ruptures (controversial in humans)

Additional info: These notes expand upon the original slides by providing definitions, examples, and tables for classification, as well as context for tissue functions and locations.