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. In human anatomy and physiology, understanding tissues is fundamental to comprehending how organs and organ systems function.

• Definition: A tissue is a group of cells with a common structure and function.

• Histology: The study of tissues.

• Types of Primary Tissues:

  • Epithelial tissue – Covers surfaces

  • Connective tissue – Supports and connects

  • Muscle tissue – Produces movement

  • Nerve tissue – Controls and communicates

Studying Human Tissue: Microscopy

Microscopy is essential for studying tissue structure. Tissue samples are prepared through fixation, sectioning, and staining to enhance contrast and reveal cellular details.

• Fixation: Preserves tissue structure.

• Cutting: Thin slices allow light or electron transmission.

• Staining: Enhances contrast for visualization.

Epithelial Tissue

Overview and Functions

Epithelial tissue forms boundaries and covers body surfaces, lines cavities, and forms glands. It is classified by cell layers and cell shapes.

• Functions:

  • Protection

  • Absorption

  • Filtration

  • Excretion

  • Secretion

  • Sensory reception

Five Characteristics of Epithelial Tissues

• Polarity: Distinct apical (top) and basal (bottom) surfaces.

• 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; nutrients diffuse from underlying tissues; supplied by nerve fibers.

• Regeneration: High capacity for renewal via cell division.

Polarity in Epithelial Tissue

Epithelial cells exhibit polarity, with distinct structural and functional differences between the apical and basal surfaces.

• Apical surface: Exposed to exterior or cavity; may have microvilli (increase surface area) or cilia (movement).

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

Support by Connective Tissue

• Basement membrane: Composed of basal lamina (glycoproteins, collagen) and reticular lamina (collagen fibers).

• Provides structural support and acts as a filter.

Specialized Contacts

• Tight junctions: Prevent leakage between cells.

• Desmosomes: Anchor cells together.

Regeneration

• High regenerative capacity due to frequent cell division.

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

Classification of Epithelia

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

• Number of layers:

  • Simple epithelium: Single layer

  • Stratified epithelium: Multiple layers

• Cell shapes:

  • Squamous: Flat, scale-like

  • Cuboidal: Cube-shaped

  • Columnar: Tall, column-like

Cells of Epithelial Tissues

• Squamous cells

• Cuboidal cells

• Columnar cells

• Nucleus shape: Corresponds to cell shape

Simple Epithelia

Simple epithelia consist of a single cell layer and are specialized for absorption, secretion, filtration, and diffusion.

• Simple squamous epithelium:

  • Flat cells; rapid diffusion and filtration

  • Locations: air sacs of lungs, lining of heart, blood vessels, kidney glomeruli

  • Endothelium: Lines blood vessels and heart

  • Mesothelium: Lines serous membranes in ventral body cavity

• Simple cuboidal epithelium:

  • Single layer of cube-shaped cells

  • Secretion and absorption

  • Locations: smallest ducts of glands, kidney tubules

• Simple columnar epithelium:

  • Single layer of tall, closely packed cells

  • Absorption and secretion

  • Locations: digestive tract, gallbladder, uterine tubes

• Pseudostratified columnar epithelium:

  • Cells vary in height; nuclei at different levels

  • Secretion and movement of mucus

  • Locations: trachea, upper respiratory tract

Stratified Epithelial Tissues

Stratified epithelia have two or more layers and are more durable, providing protection.

• Stratified squamous epithelium:

  • Most widespread; surface cells are squamous, deeper cells cuboidal/columnar

  • Located in areas subject to 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

  • Only apical layer is columnar

• Transitional epithelium:

  • Lines hollow urinary organs

  • Basal cells are cuboidal/columnar; apical cells vary in appearance

  • Allows stretching as organs fill

Glandular Epithelium

Glandular epithelia consist of cells that make and secrete a product called a secretion. Glands are classified as endocrine or exocrine.

• Endocrine glands:

  • Ductless; secrete hormones into blood or lymph

  • Target organs respond in specific ways

• Exocrine glands:

  • Secrete products into ducts onto body surfaces or cavities

  • Include sweat, oil, and salivary glands

• Unicellular exocrine glands:

  • Single cells (e.g., goblet cells) that produce mucus

  • Found in intestinal and respiratory tracts

• Multicellular exocrine glands:

  • Composed of a duct and secretory unit

  • Classified by structure and type of secretion

Classification of Multicellular Glands

Multicellular glands are classified by duct structure and mode of secretion.

Type	Structure	Mode of Secretion	Example
Simple gland	Unbranched duct	Merocrine (secrete by exocytosis)	Salivary glands
Compound gland	Branched duct	Holocrine (accumulate products, then rupture)	Sebaceous (oil) glands
Apocrine gland	May be simple or compound	Accumulate products, apex ruptures (rare in humans)	Mammary glands (controversial)

Additional info: Apocrine secretion is rare in humans; most human glands are merocrine or holocrine.

Summary Table: Epithelial Tissue Types

Type	Cell Layers	Cell Shape	Main Functions	Locations
Simple squamous	1	Flat	Diffusion, filtration	Air sacs of lungs, lining of heart, blood vessels
Simple cuboidal	1	Cube	Secretion, absorption	Kidney tubules, small glands
Simple columnar	1	Tall	Absorption, secretion	Digestive tract, uterine tubes
Pseudostratified columnar	1 (appears multilayered)	Tall, varied	Secretion, movement of mucus	Trachea, upper respiratory tract
Stratified squamous	Multiple	Flat (surface)	Protection	Skin, mouth, esophagus
Stratified cuboidal	Multiple	Cube	Protection	Sweat, mammary glands
Stratified columnar	Multiple	Tall	Protection, secretion	Pharynx, male urethra
Transitional	Multiple	Varied	Stretching	Urinary bladder, ureters

Key Equations and Concepts

• Diffusion rate in simple squamous epithelium: $J = -D \frac{dC}{dx}$ Where $J$ is the diffusion flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Conclusion

Epithelial tissue is a fundamental component of the human body, providing protection, absorption, secretion, and filtration. Its classification by cell shape and layers is essential for understanding its diverse functions and locations throughout the body.