Chapter 4: Tissue—The Living Fabric (Study Notes)

Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells with similar structure and function, essential for maintaining homeostasis in the human body. The study of tissues is known as histology. There are four basic tissue types: epithelial, connective, muscle, and nervous tissue.

• Epithelial tissue: Covers surfaces and lines cavities.

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

• Muscle tissue: Contracts to produce movement.

• Nervous tissue: Communicates and regulates body functions.

Preparation of Tissue Samples for Microscopy

Fixation, Sectioning, and Staining

To study tissues under a microscope, samples must be fixed (preserved), sectioned (sliced thinly), and stained (colored for contrast). Light microscopy uses colored dyes, while electron microscopy uses heavy metal salts. Transmission electron microscopy (TEM) shows internal sections, and scanning electron microscopy (SEM) shows surfaces.

Epithelial Tissue

Definition and Functions

Epithelial tissue is a sheet of cells covering body surfaces or lining cavities. It exists in two main forms: covering and lining epithelium (e.g., skin, lining of organs) and glandular epithelium (e.g., glands). Its main functions include protection, absorption, filtration, excretion, secretion, and sensory reception.

Special Characteristics of Epithelial Tissue

• Polarity: Cells have distinct apical (top) and basal (bottom) surfaces. The apical surface may have microvilli, while the basal surface attaches to the basal lamina.

• Specialized contacts: Cells are tightly joined by junctions (tight junctions, desmosomes).

• Supported by connective tissue: The basement membrane (basal and reticular lamina) reinforces the sheet.

• Avascular but innervated: No blood vessels, but supplied by nerves; nutrients diffuse from underlying tissues.

• Regeneration: High capacity for renewal, especially in areas exposed to friction or hostile environments.

Classification of Epithelial Tissue

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

• Simple epithelium: Single layer, ideal for absorption and filtration.

• Stratified epithelium: Multiple layers, ideal for protection.

• Squamous: Flattened, scale-like cells.

• Cuboidal: Cube-shaped cells.

• Columnar: Tall, column-like cells.

Types of Simple Epithelia

• Simple squamous epithelium: Thin, flat cells for rapid diffusion; found in kidneys, lungs, blood vessels (endothelium), and serous membranes (mesothelium).

• Simple cuboidal epithelium: Cube-shaped cells for secretion and absorption; found in kidney tubules and gland ducts.

• Simple columnar epithelium: Tall cells, often with microvilli or cilia; involved in absorption and secretion; found in digestive tract, gallbladder, bronchi, and uterine tubes.

• Pseudostratified columnar epithelium: Appears multi-layered but is single-layered; often ciliated; functions in secretion and movement of mucus; found in respiratory tract and ducts of large glands.

Summary Table: Epithelial Tissue Types

Type	Structure	Function	Location
Simple Squamous	Single layer, flat	Diffusion, filtration	Kidney, lungs, vessels
Simple Cuboidal	Single layer, cube	Secretion, absorption	Kidney tubules, glands
Simple Columnar	Single layer, tall	Absorption, secretion	Digestive tract, bronchi
Pseudostratified Columnar	Single layer, varied height	Mucus secretion, movement	Respiratory tract

Additional info:

Stratified epithelia (not shown in images) provide protection and are found in areas of high abrasion, such as skin and oral cavity. Transitional epithelium lines urinary organs and can stretch.