Chapter 3: Tissues – Structure, Function, and Classification

Introduction to Tissues

Overview of Primary Tissue Types

All cells in the human body are organized into four primary tissue types, each with specialized functions essential for maintaining homeostasis and supporting life.

• Epithelial tissue: Covers exposed surfaces, lines internal passageways, and forms glands.

• Connective tissue: Fills internal spaces, provides structural support, stores energy, and transports materials.

• Muscular tissue: Contracts to produce movement.

• Neural tissue: Conducts electrical impulses and processes information.

Epithelial Tissue

Characteristics of Epithelial Tissue

• Cellularity: Cells are tightly bound together with very little extracellular space.

• Arrangement: Cells are organized in single (simple) or multiple (stratified) layers.

• Polarity: Each cell has an apical surface (faces away from the basement membrane) and a basal surface (attached to the basement membrane).

• Avascularity: Epithelial tissues lack blood vessels.

• Regeneration: High capacity for renewal via stem cell division near the basal lamina.

Functions of Epithelial Tissue

• Physical protection: Shields underlying tissues from abrasion, dehydration, and chemical damage.

• Control of permeability: Regulates entry and exit of substances.

• Absorption: Uptake of nutrients and other substances.

• Secretion: Production and release of substances such as enzymes, hormones, and mucus.

Specializations of Epithelial Cells

• Microvilli: Increase surface area for absorption (e.g., intestinal lining).

• Cilia: Move substances across the apical surface (e.g., respiratory tract).

Maintaining Epithelial Integrity

• Intercellular connections: Cell junctions, cell adhesion molecules (CAMs), and intercellular cement provide strength and stability.

• Basement membrane:

  • Basal lamina: Glycoproteins, proteoglycans, and microfilaments anchor the epithelium.

  • Reticular lamina: Protein fibers connect epithelium to underlying connective tissue.

• Self-renewal: Stem cells near the basal lamina continually divide to replace lost or damaged cells.

Classification of Epithelia

• Simple epithelium: One layer of cells; all cells touch the basement membrane.

• Stratified epithelium: Two or more layers; only basal cells contact the basement membrane. Named for the shape of cells in the apical layer.

Shapes of epithelial cells:

• Squamous: Flat and thin.

• Cuboidal: Cube-shaped.

• Columnar: Taller than wide, column-like.

Types of Epithelia and Their Functions

• Simple squamous epithelium:

  • Location: Serous membranes (mesothelium), inner lining of heart and blood vessels (endothelium), alveoli of lungs.

  • Function: Reduces friction, allows diffusion and filtration, absorbs and secretes material.

• Stratified squamous epithelium:

  • Location: Keratinized (epidermis of skin); non-keratinized (oral cavity, esophagus, vagina, rectum).

  • Function: Protection against abrasion, pathogens, and chemical attack.

• Simple cuboidal epithelium:

  • Location: Thyroid gland, ducts, kidney tubules.

  • Function: Secretion and absorption.

• Stratified cuboidal epithelium (rare):

  • Location: Ducts of sweat and mammary glands.

  • Function: Protection, secretion, absorption.

• Simple columnar epithelium:

  • Location: Lining of stomach, intestines, gallbladder, uterine tubes, collecting ducts of kidneys.

  • Function: Secretion, absorption, protection.

• Stratified columnar epithelium (rare):

  • Location: Pharynx, epiglottis, anus, mammary glands, salivary glands, urethra.

  • Function: Protection.

• Pseudostratified ciliated columnar epithelium:

  • Location: Nasal cavity, trachea, bronchi.

  • Function: Protection, secretion; cilia move mucus.

• Transitional epithelium:

  • Location: Urinary bladder, renal pelvis, ureters.

  • Function: Permits expansion and recoil after stretching.

Glandular Epithelia

• Exocrine glands: Secrete products into ducts.

  • Serous glands: Watery secretions with enzymes.

  • Mucous glands: Secrete mucins (mucus).

  • Mixed exocrine glands: Both serous and mucous secretions.

• Endocrine glands: Ductless; secrete hormones directly into the bloodstream.

Structure of Glands

• Simple glands: Tubular (tube-shaped) or alveolar/acinar (blind pocket).

• Compound glands: Tubuloalveolar (combines tubular and alveolar).

Modes of Secretion

• Eccrine (merocrine): Secretion by exocytosis (e.g., salivary glands).

• Apocrine: Shedding of the apical portion of the cell (e.g., mammary glands).

• Holocrine: Cell bursts apart, releasing contents (e.g., sebaceous glands).

Connective Tissue

General Features

• Most abundant tissue type in the body.

• All connective tissues have three main components:

  • Specialized cells

  • Extracellular protein fibers (collagen, reticular, elastic)

  • Ground substance (fluid or semi-solid matrix)

• Matrix = fibers + ground substance; usually more matrix than cells (except adipose tissue).

Functions of Connective Tissue

• Structural framework for the body

• Transport of fluids and dissolved materials

• Protection of organs

• Support, surround, and connect other tissues

• Energy storage (e.g., adipose tissue)

• Defense against microorganisms

Categories of Connective Tissue

1. Connective Tissue Proper: Matrix of loose or dense fibers

2. Fluid Connective Tissue: Matrix is liquid (blood, lymph)

3. Supporting Connective Tissue: Matrix of fibers and, sometimes, insoluble calcium salts (bone, cartilage)

Connective Tissue Proper

Cells

• Fixed cells: Maintenance, repair, energy storage

• Wandering cells: Defense and repair of damaged tissue

Fibers

• Collagen fibers: Long, unbranched, strong; provide tensile strength

• Reticular fibers: Highly branched; form supportive mesh in organs like spleen and liver

• Elastic fibers: Branched, wavy; contain elastin for flexibility

Types of Connective Tissue Proper

• Loose connective tissue: Areolar, adipose, reticular

• Dense connective tissue: Dense regular, dense irregular

Fluid Connective Tissue

• Blood: Plasma (matrix) + formed elements (erythrocytes, leukocytes, platelets)

• Lymph: Plasma that has left blood vessels; contains immune cells

Supporting Connective Tissue

• Cartilage: Chondrocytes in a gel matrix of chondroitin sulfate; avascular; types include hyaline, elastic, and fibrocartilage

• Bone: Osteocytes in a solid matrix of calcium phosphate and collagen fibers; vascularized; organized as spongy (trabeculae) or compact (osteons)

Summary Table: Epithelial Tissue Types

Type	Location	Function
Simple squamous	Serous membranes, alveoli, endothelium	Diffusion, filtration, secretion
Stratified squamous	Epidermis, oral cavity, esophagus, vagina	Protection
Simple cuboidal	Kidney tubules, glands	Secretion, absorption
Stratified cuboidal	Sweat/mammary ducts (rare)	Protection, secretion
Simple columnar	GI tract, gallbladder, uterine tubes	Absorption, secretion
Stratified columnar	Pharynx, anus, glands (rare)	Protection
Pseudostratified ciliated columnar	Respiratory tract	Secretion, movement of mucus
Transitional	Urinary bladder, ureters	Stretching

Summary Table: Connective Tissue Types

Type	Main Cells	Matrix/Fibers	Function
Areolar	Fibroblasts	Collagen, elastic, reticular	Cushioning, support
Adipose	Adipocytes	Few fibers	Energy storage, insulation
Reticular	Reticular cells	Reticular fibers	Supportive framework
Dense regular	Fibroblasts	Parallel collagen fibers	Tendons, ligaments
Dense irregular	Fibroblasts	Irregular collagen fibers	Dermis, organ capsules
Blood	RBCs, WBCs, platelets	Plasma	Transport, immunity
Cartilage	Chondrocytes	Collagen, elastic fibers	Support, flexibility
Bone	Osteocytes	Collagen, calcium salts	Support, protection

Practice Questions

• What are the three types of cells found in epithelial tissue?

• How many layers of cells are found in a simple tissue?

• What surface of the cell is attached to the basement membrane?

• What type of tissue is found in the epidermis (outer layer of skin)?

Additional info: This guide covers the essential structure, classification, and function of epithelial and connective tissues, as outlined in a typical Anatomy & Physiology curriculum.