Tissues and the Integumentary System: Structure, Function, and Classification

Tissues: Structure and Function

Introduction to Tissues

Tissues are collections of specialized cells and cell products that perform specific functions. The four primary tissue types in the human body are epithelial, connective, muscle, and nervous tissue. Each type has distinct structural and functional characteristics that contribute to the overall physiology of the body.

Epithelial Tissue

General Characteristics

• Epithelial tissue covers exposed surfaces, lines internal passageways and chambers, and forms glands.

• Functions include protection, control of permeability, sensation, and secretion.

• Epithelia are avascular and rely on diffusion or absorption for nutrient delivery.

• Cells are tightly bound by specialized junctions and attached to a basement membrane.

• High regenerative capacity due to rapid stem cell division.

Specializations of Epithelial Cells

• Polarity: Apical (exposed) and basal (attached) surfaces differ in structure and function.

• Microvilli increase surface area for absorption/secretion; cilia move substances across surfaces.

• Cell junctions include tight junctions, gap junctions, and desmosomes, providing integrity and communication.

Classification of Epithelia

• By layers: Simple (one layer) or stratified (multiple layers).

• By shape: Squamous (flat), cuboidal (cube-shaped), columnar (tall).

Glandular Epithelia

• Endocrine glands release hormones into the bloodstream (ductless).

• Exocrine glands secrete onto epithelial surfaces or into ducts.

• Exocrine glands are classified by structure (unicellular/multicellular, simple/compound) and mode of secretion (merocrine, apocrine, holocrine).

Connective Tissue

General Characteristics

• Connective tissue supports, connects, and protects other tissues and organs.

• All connective tissues have specialized cells, extracellular protein fibers, and ground substance (matrix).

• Types include connective tissue proper, fluid connective tissues (blood, lymph), and supporting connective tissues (cartilage, bone).

Connective Tissue Proper

• Loose connective tissue (areolar, adipose, reticular) and dense connective tissue (regular, irregular, elastic).

• Contains fibroblasts, fibrocytes, adipocytes, macrophages, mast cells, and other immune cells.

• Fibers: Collagen (strength), reticular (support), elastic (flexibility).

Supporting Connective Tissues: Cartilage and Bone

• Cartilage: Chondrocytes in lacunae, avascular, flexible matrix with chondroitin sulfate.

• Bone: Osteocytes in lacunae, highly vascular, rigid matrix with calcium salts and collagen fibers.

Characteristic	Cartilage	Bone
Cells	Chondrocytes in lacunae	Osteocytes in lacunae
Ground substance	Chondroitin sulfate in proteoglycans and water	Small volume of liquid; crystals of calcium salts (calcium phosphate and calcium carbonate)
Fibers	Collagen, elastic, and reticular fibers (proportions vary)	Collagen fibers predominate
Vascularity	None	Extensive
Covering	Perichondrium (two layers)	Periosteum (two layers)
Strength	Limited; bends easily, hard to break	Strong; resists distortion until breaking point
Oxygen demand	Low	High
Nutrient delivery	By diffusion through matrix	By diffusion through cytoplasm and interstitial fluid in canaliculi
Growth	Interstitial and appositional	Appositional only
Repair capabilities	Limited	Extensive

Muscle Tissue

Types of Muscle Tissue

• Skeletal muscle: Long, multinucleate, striated, voluntary; moves skeleton.

• Cardiac muscle: Short, branched, striated, involuntary; found in heart, connected by intercalated discs.

• Smooth muscle: Short, spindle-shaped, non-striated, involuntary; found in walls of hollow organs.

Nervous Tissue

Structure and Function

• Specialized for conduction of electrical impulses.

• Contains neurons (transmit signals) and neuroglia (supporting cells).

• Neurons have a cell body, dendrites (receive signals), and an axon (transmits signals).

The Integumentary System

Overview and Functions

• Composed of the cutaneous membrane (epidermis and dermis) and accessory structures (hair, glands, nails).

• Functions: protection, excretion, temperature regulation, vitamin D3 synthesis, lipid storage, sensory detection, immune coordination.

Cutaneous Membrane

• Epidermis: Stratified squamous epithelium; provides protection, prevents water loss, and keeps out pathogens.

• Dermis: Papillary layer (areolar tissue, capillaries, nerves) and reticular layer (dense irregular connective tissue, collagen, and elastic fibers).

Accessory Structures

• Hair follicles: Produce hair for protection and sensation.

• Exocrine glands: Sebaceous (oil) and sweat glands; assist in thermoregulation and waste excretion.

• Nails: Protect tips of fingers and toes.

Summary Table: Simple and Compound Glands

Type	Structure	Examples
Simple Tubular	Unbranched, tube-like	Intestinal glands
Simple Coiled Tubular	Coiled tube	Merocrine sweat glands
Simple Branched Tubular	Branched tubes	Gastric glands, mucous glands of esophagus, tongue, duodenum
Simple Alveolar (Acinar)	Unbranched, sac-like	Not found in adults
Simple Branched Alveolar	Branched sacs	Sebaceous (oil) glands
Compound Tubular	Branched, tube-like	Mucous glands (mouth), bulbourethral glands (male reproductive system), testes (seminiferous tubules)
Compound Alveolar (Acinar)	Branched, sac-like	Mammary glands
Compound Tubulo-Alveolar	Branched, tubes and sacs	Salivary glands, glands of respiratory passages, pancreas

Summary

• Tissues are organized groups of cells with specialized functions.

• Epithelial tissue covers surfaces and forms glands; connective tissue supports and connects; muscle tissue contracts for movement; nervous tissue transmits impulses.

• The integumentary system protects, regulates, and senses the external environment, and is composed of the skin and its accessory structures.