Organization and Classification of Human Tissues: Study Notes

4-1 The Four Tissue Types

Overview of Tissue Types

Tissues are groups of specialized cells and cell products that perform specific functions in the human body. The study of tissues is known as histology.

• Epithelial tissue: Covers surfaces, lines cavities, and forms glands.

• Connective tissue: Fills spaces, supports tissue, stores energy, and transports materials.

• Muscle tissue: Contracts to produce movement.

• Nervous tissue: Transmits electrical impulses for communication.

Histology is the study of tissues.

4-2 Epithelial Tissue

General Characteristics

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is characterized by several key features:

• Avascular: Lacks blood vessels; relies on diffusion for nutrients.

• Attached to underlying connective tissue by a basement membrane (composed of basal lamina and reticular lamina).

• Polarity: Has an apical (exposed) surface and a basal (attached) surface.

  • Apical surface may have microvilli (increase absorption) or cilia (move materials).

  • Basal surface attaches to connective tissue.

• Cells are replaced by stem cells (short life span).

• Functions include:

  • Physical protection

  • Control permeability

  • Provide sensation (contain sensory nerves)

  • Produce secretions (via glands)

• Cell Junctions:

  • Tight junctions: Prevent leakage.

  • Gap junctions: Allow communication between cells.

  • Desmosomes: Strong connections; resist stress.

4-3 Classification of Epithelia

By Layers

• Simple: Single layer; good for diffusion, absorption, secretion.

• Stratified: Multiple layers; good for protection.

By Shape

• Squamous: Flat, thin (diffusion, absorption).

• Cuboidal: Cube-shaped (secretion, absorption).

• Columnar: Tall, thin (absorption, protection).

Glands

• Exocrine glands: Secrete onto surfaces/ducts (e.g., sweat, saliva).

• Endocrine glands: Release hormones into blood.

• Unicellular glands: Goblet cells, mucous cells (secrete mucin).

• Multicellular glands: Large structures (salivary, mammary).

4-4 Connective Tissue Overview

Functions and Components

Connective tissue provides framework, transport, protection, support, energy storage, and defense.

• Specialized cells: Various cell types depending on tissue subtype.

• Matrix: Extracellular fibers + ground substance (majority of tissue volume).

Categories

• Connective tissue proper: Many cell types, fibers in syrupy ground substance.

• Fluid connective tissue: Blood, lymph.

• Supporting connective tissue: Cartilage, bone.

4-5 Connective Tissue Proper

Cell Types and Fibers

• Cell types: Fibroblasts, fibrocytes, macrophages, adipocytes (fat), mesenchymal cells (stem), mast cells, lymphocytes, microphages.

• Fibers:

  • Collagen: Strong, resist force.

  • Reticular: Branching, form supportive networks.

  • Elastic: Stretch and return.

Subtypes

• Loose connective tissue (areolar, adipose, reticular): Fills space, cushions, supports.

• Dense connective tissue (dense regular, dense irregular, elastic): Provides strength, resists tension.

4-6 Fluid Connective Tissues (Blood & Lymph)

Blood

• Formed elements: Red blood cells (RBCs, carry O2), white blood cells (WBCs, defense), platelets (clotting).

• Plasma: Fluid matrix.

Lymph

• Formed from interstitial fluid.

• Monitored by immune system.

• Returns to cardiovascular system.

4-7 Supporting Connective Tissue (Cartilage & Bone)

Cartilage

• Cells: Chondrocytes in lacunae.

• Avascular (nutrients by diffusion).

• Types:

  • Hyaline: Stiff, flexible (ribs, nose, trachea).

  • Elastic: Flexible (ear, epiglottis).

  • Fibrocartilage: Very strong (discs, pubic symphysis).

• Growth: Interstitial (within tissue) & appositional (surface).

Bone (Osseous Tissue)

• Cells: Osteocytes in lacunae, connected by canaliculi.

• Highly vascularized; supports and protects.

4-8 Tissue Membranes

Types of Membranes

• Mucous membranes: Line cavities open to exterior (respiratory, digestive).

• Serous membranes: Line sealed cavities (pleura, pericardium, peritoneum); secrete serous fluid.

• Cutaneous membrane: Skin.

• Synovial membranes: Line joints; secrete synovial fluid.

4-9 Muscle Tissue

Types of Muscle Tissue

• Skeletal muscle: Striated, voluntary, multinucleate, regenerates via myosatellite cells.

• Cardiac muscle: Striated, involuntary, only in heart, pacemaker cells, intercalated discs.

• Smooth muscle: Non-striated, involuntary, regenerates well, found in walls of hollow organs.

4-10 Nervous Tissue

Structure and Function

• Neurons: Transmit impulses.

  • Structure: Cell body, dendrites (receive), axon (sends).

• Neuroglia (glial cells): Support, protect, supply nutrients.

4-11 Tissue Injury & Repair

Inflammation and Repair

• Inflammation: Isolates injury, removes pathogens/damaged cells.

• Repair processes restore tissue integrity.

4-12 Aging & Tissues

Effects of Aging

• Tissue repair slows with age.

• Collagen/elastic fibers decrease, leading to wrinkles and weaker tissues.

• Higher cancer risk (75% linked to environment: smoking, chemical exposure).