The Tissue Level of Organization: Structure and Function of Body Tissues

The Tissue Level of Organization

Introduction to Tissues and Histology

Tissues are collections of specialized cells and cell products that perform specific, limited functions. The study of tissues is known as histology. Understanding tissues is essential for comprehending how organs and organ systems function in the human body.

Epithelial Tissue

Overview and Characteristics

Epithelial tissue includes layers of cells that cover internal or external surfaces and glands that produce secretions. Key characteristics include:

• Cells bound closely together

• Free (apical) surface exposed to the environment

• Attachment to underlying connective tissue by a basement membrane

• Avascular (lacking blood vessels)

• Continual replacement or regeneration of cells

Locations and Functions

Epithelia cover both external and internal body surfaces, forming selective barriers and lining internal cavities and passageways. Major functions include:

• Physical protection

• Control of permeability

• Provision of sensation

• Production of specialized secretions (glandular epithelium)

Glandular Epithelium

Gland cells are classified by where their secretions are discharged:

• Exocrine glands: Secrete onto epithelial surfaces via ducts

• Endocrine glands: Release hormones into interstitial fluid and blood (ductless)

Intercellular Connections (Cell Junctions)

Cell junctions allow firm attachment between epithelial cells and to the basement membrane. Major types include:

• Tight junctions: Prevent passage of substances between cells

• Adhesion belts: Reinforce tight junctions

• Gap junctions: Permit communication between cells

• Desmosomes: Provide strong attachment between cells

Classification of Epithelia

Epithelia are classified by the number of cell layers and the shape of the cells at the apical surface:

	Squamous	Cuboidal	Columnar
Simple	Simple squamous epithelium	Simple cuboidal epithelium	Simple columnar epithelium
Stratified	Stratified squamous epithelium	Stratified cuboidal epithelium	Stratified columnar epithelium

Examples of Epithelial Types

• Simple squamous epithelium: Thin, flat cells; found in alveoli of lungs, lining of heart and blood vessels. Function: diffusion and filtration.

• Simple cuboidal epithelium: Cube-shaped cells; found in kidney tubules, glands. Function: secretion and absorption.

• Simple columnar epithelium: Tall, column-like cells; found in digestive tract lining. Function: absorption and secretion.

• Stratified squamous epithelium: Multiple layers; found in skin, mouth, esophagus. Function: protection.

• Pseudostratified columnar epithelium: Appears layered but all cells touch basement membrane; found in respiratory tract. Function: protection, secretion.

• Transitional epithelium: Appearance changes with stretching; found in urinary bladder. Function: permits expansion and recoil.

Modes of Exocrine Secretion

Exocrine glands secrete by three main methods:

• Merocrine secretion: Product released by exocytosis (e.g., salivary glands)

• Apocrine secretion: Involves loss of cytoplasm with secretion (e.g., mammary glands)

• Holocrine secretion: Entire cell bursts, releasing contents (e.g., sebaceous glands)

Method of Secretion	Description	Examples
Merocrine	Secretion occurs through exocytosis	Saliva from salivary glands, sweat
Apocrine	Secretion occurs through loss of cytoplasm	Milk in breasts, underarm perspiration
Holocrine	Secretion occurs through loss of entire cell	Sebaceous (oil) glands

Connective Tissue

Overview and Characteristics

Connective tissue is the most diverse tissue type, providing a structural framework, support, and protection for other tissues. It consists of specialized cells, extracellular protein fibers, and ground substance (fluid).

Cells and Fibers of Connective Tissue Proper

• Fibroblasts: Produce fibers and ground substance

• Fibrocytes: Maintain fibers

• Macrophages: Phagocytize pathogens and debris

• Adipocytes: Store fat

• Mast cells: Release histamine and heparin during inflammation

Types of fibers:

• Collagen fibers: Strong, flexible, most common

• Elastic fibers: Stretch and return to original length

• Reticular fibers: Form supportive networks

Types of Connective Tissue Proper

• Loose connective tissue: More ground substance, fewer fibers; supports epithelia, fills spaces, stores lipids

  • Areolar tissue: Cushions organs, provides support

  • Adipose tissue: Stores energy, insulates, cushions

  • Reticular tissue: Provides supporting framework

• Dense connective tissue: More fibers, less ground substance; resists tension

  • Dense regular connective tissue: Parallel collagen fibers; tendons, ligaments

  • Dense irregular connective tissue: Interwoven fibers; dermis, organ capsules

Fluid and Supporting Connective Tissues

• Fluid connective tissues: Blood and lymph; transport cells and dissolved materials

• Supporting connective tissues: Cartilage and bone; provide strong framework

Cartilage

• Hyaline cartilage: Most common; closely packed collagen fibers; found in joints

• Elastic cartilage: Contains elastic fibers; flexible; found in ear

• Fibrocartilage: Densely woven collagen fibers; durable; found in intervertebral discs

Bone (Osseous Tissue)

Bone tissue has a hard matrix of calcium compounds and flexible collagen fibers. Osteocytes are arranged around central canals and communicate via canaliculi. The periosteum covers bone surfaces except at joints.

Feature	Cartilage	Bone
Cells	Chondrocytes in lacunae	Osteocytes in lacunae
Ground substance	Chondroitin sulfate gel	Small volume of liquid with calcium salts
Fibers	Collagen, elastic, reticular	Collagen fibers (predominate)
Vascularity	Avascular	Vascular
Repair	Limited	Extensive

Tissue Membranes

Types of Tissue Membranes

Tissue membranes are physical barriers that line or cover body surfaces. They consist of an epithelium and supporting connective tissue. Types include:

• Mucous membranes: Line passageways open to exterior; kept moist by secretions

• Serous membranes: Line sealed internal cavities; secrete serous fluid to reduce friction

• Cutaneous membrane: Skin; thick, waterproof, dry

• Synovial membranes: Line joint cavities; produce synovial fluid for lubrication

Muscle Tissue

Types and Functions

Muscle tissue is specialized for contraction and movement. There are three types:

• Skeletal muscle: Voluntary, striated, moves skeleton

• Cardiac muscle: Involuntary, striated, found in heart, intercalated discs for coordination

• Smooth muscle: Involuntary, nonstriated, found in walls of organs

Nervous Tissue

Structure and Function

Nervous tissue is specialized for transmitting electrical impulses. It is concentrated in the brain and spinal cord and is essential for sensing the environment and controlling responses.

• Neurons: Nerve cells that transmit signals

• Neuroglia: Supporting cells that protect and nourish neurons

Tissue Injury and Repair

Inflammation and Regeneration

Tissues respond to injury through inflammation (first response) and regeneration (repair). Inflammation involves swelling, heat, redness, and pain, triggered by trauma or infection. Mast cells release chemicals that increase blood flow and attract immune cells. Regeneration replaces damaged tissue; some tissues regenerate well (epithelia, connective), others poorly (cardiac, neural).

Aging and Tissue Changes

With age, tissue repair slows, and the risk of cancer increases. Epithelia thin, connective tissues become fragile, bones brittle, and cartilage less resilient. Cancer rates rise due to accumulated mutations from environmental exposures.