Tissue: The Living Fabric – Study Notes

cChapter 4: Tissue – The Living Fabric

1. Introduction to Tissues, Organs, and Organ Systems

Tissues are groups of similar cells that perform a common function. Organs are structures composed of at least two tissue types that work together to perform specific, complex functions. Organ systems are groups of organs that work together to accomplish a common purpose.

• Tissue Types: Epithelial, Connective, Muscle, Nervous

• Organs: Composed of multiple tissue types (e.g., stomach, heart)

• Organ Systems: Groups of organs working together (e.g., digestive system)

2. Epithelial Tissue: Characteristics and Classification

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is specialized for protection, absorption, filtration, excretion, secretion, and sensory reception.

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

• Cellularity: Composed almost entirely of closely packed cells.

• Specialized Contacts: Cells are bound together by tight junctions and desmosomes.

• Supported by Connective Tissue: Rest on a basement membrane.

• Avascular but Innervated: Contains no blood vessels but supplied by nerve fibers.

• Regeneration: High regenerative capacity.

Classification of Epithelia

• By Layers: Simple (one layer), Stratified (multiple layers)

• By Shape: Squamous (flat), Cuboidal (cube-like), Columnar (tall)

Type	Layers	Shape	Main Function
Simple Squamous	1	Flat	Diffusion, filtration
Simple Cuboidal	1	Cube-like	Secretion, absorption
Simple Columnar	1	Tall	Absorption, secretion
Pseudostratified Columnar	1 (appears multi-layered)	Varied	Secretion, propulsion (ciliated)
Stratified Squamous	Multiple	Flat (surface)	Protection
Stratified Cuboidal	2+	Cube-like	Protection
Stratified Columnar	2+	Tall	Protection, secretion
Transitional	Multiple	Varied	Stretching (urinary organs)

Examples and Locations

• Simple Squamous: Air sacs of lungs, lining of heart, blood vessels

• Simple Cuboidal: Kidney tubules, ducts of small glands

• Simple Columnar: Digestive tract, gallbladder

• Pseudostratified Columnar: Trachea, upper respiratory tract

• Stratified Squamous: Skin (keratinized), mouth, esophagus (non-keratinized)

• Transitional: Urinary bladder, ureters

3. Glandular Epithelium

Glandular epithelia form glands that secrete substances into ducts (exocrine) or into the blood (endocrine).

• Endocrine Glands: Ductless; secrete hormones directly into blood (e.g., thyroid, pituitary)

• Exocrine Glands: Secrete products into ducts (e.g., sweat, salivary, mammary glands)

Types of Exocrine Glands

• Unicellular: Goblet cells (secrete mucus)

• Multicellular: Composed of a duct and secretory unit; classified by duct structure (simple/compound) and secretory unit shape (tubular/alveolar)

Modes of Secretion

• Merocrine: Secrete by exocytosis (e.g., sweat, pancreas)

• Holocrine: Accumulate products until cell ruptures (e.g., sebaceous glands)

• Apocrine: Accumulate products at apex, then pinch off (rare in humans)

4. Connective Tissue: Characteristics and Types

Connective tissue supports, protects, and binds other tissues. It is the most abundant and widely distributed tissue type in the body.

• Common Features: Mesenchyme origin, varying vascularity, extracellular matrix (ground substance + fibers)

Structural Elements

• Ground Substance: Fills space between cells; contains interstitial fluid, cell adhesion proteins, proteoglycans

• Fibers: Collagen (strength), elastic (stretch), reticular (support)

• Cells: Fibroblasts (connective tissue proper), chondroblasts (cartilage), osteoblasts (bone), hematopoietic stem cells (blood)

Types of Connective Tissue

Type	Main Cells	Function	Location
Loose (Areolar)	Fibroblasts	Cushions organs, inflammation	Under epithelia
Adipose	Adipocytes	Energy storage, insulation	Under skin, around organs
Reticular	Reticular cells	Support for lymphoid organs	Lymph nodes, spleen
Dense Regular	Fibroblasts	Attach muscles to bones, withstand tension	Tendons, ligaments
Dense Irregular	Fibroblasts	Withstand tension in many directions	Dermis, joint capsules
Elastic	Fibroblasts	Allows recoil	Walls of arteries, vertebral ligaments
Cartilage	Chondroblasts/cytes	Support, cushioning	Joints, ear, nose, trachea
Bone	Osteoblasts/cytes	Support, protection, mineral storage	Bones
Blood	Hematopoietic cells	Transport gases, nutrients, wastes	Blood vessels

5. Membranes

• Cutaneous Membrane: Skin; dry, protective outer covering

• Mucous Membranes: Line body cavities open to exterior; moist, secrete mucus (e.g., digestive, respiratory tracts)

• Serous Membranes: Line closed ventral body cavities; secrete serous fluid (e.g., pleura, pericardium, peritoneum)

6. Muscle Tissue: Structure, Function, and Types

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

• Skeletal Muscle: Voluntary, striated, attached to bones, multinucleated

• Cardiac Muscle: Involuntary, striated, found in heart, intercalated discs

• Smooth Muscle: Involuntary, non-striated, found in walls of hollow organs

7. Nervous Tissue: Structure and Function

Nervous tissue is specialized for communication via electrical signals. It consists of neurons (transmit impulses) and supporting cells (neuroglia).

• Neurons: Generate and conduct nerve impulses

• Neuroglia: Support, insulate, and protect neurons

8. Tissue Repair: Steps in Basic Tissue Repair

1. Inflammation: Release of inflammatory chemicals, increased permeability, clotting

2. Organization: Restoration of blood supply, formation of granulation tissue

3. Regeneration and Fibrosis: Surface epithelium regenerates, underlying tissue forms scar (fibrosis)

Example: Skin wound healing involves inflammation, formation of granulation tissue, and regeneration of epithelium.