Chapter 4: Tissue - The Living Fabric (Anatomy & Physiology 2A Study Notes)

Tissue: The Living Fabric

What is a Tissue and How is it Studied?

A tissue is a group of cells with similar structure and function, working together to perform specific activities. The study of tissues is called histology, which involves examining tissue samples under a microscope to understand their structure and function.

• Definition: Tissue is a collection of similar cells and their extracellular matrix.

• Histology: The microscopic study of tissues.

• Applications: Used in medical diagnosis and research.

Epithelial Tissue

Functions of Epithelial Tissue

Epithelial tissue covers body surfaces, lines internal cavities, and forms glands. It serves several essential functions:

• Protection: Shields underlying tissues from injury, pathogens, and dehydration.

• Absorption: Takes in nutrients and substances (e.g., intestinal lining).

• Filtration: Selectively allows passage of materials (e.g., kidney tubules).

• Excretion: Removes waste products.

• Secretion: Produces substances like mucus, hormones, and enzymes.

• Sensory Reception: Contains nerve endings for sensation.

Special Characteristics of Epithelial Tissues

• Polarity: Epithelial cells have an apical (top) surface and a basal (bottom) surface, each with distinct structures and functions.

• Specialized Contacts: Cells are tightly joined by junctions (tight junctions, desmosomes) to form continuous sheets.

• Supported by Connective Tissue: The basal surface rests on a basement membrane, which anchors the epithelium to underlying connective tissue.

• Avascular but Innervated: Epithelial tissues lack blood vessels but receive nutrients by diffusion and are supplied with nerves.

• Regeneration: High capacity for renewal, especially in areas subject to abrasion.

Classification of Epithelial Tissues

Epithelial tissues are classified by cell shape and number of layers:

• Cell Shapes: Squamous (flat), Cuboidal (cube-shaped), Columnar (tall).

• Layers: Simple (one layer), Stratified (multiple layers), Pseudostratified (appears layered but is not).

Types of Epithelial Tissue

• Simple Squamous Epithelium

  • Description: Single layer of flat cells.

  • Functions: Diffusion, filtration.

  • Locations: Alveoli of lungs, lining of blood vessels (endothelium), serous membranes (mesothelium).

• Endothelium

  • Special Characteristics: Lines blood vessels and heart; provides a smooth surface for blood flow.

• Mesothelium

  • Special Characteristics: Lines serous cavities (pleura, pericardium, peritoneum); produces serous fluid.

• Simple Cuboidal Epithelium

  • Description: Single layer of cube-shaped cells.

  • Functions: Secretion, absorption.

  • Locations: Kidney tubules, glands.

• Simple Columnar Epithelium

  • Description: Single layer of tall, column-shaped cells.

  • Functions: Absorption, secretion of mucus and enzymes.

  • Locations: Digestive tract lining, gallbladder.

• Pseudostratified Columnar Epithelium

  • Description: Single layer, appears stratified due to varying cell heights.

  • Functions: Secretion, movement of mucus (ciliated).

  • Locations: Respiratory tract (trachea).

• Stratified Squamous Epithelium

  • Description: Multiple layers of flat cells.

  • Functions: Protection against abrasion.

  • Locations: Skin, mouth, esophagus.

• Transitional Epithelium

  • Description: Multiple layers, cells change shape when stretched.

  • Functions: Allows distension.

  • Locations: Urinary bladder.

Glandular Epithelial Tissue

• Glands: Structures that produce and secrete substances.

• Endocrine Glands: Release hormones directly into the bloodstream; ductless (e.g., thyroid, pituitary).

• Exocrine Glands: Release products via ducts to body surfaces or cavities (e.g., sweat, salivary glands).

• Unicellular Exocrine Glands: Single-celled, e.g., goblet cells producing mucus.

• Multicellular Exocrine Glands: Composed of many cells, with ducts and secretory units.

Connective Tissue (C.T.)

Main Classes of Connective Tissue

• Connective Tissue Proper

• Cartilage

• Bone

• Blood

General Functions of Connective Tissue

• Support: Provides structural framework.

• Protection: Shields organs (e.g., bone protects brain).

• Insulation: Stores energy (e.g., adipose tissue).

• Transportation: Moves substances (e.g., blood).

Common Characteristics of Connective Tissue

• Origin: All arise from mesenchyme (embryonic tissue).

• Varying Vascularity: Some are highly vascular (bone), others avascular (cartilage).

• Extracellular Matrix: Composed of ground substance and fibers; separates cells.

Structural Elements of Connective Tissue

• Ground Substance: Gel-like material containing water, proteins, and polysaccharides.

• Fibers: Collagen (strength), Elastic (stretch), Reticular (support).

• Cells: Fibroblasts (produce fibers), adipocytes (store fat), chondrocytes (cartilage), osteocytes (bone), blood cells.

Connective Tissue Proper

• Areolar Connective Tissue

  • Description: Loose arrangement of fibers and cells.

  • Functions: Supports and binds tissues, holds fluids.

  • Locations: Under epithelia, around organs.

• Adipose Tissue

  • Description: Contains fat cells (adipocytes).

  • Functions: Stores energy, insulates, cushions.

  • Locations: Under skin, around kidneys.

• Reticular Connective Tissue

  • Description: Network of reticular fibers.

  • Functions: Supports lymphoid organs.

  • Locations: Lymph nodes, spleen.

• Dense Regular Connective Tissue

  • Description: Parallel collagen fibers.

  • Functions: Withstands tension in one direction.

  • Locations: Tendons, ligaments.

• Dense Irregular Connective Tissue

  • Description: Irregularly arranged collagen fibers.

  • Functions: Withstands tension in multiple directions.

  • Locations: Dermis of skin, joint capsules.

• Dense Elastic Connective Tissue

  • Description: Contains many elastic fibers.

  • Functions: Allows stretch and recoil.

  • Locations: Walls of large arteries, bronchial tubes.

Cartilage Connective Tissue

• Hyaline Cartilage

  • Description: Glassy, smooth matrix.

  • Functions: Supports, cushions.

  • Locations: Ends of bones, nose, trachea.

• Elastic Cartilage

  • Description: Contains elastic fibers.

  • Functions: Maintains shape, flexibility.

  • Locations: External ear, epiglottis.

• Fibrocartilage

  • Description: Dense collagen fibers.

  • Functions: Absorbs shock.

  • Locations: Intervertebral discs, pubic symphysis.

Bone Connective Tissue

• Description: Hard, calcified matrix with osteocytes.

• Functions: Supports, protects, stores minerals.

• Locations: Skeleton.

Blood Connective Tissue

• Description: Fluid matrix (plasma) with cells (erythrocytes, leukocytes, platelets).

• Functions: Transports gases, nutrients, wastes.

• Location: Blood vessels.

Muscle Tissue

General Function

Muscle tissue is specialized for contraction, enabling movement of the body and its parts.

• Skeletal Muscle Tissue

  • Description: Long, multinucleated, striated fibers.

  • Functions: Voluntary movement.

  • Locations: Attached to bones.

• Cardiac Muscle Tissue

  • Description: Branched, striated, with intercalated discs.

  • Functions: Pumps blood involuntarily.

  • Locations: Heart wall.

• Smooth Muscle Tissue

  • Description: Spindle-shaped, non-striated.

  • Functions: Involuntary movement (e.g., peristalsis).

  • Locations: Walls of hollow organs (intestines, blood vessels).

Nervous Tissue

Description, Functions, and Locations

Nervous tissue is specialized for communication and control. It consists of neurons and supporting cells (neuroglia).

• Neurons: Transmit electrical signals.

• Neuroglia: Support, protect, and nourish neurons.

• Functions: Sensory input, integration, motor output.

• Locations: Brain, spinal cord, nerves.

Covering and Lining Membranes

Types and Their Relation to Tissue Types

• Cutaneous Membrane

  • Composition: Stratified squamous epithelium (epidermis) and dense irregular connective tissue (dermis).

  • Location: Skin.

• Mucous Membrane

  • Composition: Epithelium and loose connective tissue (lamina propria).

  • Location: Lines body cavities open to exterior (digestive, respiratory, urinary tracts).

• Serous Membrane

  • Composition: Simple squamous epithelium (mesothelium) and areolar connective tissue.

  • Location: Lines closed body cavities.

  • Pleura: Surrounds lungs.

  • Pericardium: Surrounds heart.

  • Peritoneum: Surrounds abdominal organs.

Tissue Repair

Steps of Tissue Repair

• Inflammation: Damaged cells release signals; blood vessels dilate; white blood cells arrive.

• Organization: Clot replaced by granulation tissue; new capillaries form.

• Regeneration and Fibrosis: Epithelium regenerates; connective tissue forms scar if regeneration is incomplete.

Regenerative Properties of Different Tissues

• Epithelial and Connective Tissues: High regenerative capacity.

• Muscle Tissue: Moderate regeneration (skeletal muscle limited, cardiac muscle poor).

• Nervous Tissue: Very limited regeneration.

Comparison Table: Epithelial Tissue Types

Type	Description	Function	Location
Simple Squamous	Single layer, flat cells	Diffusion, filtration	Alveoli, blood vessels, serous membranes
Simple Cuboidal	Single layer, cube-shaped cells	Secretion, absorption	Kidney tubules, glands
Simple Columnar	Single layer, tall cells	Absorption, secretion	Digestive tract, gallbladder
Pseudostratified Columnar	Single layer, appears stratified	Secretion, movement of mucus	Trachea, upper respiratory tract
Stratified Squamous	Multiple layers, flat cells	Protection	Skin, mouth, esophagus
Transitional	Multiple layers, cells change shape	Distension	Urinary bladder

Comparison Table: Connective Tissue Types

Type	Description	Function	Location
Areolar	Loose fibers, many cell types	Support, bind, hold fluids	Under epithelia, around organs
Adipose	Fat cells	Energy storage, insulation	Under skin, around kidneys
Reticular	Network of reticular fibers	Support	Lymph nodes, spleen
Dense Regular	Parallel collagen fibers	Tension in one direction	Tendons, ligaments
Dense Irregular	Irregular collagen fibers	Tension in multiple directions	Dermis, joint capsules
Dense Elastic	Elastic fibers	Stretch, recoil	Arteries, bronchial tubes
Hyaline Cartilage	Glassy matrix	Support, cushion	Ends of bones, nose
Elastic Cartilage	Elastic fibers	Shape, flexibility	Ear, epiglottis
Fibrocartilage	Dense collagen	Shock absorption	Intervertebral discs
Bone	Calcified matrix	Support, protection	Skeleton
Blood	Fluid matrix	Transport	Blood vessels

Summary Table: Muscle and Nervous Tissue

Tissue Type	Description	Function	Location
Skeletal Muscle	Striated, multinucleated	Voluntary movement	Attached to bones
Cardiac Muscle	Striated, branched, intercalated discs	Pumps blood	Heart
Smooth Muscle	Non-striated, spindle-shaped	Involuntary movement	Walls of organs
Nervous Tissue	Neurons, neuroglia	Communication, control	Brain, spinal cord, nerves

Summary Table: Membranes

Membrane	Composition	Location
Cutaneous	Stratified squamous epithelium + dense irregular C.T.	Skin
Mucous	Epithelium + lamina propria	Digestive, respiratory, urinary tracts
Serous	Mesothelium + areolar C.T.	Pleura (lungs), pericardium (heart), peritoneum (abdomen)

Summary Table: Tissue Repair

Step	Description
Inflammation	Vasodilation, white blood cells arrive
Organization	Granulation tissue forms, new capillaries
Regeneration/Fibrosis	Epithelium regenerates, scar forms if needed

Additional info: Regeneration capacity varies: epithelial and connective tissues regenerate well; muscle and nervous tissues regenerate poorly.