Tissues

Introduction to Tissues

Tissues are groups of cells with similar structure that work together to perform specific functions. Understanding tissues is fundamental to the study of anatomy and physiology, as tissues form the basis for all organs and body systems.

• Tissue: A group of cells of similar structure performing a common function.

• Four basic tissue types: Epithelial (covering), Connective (support), Muscle (movement), Nervous (control).

Epithelial Tissue

Overview and Functions

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

• Protection

• Secretion

• Absorption

• Diffusion

• Filtration

• Sensory reception

Special Characteristics of Epithelia

• Cellularity: Composed almost entirely of cells with minimal extracellular material.

• Specialized contacts: Cells joined by special junctions (tight junctions, desmosomes, gap junctions).

• Polarity: Apical (top) and basal (bottom) surfaces differ in structure and function.

• Support by connective tissue: Underlying connective tissue provides support.

• Avascular but innervated: No blood vessels; nutrients diffuse from underlying tissues.

• Regeneration: High capacity for renewal by cell division.

Classification of Epithelia

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

• Simple epithelia: Single layer of cells.

• Stratified epithelia: Multiple layers of cells.

• Squamous: Cells wider than tall (flat).

• Cuboidal: Cells as wide as tall (cube-shaped).

• Columnar: Cells taller than wide (column-shaped).

Types of Epithelial Tissue

Simple Squamous Epithelium

• Description: Single layer of flat cells with disc-shaped nuclei.

• Function: Allows passage of materials by diffusion and filtration; secretes lubricating substances.

• Location: Kidney glomeruli, air sacs of lungs, lining of heart, blood vessels, lymphatic vessels, serosae.

Simple Cuboidal Epithelium

• Description: Single layer of cube-like cells with large, spherical nuclei.

• Function: Secretion and absorption.

• Location: Kidney tubules, ducts and secretory portions of small glands, ovary surface.

Simple Columnar Epithelium

• Description: Single layer of tall, column-shaped cells; may have cilia and goblet cells.

• Function: Absorption; secretion of mucus, enzymes, and other substances; ciliated type propels mucus or reproductive cells.

• Location: Nonciliated: digestive tract, gallbladder; Ciliated: small bronchi, uterine tubes, uterus.

Pseudostratified Columnar Epithelium

• Description: Single layer of cells of varying heights; nuclei at different levels; may have cilia and goblet cells.

• Function: Secretion of mucus; propulsion of mucus by cilia.

• Location: Ciliated: trachea, upper respiratory tract; Nonciliated: ducts of male reproductive tubes, large glands.

Stratified Squamous Epithelium

• Description: Several layers; basal cells cuboidal/columnar, surface cells squamous; keratinized or nonkeratinized.

• Function: Protects underlying tissues in areas subject to abrasion.

• Location: Keratinized: epidermis; Nonkeratinized: esophagus, mouth, anus, vagina, urethra.

Stratified Cuboidal Epithelium

• Description: Generally two layers of cube-shaped cells.

• Function: Protection.

• Location: Ducts of mammary glands, salivary glands, largest sweat glands.

Stratified Columnar Epithelium

• Description: Several layers; basal cells cuboidal, superficial cells columnar.

• Function: Protection and secretion.

• Location: Rare; male urethra, large ducts of some glands.

Transitional Epithelium

• Description: Resembles both stratified squamous and cuboidal; surface cells dome-shaped or squamous-like depending on stretch.

• Function: Stretches readily, permits distension of urinary organs.

• Location: Lines ureters, bladder, part of urethra.

Glands

• Endocrine glands: Ductless, secrete hormones into tissue fluid (covered in detail in Endocrine System chapter).

• Exocrine glands: Secrete products into ducts that open onto epithelial surfaces (e.g., sweat, oil, salivary glands).

• Unicellular exocrine glands: Goblet cells produce mucin, which forms mucus for protection and lubrication.

Cell Junctions

• Tight junctions (zona occludens): Seal adjacent cells to prevent passage of molecules between them.

• Desmosomes: Anchor cells together, providing mechanical stability.

• Gap junctions: Allow communication between cells via passage of ions and small molecules.

Surface Features

• Microvilli: Increase surface area for absorption; abundant in small intestine and kidney tubules.

• Cilia: Motile extensions that move substances across cell surfaces; found in respiratory tract and uterine tubes.

Connective Tissue

Overview and Functions

Connective tissue is the most diverse and abundant tissue type, providing support, storage, and protection throughout the body.

• Main classes: Connective tissue proper, cartilage, bone tissue, blood.

• Functions: Support, nutrient storage, defense, transport, and tissue repair.

Special Characteristics

• Few cells, abundant extracellular matrix (ground substance + fibers).

• Common embryonic origin: mesenchyme.

Structural Elements

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

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

Comparison of Connective Tissue Classes

Connective Tissue Proper

• Loose connective tissue: Areolar, adipose, reticular.

• Dense connective tissue: Dense irregular, dense regular, elastic.

Areolar Connective Tissue

• Description: Gel-like matrix with all three fiber types; contains fibroblasts, macrophages, mast cells, and white blood cells.

• Function: Wraps and cushions organs; holds tissue fluid; important in inflammation.

• Location: Widely distributed under epithelia, packages organs, surrounds capillaries.

Adipose Tissue

• Description: Closely packed adipocytes (fat cells); nucleus pushed to side by fat droplet.

• Function: Stores energy, insulates, supports and protects organs.

• Location: Under skin, around kidneys and eyeballs, within abdomen, in breasts.

Reticular Connective Tissue

• Description: Network of reticular fibers in loose ground substance.

• Function: Forms soft internal skeleton (stroma) supporting other cell types.

• Location: Lymph nodes, bone marrow, spleen.

Dense Irregular Connective Tissue

• Description: Irregularly arranged collagen fibers; some elastic fibers; fibroblasts are major cell type.

• Function: Withstands tension in many directions; provides structural strength.

• Location: Dermis of skin, submucosa of digestive tract, fibrous capsules of organs and joints.

Dense Regular Connective Tissue

• Description: Parallel collagen fibers; few elastic fibers; fibroblasts between fibers.

• Function: Attaches muscles to bones or muscles; attaches bones to bones; withstands stress in one direction.

• Location: Tendons, ligaments, aponeuroses.

Elastic Connective Tissue

• Description: Dense regular connective tissue with high proportion of elastic fibers.

• Function: Allows recoil after stretching; maintains pulsatile flow of blood in arteries.

• Location: Walls of large arteries, certain ligaments, bronchial tubes.

Cartilage

• General features: Firm, flexible; no blood vessels or nerves; matrix up to 80% water; chondrocytes in lacunae.

• Types: Hyaline, elastic, fibrocartilage.

Hyaline Cartilage

• Description: Amorphous but firm matrix; collagen fibers not visible; chondrocytes in lacunae.

• Function: Supports, reinforces, cushions, resists compressive stress.

• Location: Embryonic skeleton, ends of long bones, costal cartilages, nose, trachea, larynx.

Elastic Cartilage

• Description: Similar to hyaline but with more elastic fibers.

• Function: Maintains shape while allowing flexibility.

• Location: External ear, epiglottis.

Fibrocartilage

• Description: Matrix less firm than hyaline; thick collagen fibers predominate.

• Function: Tensile strength, absorbs compressive shock.

• Location: Intervertebral discs, pubic symphysis, knee joint discs.

Bone Tissue

• Description: Hard, calcified matrix with collagen fibers; osteocytes in lacunae; well vascularized.

• Function: Supports and protects organs; provides levers for muscles; stores calcium and fat; marrow is site for blood cell formation.

• Location: Bones.

Blood Tissue

• Description: Red and white blood cells in fluid matrix (plasma).

• Function: Transports gases, nutrients, wastes, and other substances.

• Location: Within blood vessels.

Covering and Lining Membranes

• Cutaneous membrane: Skin; covers body surface.

• Mucous membranes: Line hollow organs open to exterior; composed of epithelial sheet and lamina propria.

• Serous membranes: Line closed cavities (pleural, peritoneal, pericardial); produce serous fluid.

Muscle Tissue

Overview

Muscle tissue is specialized for contraction and movement. It contains myofilaments (actin and myosin) and is classified into three types:

• Skeletal muscle: Voluntary movement; attached to bones.

• Cardiac muscle: Involuntary; found in heart walls.

• Smooth muscle: Involuntary; found in walls of hollow organs.

Skeletal Muscle Tissue

• Description: Long, cylindrical, multinucleate cells with striations.

• Function: Voluntary movement, locomotion, facial expression.

• Location: Skeletal muscles attached to bones or skin.

Cardiac Muscle Tissue

• Description: Branching, striated, generally uninucleate cells connected at intercalated discs.

• Function: Contracts to propel blood; involuntary control.

• Location: Walls of the heart.

Smooth Muscle Tissue

• Description: Spindle-shaped cells with central nuclei; no striations; form sheets.

• Function: Propels substances along internal passageways; involuntary control.

• Location: Walls of hollow organs.

Nervous Tissue

Overview

Nervous tissue is the main component of the nervous system, including the brain, spinal cord, and nerves. It consists of neurons and supporting cells (neuroglia).

• Neurons: Generate and conduct electrical impulses.

• Neuroglia: Support, insulate, and protect neurons.

• Function: Transmit electrical signals from sensory receptors to effectors.

• Location: Brain, spinal cord, nerves.

Additional info: This guide expands on the provided lecture slides and textbook images, offering definitions, examples, and context for each tissue type. The included tables and images are directly relevant to the structure and function of tissues as covered in a college-level Anatomy & Physiology course.