Tissues: Structure, Function, and Classification in Anatomy & Physiology

Tissues in Anatomy & Physiology

Introduction to Tissues

Tissues are groups of cells similar in structure and function, forming the basic building blocks of organs and systems in the body. Histology is the study of tissues.

• Definition: A tissue is a group of cells with a common structure and function.

• Histology: The microscopic study of tissue structure.

Types of Tissues

There are four primary tissue types in the human body, each with distinct functions and characteristics.

• Epithelial tissue

• Connective tissue

• Muscular tissue

• Nervous tissue

Epithelial Tissue

Overview and Functions

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It serves as a protective barrier and is involved in absorption, secretion, and sensation.

• Main forms: Covering/lining and glandular epithelium

• Functions:

  • Protection

  • Absorption

  • Filtration

  • Excretion

  • Secretion

  • Sensory reception

Characteristics of Epithelial Tissue

• Cells fit closely together

• Apical (free) surface

• Basement membrane

• Avascular (no blood vessels)

• Innervated (has nerves)

• Regenerates easily

Classification of Epithelial Tissue

• Arrangement/Layers:

  • Simple (single layer)

  • Stratified (multiple layers)

  • Pseudostratified (appears layered, but all cells touch basement membrane)

• Shape:

  • Squamous (flat)

  • Cuboidal (cube-shaped)

  • Columnar (tall, column-shaped)

  • Transitional (variable shape)

Types of Epithelial Tissue

• Simple squamous epithelium

  • Single layer, flat cells

  • Areas of low wear and tear (kidneys, lungs, lining blood vessels, serous membranes)

  • Functions: Filtration, diffusion, osmosis

• Simple cuboidal epithelium

  • Single layer, cube-shaped cells

  • Kidneys, pancreas, glands (mammary, salivary)

  • Functions: Secretion, absorption

• Simple columnar epithelium

  • Single layer, column-shaped cells

• Pseudostratified columnar epithelium

  • Lines upper respiratory tract, ducts, epididymis

  • Not all cells reach apical surface

  • Ciliated/nonciliated

    • Function: Secretion, movement (e.g., ciliated in respiratory tract)

    • Contains goblet cells (mucus secretion)

• Stratified squamous epithelium

  • 2+ layers

  • Areas of high wear and tear (protection)

  • Keratized: Resists friction, repels bacteria, waterproof (epidermal layer of skin)

  • Nonkeratinized: Remains moist (tongue, mouth, esophagus, vagina)

• Stratified cuboidal and columnar epithelium

  • Rare, 2 layers

  • Ducts of large glands (sweat glands), urethra, conjunctiva of eye

  • Functions: Protection, secretion, absorption

• Transitional epithelium

  • Layers, cell shapes vary with organ state

  • Bladder (expansion/distension, protection)

Glandular Epithelium

• Gland: Cells that make and secrete a product

• Endocrine glands: Secrete hormones directly into blood (thyroid, pituitary, adrenal, etc.)

• Exocrine glands: Secrete products via ducts (mucus, sweat, oil, digestive enzymes)

• Unicellular exocrine glands: Goblet cells

• Multicellular exocrine glands: Sweat glands, salivary glands

• Modes of secretion:

  • Merocrine (exocytosis, e.g., sweat glands)

  • Holocrine (rupture, e.g., oil glands)

Connective Tissue

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue type, providing support, protection, and insulation. It consists of cells embedded in an extracellular matrix.

• Extracellular matrix: Nonliving material between cells

• Many types of cells

• Functions:

  • Binding and support

  • Protection

  • Insulating

  • Storing energy

  • Transport

Cells of Connective Tissue

• Fibroblasts

• Adipocytes

• Immune cells: Macrophages, plasma cells, mast cells, white blood cells

Matrix of Connective Tissue

• Ground substance: Fluid to calcified

• Fibers: Collagen, elastic, reticular

Types of Connective Tissue

• Loose connective tissues: Areolar, adipose, reticular

• Dense connective tissues: Dense regular, dense irregular, elastic

• Cartilage: Hyaline, fibrocartilage, elastic

• Bone

• Blood

Loose Connective Tissue

• Areolar CT: Fibroblasts, macrophages, plasma cells, mast cells, adipocytes, WBCs; fibers: collagen, elastic, reticular; under skin, around organs, between muscles

• Adipose CT: Adipocytes; under skin, around blood vessels and organs, between muscles; types: yellow and brown

• Reticular CT: Network of fibers and cells; supportive stroma for liver, spleen, bone marrow

Dense Connective Tissue

• Dense regular CT: Collagen fibers running parallel (tendons, ligaments, aponeuroses)

• Dense irregular CT: Collagen fibers arranged irregularly (dermis, heart valves, periosteum, perichondrium)

• Elastic CT: Branching elastic fibers (lungs, blood vessels, vocal cords)

Cartilage

• General features: Strong, supportive; collagen or elastic fibers in ground substance; cells in lacunae (chondrocytes); surrounded by perichondrium; avascular; three types

• Hyaline cartilage: Collagen fibers in clear matrix; chondrocytes/lacunae in clusters; ends of long bones and ribs, trachea, bronchi; fetal skeleton

• Elastic cartilage: Elastic fibers; strength and elasticity; external ear, epiglottis, Eustachian tube

• Fibrocartilage: Collagen fiber bundles; strength and shock absorption; intervertebral discs, pubic symphysis, menisci of knee

Bone

• Support, protection

• Osteocytes in lacunae

• Lamellae: Calcium, collagen

• Blood supply

Blood

• Liquid matrix: Plasma

• Cells and cell fragments: RBCs, WBCs, platelets

Muscle Tissue

Overview and Types

Muscle tissue is specialized for movement and force generation. It contains contractile proteins called myofilaments.

• Skeletal muscle: Muscle cells (fibers) are long, parallel, striated, multinucleated; attach bone to bone or skin; voluntary

• Cardiac muscle: Heart wall; striated, branching, joined via intercalated discs; one nucleus (usually); involuntary

• Smooth muscle: No striations, spindle-shaped; walls of hollow internal structures; one nucleus; involuntary

Nervous Tissue

Overview and Functions

Nervous tissue is responsible for transmitting electrical impulses and supporting neural function.

• Neurons: Convert stimuli to nerve impulses

• Neuroglia: Support, insulate, protect neurons; do not send impulses

Covering and Lining Membranes

Types of Membranes

Membranes consist of an epithelium bound to an underlying layer of connective tissue. They serve protective and functional roles.

• Cutaneous: Skin; keratinized stratified squamous epithelium

• Mucous: Lines body cavities opening to exterior (digestive, respiratory, urinary); protection, absorption, secretion

• Serous: Lines body cavities not open to exterior; closed sac covering organs; simple squamous and areolar CT; heart (pericardium), abdomen/viscera (peritoneum)

• Synovial: Lines joint cavities; CT only; areolar CT and elastic fibers

Tissue Repair

Regeneration and Fibrosis

Tissue repair involves regeneration (replacement with same tissue type) or fibrosis (scar tissue formation).

• Regeneration: Replaces destroyed tissues with same type; superficial skin damage

• Fibrosis: Fibrous CT forms scar tissue; may impair function; severe skin damage, muscle injury, burns

Ability of Tissues to Repair

• Good: Epithelial, smooth muscle

• Moderate: CT except cartilage

• Poor: Cartilage, neural tissue, muscle tissue

Summary Table: Types of Tissues and Key Features

Type	Main Function	Key Features	Examples
Epithelial	Protection, absorption, secretion	Cells closely packed, avascular, regenerates	Skin, lining of GI tract, glands
Connective	Support, binding, transport	Extracellular matrix, various cell types	Bone, blood, cartilage, adipose
Muscular	Movement, force	Contractile proteins, excitable	Skeletal muscle, heart, smooth muscle
Nervous	Impulse transmission	Neurons, neuroglia	Brain, spinal cord, nerves

Key Equations and Concepts

• Osmosis: Movement of water across a semipermeable membrane from low solute concentration to high solute concentration.

• Diffusion: Movement of molecules from high to low concentration.

• Filtration: Movement of water and solutes through a membrane due to hydrostatic pressure.

Example equation for diffusion rate:

$J = -D \frac{dC}{dx}$

Where $J$ is the diffusion flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Additional info: Some details, such as the specific cell types and functions, have been expanded for clarity and completeness.