Anatomy & Physiology: Tissue Types, Structure, and Function

Tissues: Structure, Function, and Classification

Career Connection: The Role of Tissues in Organisms

Tissues are groups of similar cells that perform common functions, forming the foundation for organ structure and function in multicellular organisms.

• Unicellular organisms: Single cells perform all life functions independently.

• Multicellular organisms: Specialized cells form communities, working together to maintain homeostasis.

• Cell specialization: Allows for complex body functions but can be detrimental if specialized cells are lost.

• Organs: Composed of multiple tissue types; their structure and function depend on tissue arrangement.

• Histology: The study of tissues, providing a basis for understanding organ physiology and anatomy.

Major Tissue Types

The four primary tissue types in the human body are:

• Epithelial tissue: Covers surfaces, forms boundaries, protects, secretes, absorbs, and filters. Found in the lining of digestive tract organs, glands, and the epidermis.

• Connective tissue: Supports, protects, and binds other tissues. Found in bones, tendons, fat, and other soft padding tissue.

• Muscle tissue: Contracts to cause movement. Includes skeletal muscle (attached to bones), cardiac muscle (heart), and smooth muscle (walls of hollow organs).

• Nervous tissue: Provides internal communication. Found in the brain, spinal cord, and nerves.

Preparation of Animal Tissue for Microscopy

Histological study requires tissues to be fixed, sliced, and stained for microscopic examination.

• Fixation: Preserves tissue to prevent decay.

• Sectioning: Slices tissue thinly to allow light or electrons to pass through.

• Staining: Enhances contrast between structures.

Staining Techniques

• Light microscopy: Uses synthetic dyes (acidic or basic) to highlight anatomical structures.

• Transmission electron microscopy (TEM): Uses heavy metal salts to stain tissue sections, creating contrast with electron beams. Images are in shades of gray but can be colorized.

• Scanning electron microscopy (SEM): Provides 3D images of tissue surfaces in shades of gray.

Artifacts: Distortions introduced during tissue preparation that may alter the tissue's original condition.

Epithelial Tissue

Epithelial tissue is a sheet of cells that covers body surfaces, lines cavities, and forms glands.

Types of Epithelial Tissue

• Covering and lining epithelium: Forms the outer layer of the skin, lines open cavities of the urogenital, digestive, and respiratory systems, and covers the walls and organs of the closed ventral body cavity.

• Glandular epithelium: Forms the glands of the body.

Functions of Epithelial Tissue

• Protection

• Absorption

• Filtration

• Excretion

• Secretion

• Sensory reception

Special Characteristics of Epithelial Tissue

• Polarity: Has an apical (exposed) surface and a basal (attached) surface, exhibiting apical-basal polarity.

• Apical surface: May have microvilli (increase surface area) or cilia (propel substances).

• Basal surface: Supported by the basal lamina, a noncellular adhesive sheet.

• Specialized contacts: Cells form continuous sheets connected by tight junctions and desmosomes.

• Supported by connective tissue: Epithelial sheets rest on connective tissue, separated by the basement membrane (basal lamina + reticular lamina).

• Avascular but innervated: No blood vessels, but supplied by nerve fibers.

• Regeneration: High capacity for regeneration if adequately nourished.

Classification of Epithelial Tissue

• By layers:

  • Simple epithelium: Single cell layer; absorption, secretion, filtration.

  • Stratified epithelium: Multiple layers; protection.

• By cell shape:

  • Squamous: Flat, scale-like.

  • Cuboidal: Box-like, as tall as wide.

  • Columnar: Tall, column-shaped.

Types and Functions of Epithelial Tissue

• Simple squamous: Diffusion and filtration; found in air sacs of lungs, lining of heart.

• Simple cuboidal: Secretion and absorption; found in kidney tubules, ducts of small glands.

• Simple columnar: Absorption and secretion; found in digestive tract.

• Pseudostratified columnar: Secretion, particularly of mucus; found in trachea.

• Stratified squamous: Protection; found in skin, mouth.

• Transitional: Stretches; found in bladder.

Glands

• Definition: One or more cells that make and secrete a product.

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

• Endocrine glands: Secrete hormones into the bloodstream (e.g., pituitary gland).

• Unicellular glands: Single-celled (e.g., goblet cells).

• Multicellular glands: Composed of multiple cells (e.g., salivary glands).

• Classification of multicellular exocrine glands:

  • Structural: Simple (unbranched ducts) vs. compound (branched ducts).

  • Functional: Merocrine (secrete by exocytosis) vs. holocrine (accumulate products until rupture).

Connective Tissue

Connective tissue is the most abundant and widely distributed tissue type, providing support, protection, and insulation.

Characteristics

• Common elements: ground substance, fibers, and cells.

• Originates from embryonic mesenchyme.

• Varying degrees of vascularity (blood supply).

Types and Functions

• Loose connective tissue: Supports and binds other tissues (e.g., areolar, adipose).

• Dense connective tissue: Provides strength (e.g., tendons, ligaments).

• Cartilage: Supports and cushions (e.g., hyaline, elastic, fibrocartilage).

• Bone: Supports and protects body structures.

• Blood: Transports nutrients, wastes, and gases.

Structural Components

• Ground substance: Unstructured material filling space between cells.

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

• Cells: Fibroblasts, chondroblasts, osteoblasts, adipocytes, white blood cells, mast cells, macrophages.

Muscle Tissue

Muscle tissue is responsible for body movement through contraction.

• Skeletal muscle: Attached to bones; voluntary movement.

• Cardiac muscle: Walls of the heart; involuntary movement.

• Smooth muscle: Walls of hollow organs; involuntary movement.

Nervous Tissue

Nervous tissue is specialized for communication and control, forming the brain, spinal cord, and nerves.

• Neurons: Transmit electrical signals from sensory receptors to effectors (muscles, glands).

• Supporting cells (neuroglia): Support, insulate, and protect neurons.

Body Membranes

Body membranes are simple organs composed of epithelial and connective tissue layers.

• Cutaneous membrane: Dry; covers the body surface (skin).

• Mucous membrane: Wet; lines body cavities open to the exterior (e.g., digestive tract).

• Serous membrane: Wet; lines closed body cavities (e.g., pleura, pericardium).

Tissue Repair

Tissue repair involves restoring tissue structure and function after injury.

• Process: Inflammation → Organization → Regeneration

• Regeneration: Replacement of destroyed tissue with the same kind of tissue.

• Fibrosis: Replacement with scar tissue (dense connective tissue).

Stages of Skin Wound Repair

1. Inflammation: Blood vessels dilate, immune cells arrive, clot forms.

2. Organization: Granulation tissue forms, new blood vessels develop, macrophages clean debris.

3. Regeneration and fibrosis: Epithelium regenerates, underlying area may scar.

Summary Table: Major Tissue Types and Functions

Tissue Type	Main Function	Location
Epithelial	Protection, absorption, secretion, filtration	Skin, lining of GI tract, glands
Connective	Support, protection, binding	Bones, tendons, fat, blood
Muscle	Movement	Skeletal muscles, heart, walls of hollow organs
Nervous	Internal communication	Brain, spinal cord, nerves

Additional info: This guide expands on the provided notes with definitions, examples, and a summary table for clarity and exam preparation.