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

Chapter 4: Tissues

Biological Levels of Organization

The tissue level is one of the fundamental levels of biological organization, situated between cells and organs. Tissues are groups of similar cells that perform a common function.

• Definition: A tissue is a group of structurally similar cells and their extracellular matrix, working together to perform specific functions.

• Hierarchy: Cells → Tissues → Organs → Organ Systems → Organism

• Example: Muscle tissue contracts to produce movement; nervous tissue transmits electrical impulses.

Main Tissue Types

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

• Epithelial Tissue: Covers body surfaces, lines cavities, and forms glands.

• Connective Tissue: Supports, binds, and protects other tissues and organs.

• Muscle Tissue: Responsible for movement through contraction.

• Nervous Tissue: Initiates and transmits electrical impulses for communication.

Epithelial Tissue

Definition and Locations

Epithelial tissue consists of closely packed cells that form continuous sheets, covering surfaces and lining cavities throughout the body.

• Locations: Skin (epidermis), lining of the digestive tract, respiratory tract, blood vessels, and glands.

Characteristics and Functions

• Cellularity: Composed almost entirely of tightly packed cells with minimal extracellular matrix.

• Polarity: Has an apical surface (exposed to body exterior or cavity) and a basal surface (attached to underlying connective tissue).

• Attachment: The basement membrane anchors epithelium to connective tissue.

• Avascularity: Lacks blood vessels; nutrients diffuse from underlying tissues.

• Regeneration: High capacity for renewal due to frequent cell division.

• Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

Specializations of the Apical Surface

• Microvilli: Increase surface area for absorption (e.g., small intestine).

• Cilia: Move substances across the surface (e.g., respiratory tract).

Basement Membrane

• Definition: A thin, fibrous layer that attaches epithelium to underlying connective tissue.

• Function: Provides support and regulates exchange of materials.

Intercellular Junctions

• Tight Junctions: Prevent leakage between cells.

• Desmosomes: Provide mechanical strength by anchoring cells together.

• Gap Junctions: Allow communication between adjacent cells.

Criteria for Identifying Epithelial Tissue

• Cell Shape: Squamous (flat), cuboidal (cube-shaped), columnar (tall).

• Cell Layers: Simple (one layer), stratified (multiple layers).

Comparison Table: Epithelial Tissue Types

Type	Main Function	Location
Simple Squamous	Diffusion, filtration	Alveoli of lungs, lining of blood vessels
Simple Cuboidal	Secretion, absorption	Kidney tubules, glands
Simple Columnar	Absorption, secretion	Digestive tract lining
Stratified Squamous	Protection	Skin, mouth, esophagus
Stratified Cuboidal	Protection, secretion	Some ducts of glands
Stratified Columnar	Protection, secretion	Male urethra, some gland ducts

Simple vs. Stratified Epithelium

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

• Stratified: Multiple layers; functions in protection.

Glands

• Definition: Structures composed of epithelial cells that produce and secrete substances.

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

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

Connective Tissue

General Characteristics and Functions

• Most abundant tissue type in the body.

• Functions: Support, protection, insulation, storage, and transport.

• Components: Cells, fibers, and ground substance (matrix).

General Components of Connective Tissue

Component	Description	Function
Cells	Fibroblasts, adipocytes, chondrocytes, osteocytes, blood cells	Produce matrix, store energy, immune defense
Fibers	Collagen, elastic, reticular	Strength, elasticity, support
Ground Substance	Gel-like material	Fills space, supports cells and fibers

Types of Connective Tissue

Type	Main Function	Location
Areolar	Wraps and cushions organs	Under epithelia, around organs
Adipose	Stores fat, insulates, protects	Under skin, around kidneys
Reticular	Supports other cells	Lymphoid organs
Dense Regular	Attaches muscles to bones	Tendons, ligaments
Dense Irregular	Withstands tension	Dermis of skin
Cartilage (Hyaline)	Supports, reinforces	Ends of long bones, nose, trachea
Bone	Supports, protects, stores minerals	Skeletal system
Blood	Transports gases, nutrients, wastes	Within blood vessels

Types of Membranes

Body membranes are thin layers of tissue that cover surfaces, line cavities, and divide spaces or organs.

Type	Location	Purpose	Composition
Mucous	Lines body cavities open to exterior (e.g., digestive, respiratory tracts)	Secretion, protection	Epithelial + connective
Serous	Lines closed body cavities (e.g., peritoneum, pleura)	Reduces friction	Epithelial + connective
Cutaneous	Skin	Protection	Epithelial + connective
Synovial	Lines joint cavities	Lubrication	Connective only

Muscle Tissue

General Function

• Function: Produces movement by contracting; maintains posture; generates heat.

Types of Muscle Tissue

Type	Control	Striations	Nuclei	Location
Skeletal	Voluntary	Striated	Multinucleate	Attached to bones
Cardiac	Involuntary	Striated	Uninucleate (usually)	Heart
Smooth	Involuntary	Non-striated	Uninucleate	Walls of hollow organs

Nervous Tissue

General Function

• Function: Receives, processes, and transmits electrical signals throughout the body.

Cell Types

• Neurons: Conduct electrical impulses; main functional unit.

• Neuroglial Cells: Support, protect, and nourish neurons.

Neuron Structure and Function

• Cell Body: Contains nucleus and organelles.

• Dendrites: Receive signals.

• Axon: Transmits impulses away from cell body.

• Location: Brain, spinal cord, nerves.

Inflammation and Tissue Repair

• Inflammation: A protective response to injury or infection, characterized by redness, heat, swelling, pain, and loss of function.

• Role in Repair: Removes damaged cells and pathogens, initiates tissue repair.

Aging and Tissue Structure

• Aging Effects: Tissues may lose elasticity, repair more slowly, and accumulate damage with age.

• Examples: Thinner skin, reduced bone density, decreased muscle mass.