Tissues: Structure and Function

Basic Components of Tissues

Tissues are groups of cells with similar structure and function, organized to perform specific activities. All tissues share two basic components:

• Cells: Discrete populations of cells related in structure and function.

• Extracellular Matrix (ECM): Non-cellular material surrounding cells, providing structural and biochemical support.

Epithelial Tissue

Characteristics and Functions

Epithelial tissue consists of sheets of tightly packed cells attached to a basement membrane (ECM). It covers and lines all body cavities and surfaces.

• Protection: Shields underlying tissues from mechanical and chemical injury.

• Immune Defense: Acts as a barrier to pathogens.

• Sensation: Detects changes in the environment.

• Secretion: Forms glands that release substances.

• Transport: Moves substances across tissue layers.

Types of Cell Junctions in Epithelial Tissue

• Tight Junctions: Hold cells tightly together, preventing passage of substances (like a zipper).

• Desmosomes: Integral proteins linking cells (like a button); allow extracellular fluid to pass through.

• Gap Junctions: Small pores formed by protein channels; allow small substances to pass between cells.

Classification of Epithelial Cells

• Squamous cells: Flat and thin.

• Cuboidal cells: Cube-shaped.

• Columnar cells: Tall and column-like.

Additional info: Epithelial cells can be simple (one layer) or stratified (multiple layers).

Types of Epithelia

• Simple Epithelium: One layer; allows for rapid diffusion and transport.

• Stratified Epithelium: Multiple layers; provides protection against mechanical stress.

• Transitional Epithelium: Specialized for stretching; found in urinary system.

Glandular Epithelium

• Exocrine glands: Ducts secrete products onto surfaces.

• Endocrine glands: Lack ducts; secrete products directly into blood.

Three types of secretion: merocrine (exocytosis), holocrine (cell ruptures), apocrine (portion of cytoplasm pinched off).

Connective Tissue

Functions and Components

Connective tissue connects, supports, and protects other tissues and organs. It is characterized by abundant ECM and various cell types.

• Support: Provides structural framework.

• Protection: Cushions and insulates organs.

• Transport: Moves substances through the body.

• Binding: Connects tissues and organs.

Ground Substance and Protein Fibers

• Ground Substance: Gel-like fluid with water, ions, nutrients, and macromolecules (glycosaminoglycans, proteoglycans, glycoproteins).

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

Types of Connective Tissue Proper

• Loose Connective Tissue: Areolar; mostly ground substance, supports immune cells.

• Dense Connective Tissue: Mostly protein fibers; regular (collagenous), irregular, elastic.

• Reticular Tissue: Supports small structures.

• Adipose Tissue: Fat storage, insulation, shock absorption.

Specialized Connective Tissues

• Cartilage: Flexible, found in joints, ears, nose; hyaline (most abundant), fibrocartilage (fibrous joints), elastic cartilage (ears).

• Bone: Supports body, protects organs, stores calcium, houses bone marrow.

• Blood: Transports cells and nutrients.

Muscle Tissue

Types and Functions

Muscle tissue is specialized for contraction and movement. All muscle cells are excitable, responding to chemical or electrical stimulation.

• Skeletal Muscle: Striated, voluntary movement.

• Cardiac Muscle: Striated, involuntary movement; found in heart.

• Smooth Muscle: Non-striated, involuntary movement; found in walls of hollow organs, blood vessels, skin, ducts.

Nervous Tissue

Structure and Function

Nervous tissue is responsible for generating, sending, and receiving messages throughout the body.

• Neurons: Transmit electrical signals.

• Neuroglia: Support, protect, and nourish neurons.

Membranes

Types of Body Membranes

Membranes are thin sheets of tissue that cover surfaces, line cavities, and divide spaces or organs.

• Serous Membranes: Simple squamous epithelium (mesothelium) and loose connective tissue; lines body cavities, produces serous fluid to reduce friction.

• Synovial Membranes: Lines cavities of freely movable joints; made of connective tissue, produces synovial fluid.

• Mucous Membranes: Lines hollow organs that open to the outside; contains goblet cells that secrete mucus.

• Cutaneous Membrane: Skin; outer layer of keratinized stratified squamous epithelium and deeper connective tissue.

Integumentary System (Skin)

Structure and Layers

The skin is the largest organ, consisting of two main layers:

• Epidermis: Superficial layer; keratinized stratified squamous epithelium; avascular.

• Dermis: Deep layer; loose and dense irregular connective tissue; contains blood vessels and nerves.

• Hypodermis: Subcutaneous tissue; anchors skin to muscle and bone; contains fat and blood vessels.

Functions of the Skin

• Protection

• Temperature regulation

• Sensation

• Excretion

• Vitamin D synthesis

Temperature Regulation: Feedback Mechanisms

• Positive Feedback (when body temperature rises):

  • Stimulus: Body temperature increases above normal range (97-99°F)

  • Receptor: Thermoreceptors in brain detect increase

  • Control center: Heat-loss center in hypothalamus receives signals

  • Effector/response: Nerves stimulate sweat glands and dilation of blood vessels, releasing heat

  • Return to normal: Body temperature returns to normal, feedback stops

• Negative Feedback (when body temperature falls):

  • Stimulus: Body temperature decreases below normal range (97-99°F)

  • Response: Thermoreceptors decrease body temperature, heat-loss center stops response

Summary Table: Tissue Types and Functions

Additional info: These notes expand on the original bullet points, providing definitions, examples, and context for each tissue type and membrane, suitable for college-level Anatomy & Physiology study.