Chapter 4: Tissues – Structure, Classification, and Function

Chapter 4: Tissues

Introduction to Tissues

Tissues are fundamental units in multicellular organisms, consisting of groups of cells with similar structure and function, along with associated extracellular substances (matrix). The study of tissues is known as histology.

• Tissue: A group of cells with similar structure and function, plus extracellular matrix.

• Histology: The scientific study of tissues.

Types of Tissues

Main Tissue Types

There are four primary tissue types in animals, each with distinct structures and functions:

• Epithelial Tissue

• Connective Tissue

• Muscular Tissue

• Nervous Tissue

Epithelial Tissue

Characteristics and Locations

Epithelial tissues cover body surfaces, line internal cavities, and form most glands. They are characterized by closely packed cells with minimal extracellular matrix (ECM).

• Location: Covers body surfaces (internal and external), e.g., skin, kidney, trachea, glands.

• Characteristics:

  • Cells are tightly packed (very little ECM).

  • Form most glands.

  • Have a free (apical) surface and a basal surface that attaches to underlying tissues.

Structure of Epithelial Tissue

Epithelial cells are arranged in sheets and are attached to a basement membrane, which separates them from underlying connective tissue.

• Free surface: Exposed to the body exterior or cavity.

• Basement membrane: Thin layer anchoring epithelial cells to connective tissue.

Functions of Epithelial Tissue

Epithelial tissues perform several essential functions:

• Protection: Skin protects underlying tissues.

• Barrier: Prevents entry of pathogens (e.g., skin keeps bacteria out).

• Diffusion and Filtration: Allows exchange of gases and filtration (e.g., lungs, kidneys).

• Secretion: Produces substances such as sweat (e.g., sweat glands).

• Absorption: Takes up nutrients (e.g., small intestine).

Classification of Epithelial Tissue

Epithelial tissues are classified based on two criteria: the number of cell layers and the shape of the cells.

• Number of Layers:

  • Simple: One layer of cells.

  • Stratified: Multiple layers of cells.

  • Pseudostratified: Appears layered but is a single layer with nuclei at different levels.

  • Transitional: A type of stratified epithelium that can change shape (e.g., bladder).

• Cell Shape:

  • Squamous: Flat, tile-like cells.

  • Cuboidal: Cube-shaped cells.

  • Columnar: Tall, column-like cells.

Type	Layers	Cell Shape	Example Location
Simple Squamous	One	Flat	Lungs, blood vessels, kidneys
Simple Cuboidal	One	Cube-shaped	Kidneys, glands, ovaries
Simple Columnar	One	Tall, column-like	Stomach, intestines, respiratory tract
Stratified Squamous	Multiple	Flat	Skin, mouth, throat, esophagus
Transitional	Multiple	Changes shape	Bladder

Specialized Structures of Epithelial Cells

• Cell Surfaces:

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

  • Cilia: Move materials across the cell surface (e.g., trachea).

  • Goblet cells: Produce mucus (e.g., stomach).

• Cell Junctions:

  • Tight junctions: Bind adjacent cells together (e.g., intestines).

  • Desmosomes: Mechanical links that bind cells.

  • Hemidesmosomes: Attach cells to the basement membrane.

  • Gap junctions: Channels that allow molecules to pass between cells for communication.

Glands

Glands are structures that secrete substances onto surfaces, into cavities, or into the blood.

• Endocrine glands: Secrete directly into the bloodstream (e.g., thyroid, pituitary).

• Exocrine glands: Secrete via ducts (e.g., sweat, oil glands).

Gland Type	Secretion Method	Example
Endocrine	No ducts; into blood	Thyroid gland
Exocrine	Via ducts	Sweat gland

Connective Tissue

Characteristics and Functions

Connective tissues provide support, connect body parts, and play roles in storage, transport, and protection. They are classified based on the type of extracellular matrix and their function.

• Extracellular Matrix (ECM): Non-living material between cells, containing water, fibers, and minerals.

• Components of ECM:

  • Protein fibers (collagen, reticular, elastic)

  • Ground substance (proteins and sugars)

  • Fluid

Types of Protein Fibers

• Collagen fibers: Flexible, resist stretching (look like ropes).

• Reticular fibers: Form supporting networks in organs.

• Elastic fibers: Recoil after being stretched.

Functions of Connective Tissue

• Enclose and separate organs and muscles

• Connect tissues (e.g., tendons connect muscle to bone; ligaments connect bone to bone)

• Support and movement (bones)

• Storage (bones store calcium; adipose tissue stores fat)

• Cushion and insulate (adipose tissue)

• Transport (blood)

• Protect (immune cells)

Classification of Connective Tissue

• Liquid CT: Blood and lymph

• Soft CT: Areolar and adipose tissue

• Fibrous CT (Dense): Tendons, ligaments, skin

• Hard CT: Cartilage and bone

Type	Main Features	Example Location
Liquid (Blood, Lymph)	Cells in fluid matrix	Blood vessels, lymphatic system
Soft (Areolar, Adipose)	Loose arrangement of fibers	Under skin, around organs
Fibrous (Dense)	Dense collagen fibers	Tendons, ligaments, skin
Hard (Cartilage, Bone)	Solid matrix, chondrocytes/osteocytes	Joints, skeleton

Types of Cartilage

• Hyaline cartilage: Covers ends of bones; reduces friction.

• Fibrocartilage: Between vertebrae; withstands compression.

• Elastic cartilage: Ear, tip of nose; can recoil.

Bones

• Made of osseous tissue; starts as cartilage and hardens (calcifies).

• Cells that form bone: osteoblasts.

• Types: Compact and spongy bone.

Muscle Tissue

Types and Functions

Muscle tissue produces movement through contraction of muscle fibers. There are three types:

• Skeletal muscle: Moves the body; voluntary; striated; multiple nuclei.

• Cardiac muscle: Forms the heart; involuntary; striated; intercalated discs.

• Smooth muscle: Found in organs; involuntary; non-striated; single nucleus.

Type	Striations	Control	Location
Skeletal	Yes	Voluntary	Attached to bones
Cardiac	Yes	Involuntary	Heart
Smooth	No	Involuntary	Walls of organs

Nervous Tissue

Structure and Function

Nervous tissue consists of neurons and supporting neuroglia cells. It is found in the brain, spinal cord, and peripheral nerves, and is responsible for controlling and coordinating body activities.

• Neurons: Cells that transmit electrical signals; consist of dendrites, cell body, and axon.

• Neuroglia (glial cells): Support and protect neurons.

Tissue Repair and Inflammation

Tissue Repair

Tissue repair involves the replacement of dead or damaged cells with viable cells.

• Regeneration: Replacement with the same cell type (no scar).

• Replacement: Replacement with a different cell type (scar formation).

Inflammation

Inflammation is the body's response to tissue damage, signaling defense mechanisms and initiating repair.

• Chemical mediators: Released after injury; cause dilation of blood vessels.

• Symptoms:

  • Redness: Blood vessels dilate.

  • Heat: Increased blood flow.

  • Swelling: Water and proteins accumulate.

  • Pain: Nerve endings stimulated by damage and swelling.

Summary Table: Four Major Tissue Types

Tissue Type	Main Function	Key Features	Example Location
Epithelial	Protection, absorption, secretion	Tightly packed cells, little ECM	Skin, glands, lining of organs
Connective	Support, connection, transport	Abundant ECM, various fibers	Bones, blood, tendons
Muscular	Movement	Contractile fibers	Muscles, heart, organs
Nervous	Control, coordination	Neurons, neuroglia	Brain, spinal cord, nerves

Additional info: For further study, review textbook Chapter 4 (pages 85-109) and complete related activities and quizzes as recommended in the course notes.