Chapter 4: Tissue—The Living Fabric (Anatomy & Physiology Study Notes)

Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells similar in structure that perform common or related functions, essential for maintaining homeostasis. The study of tissues is called histology. There are four basic tissue types in the human body: epithelial, connective, muscle, and nervous tissue.

• Epithelial tissue: Covers body surfaces and lines cavities.

• Connective tissue: Supports, protects, and binds other tissues together.

• Muscle tissue: Contracts to produce movement.

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

4.1 Tissue Preparation for Microscopy

Steps in Tissue Preparation

To study tissues under a microscope, samples must be:

• Fixed: Preserved with a solvent to prevent decay.

• Sectioned: Cut into thin slices to allow light or electrons to pass through.

• Stained: Treated with dyes to enhance contrast. Light microscopy uses colored dyes, while electron microscopy uses heavy metal salts.

Two main types of electron microscopy:

• Transmission Electron Microscopy (TEM): Shows internal sections of tissues.

• Scanning Electron Microscopy (SEM): Shows surface details.

4.2 Epithelial Tissue

General Features and Functions

Epithelial tissue (epithelium) is a sheet of cells that covers body surfaces or lines body cavities. It exists in two main forms:

• Covering and lining epithelium: Forms the outer layer of skin and lines open cavities and organs.

• Glandular epithelium: Forms glands that secrete substances (e.g., salivary glands).

Main functions include protection, absorption, filtration, excretion, secretion, and sensory reception.

Special Characteristics of Epithelial Tissues

• Polarity: Cells have an apical (top, exposed) and basal (bottom, attached) surface, each with distinct structures and functions.

• Specialized contacts: Cells are tightly joined by junctions (tight junctions, desmosomes) to form continuous sheets.

• Supported by connective tissue: The basement membrane (basal and reticular lamina) reinforces the epithelium.

• Avascular but innervated: Contains no blood vessels but is supplied by nerves; nutrients diffuse from underlying tissues.

• Regeneration: High capacity for renewal, especially in areas exposed to friction or hostile environments.

Classification of Epithelial Tissue

Epithelia are classified by the number of cell layers and cell shape:

• Simple epithelium: Single cell layer; found where absorption, secretion, and filtration occur.

• Stratified epithelium: Two or more layers; found in high-abrasion areas for protection.

Cell shapes:

• Squamous: Flattened and scale-like.

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

• Columnar: Tall and column-shaped.

Types of Epithelial Tissue

• Simple Squamous Epithelium: Single layer of flat cells; allows diffusion and filtration. Found in air sacs of lungs, lining of heart, blood vessels, and lymphatic vessels.

• Simple Cuboidal Epithelium: Single layer of cube-shaped cells; functions in secretion and absorption. Found in kidney tubules and ducts of small glands.

• Simple Columnar Epithelium: Single layer of tall cells; may have microvilli or cilia, and goblet cells. Functions in absorption and secretion. Found in digestive tract, gallbladder, and some ducts.

• Pseudostratified Columnar Epithelium: Appears multilayered but is a single layer; often ciliated. Functions in secretion and movement of mucus. Found in upper respiratory tract and ducts of large glands.

• Stratified Squamous Epithelium: Multiple layers; protects underlying tissues. Keratinized type forms skin; nonkeratinized type lines moist cavities (mouth, esophagus).

• Transitional Epithelium: Resembles both stratified squamous and cuboidal; stretches to allow filling of urinary organs. Found in bladder, ureters, and part of urethra.

Glandular Epithelia

A gland consists of one or more cells that make and secrete an aqueous fluid called a secretion. Glands are classified by:

• Site of product release: Endocrine (ductless, hormones) or exocrine (via ducts, e.g., sweat, saliva).

• Number of cells: Unicellular (e.g., goblet cells) or multicellular (e.g., salivary glands).

Unicellular Exocrine Glands

Goblet cells and mucous cells are the main unicellular exocrine glands, producing mucin that forms mucus for protection and lubrication.

Multicellular Exocrine Glands

These consist of a duct and a secretory unit, supported by connective tissue. They are classified by structure (simple/compound, tubular/alveolar/tubuloalveolar) and mode of secretion (merocrine, holocrine, apocrine).

4.3 Connective Tissue

General Features and Functions

Connective tissue is the most abundant and widely distributed tissue type. Its major functions include binding and support, protection, insulation, energy storage, and transportation of substances (e.g., blood).

• Four main classes: Connective tissue proper, cartilage, bone, blood.

Comparison of Connective Tissue Classes

Tissue Class	Subclasses	Cells	Matrix Components	General Features
Connective Tissue Proper	Loose (areolar, adipose, reticular); Dense (regular, irregular, elastic)	Fibroblasts, fibrocytes, defense cells, adipocytes	Gel-like ground substance; collagen, reticular, elastic fibers	Binding, resisting tension, water reservoir, energy storage
Cartilage	Hyaline, elastic, fibrocartilage	Chondroblasts, chondrocytes	Gel-like ground substance; collagen, elastic fibers	Resists compression, supports, avascular
Bone	Compact, spongy	Osteoblasts, osteocytes	Calcified ground substance; collagen fibers	Support, resists compression and tension
Blood	---	RBCs, WBCs, platelets	Plasma (fluid); no fibers	Transport of gases, nutrients, wastes

Common Characteristics of Connective Tissue

• Extracellular matrix: Nonliving material (ground substance and fibers) separates cells, allowing tissue to bear weight and withstand tension.

• Common origin: All connective tissues arise from mesenchyme (embryonic tissue).

Structural Elements of Connective Tissue

• Ground substance: Unstructured material filling space between cells; composed of interstitial fluid, cell adhesion proteins, and proteoglycans (e.g., chondroitin sulfate, hyaluronic acid).

• Fibers: Collagen (strength), elastic (stretch and recoil), reticular (supportive networks).

• Cells: "-blast" (immature, matrix-secreting) and "-cyte" (mature, maintain matrix) forms; also includes adipocytes, leukocytes, mast cells, and macrophages.

Types of Connective Tissue

• Connective Tissue Proper: Loose (areolar, adipose, reticular) and dense (regular, irregular, elastic) types.

• Cartilage: Hyaline, elastic, fibrocartilage.

• Bone: Compact and spongy.

• Blood: Fluid matrix with cells for transport.

Loose Connective Tissue

• Areolar: Supports and binds tissues, holds body fluids, defends against infection.

• Adipose: Stores energy, insulates, cushions.

• Reticular: Forms a soft internal skeleton (stroma) for lymphoid organs.

4.4 Muscle Tissue

General Features and Types

Muscle tissue is highly vascularized and responsible for movement. Muscle cells contain myofilaments (actin and myosin) for contraction. There are three types:

• Skeletal muscle: Voluntary, striated, multinucleated, attached to bones.

• Cardiac muscle: Involuntary, striated, one nucleus per cell, found in heart, connected by intercalated discs.

• Smooth muscle: Involuntary, non-striated, spindle-shaped cells, found in walls of hollow organs.

4.5 Nervous Tissue

Structure and Function

Nervous tissue is the main component of the nervous system (brain, spinal cord, nerves). It regulates and controls body functions. Two main cell types:

• Neurons: Respond to stimuli and transmit electrical signals via dendrites and axons.

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

4.6 Membranes

Types of Membranes

Membranes are organs composed of at least two tissue types. Three main types:

• Cutaneous membrane: Skin; dry, keratinized stratified squamous epithelium attached to connective tissue.

• Mucous membranes (mucosae): Line body cavities open to the exterior; moist, may secrete mucus.

• Serous membranes (serosae): Line closed ventral body cavities; moist, secrete serous fluid, consist of mesothelium and areolar connective tissue.

4.7 Tissue Repair

Steps in Tissue Repair

1. Inflammation: Release of chemicals, dilation of blood vessels, increased permeability, clotting.

2. Organization: Blood clot replaced by granulation tissue, epithelium regenerates, fibroblasts produce collagen, debris removed.

3. Regeneration and Fibrosis: Epithelium thickens, scar tissue forms, function may be restored or lost depending on tissue type.

Regenerative Capacity of Tissues

• High: Epithelial tissues, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissue.

• Moderate: Smooth muscle, dense regular connective tissue.

• Poor/None: Cardiac muscle, nervous tissue in brain and spinal cord.

Developmental Aspects of Tissues

Embryonic Germ Layers

Three primary germ layers form early in development:

• Ectoderm: Forms nerve tissue.

• Mesoderm: Forms muscle and connective tissues.

• Endoderm: Contributes to epithelial tissues.

Additional info: As the body ages, tissue repair becomes less efficient, epithelia thin, and risk of cancer increases due to DNA mutations.