Chapter 4: Tissues – Structure and Function in Anatomy & Physiology

Tissues and Histology

Introduction to Tissues

Tissues are groups of cells similar in structure that perform common or related functions. The study of tissues is known as histology. There are four basic tissue types in the human body, each with specialized roles essential for maintaining homeostasis and supporting life.

• Epithelial tissue

• Connective tissue

• Muscle tissue

• Nervous tissue

Overview of the Four Tissue Types

• Nervous tissue: Internal communication (brain, spinal cord, nerves)

• Muscle tissue: Contracts to cause movement (skeletal muscles, cardiac muscle, smooth muscle in walls of hollow organs)

• Epithelial tissue: Forms boundaries, protects, secretes, absorbs, filters (skin surface, lining of GI tract and other organs)

• Connective tissue: Supports, protects, binds other tissues (bones, tendons, fat, and other soft padding tissue)

#1 Epithelial Tissue (Epithelium)

Main Types of Epithelial Tissue

• Covering and lining epithelia: Found on external and internal surfaces

• Glandular epithelia: Secretory tissue in glands

Characteristics of Epithelial Tissue

1. Polarity: Cells have apical (upper, free) and basal (lower, attached) surfaces that differ in structure and function.

   • Apical surfaces may have microvilli (e.g., brush border of intestinal lining) or cilia (e.g., lining of trachea).

   • Basal lamina (noncellular, glycoprotein and collagen) lies adjacent to the basal surface.

2. Cellularity: Composed of closely packed cells, forming continuous sheets held together by tight junctions and desmosomes.

3. Attachment: Bound to the basal lamina and reticular lamina, forming the basement membrane (acts as a filter and scaffold for cell migration).

4. Avascular but innervated: Contains no blood vessels but is supplied by nerve fibers.

5. High regenerative capacity: Rapidly replaces lost cells by cell division.

Functions of Epithelium

• Protection: Acts as a barrier to keep substances out or in.

• Permeability: Controls what can cross into or out of the tissue.

• Sensation: Contains nerve endings for sensory reception.

• Secretion: Gland cells synthesize and secrete products such as enzymes, hormones, and mucus.

Maintaining the Integrity of Epithelia

• Intercellular connections: Cell adhesion molecules (CAMs) and proteoglycans (with hyaluronic acid) bind neighboring cells and hold large sheets together.

• Attachment to the basement membrane: Provides structural support and regulates cell behavior.

• Epithelial maintenance and repair: Essential for tissue function and regeneration.

Membrane Junctions

Specialized attachment sites are crucial for securing cells together and providing communication pathways. There are three main types:

• Tight junctions: Prevent fluids and most molecules from moving between cells (impermeable junctions).

• Desmosomes: "Spot-welds" that anchor cells together, abundant in tissues subject to mechanical stress (e.g., heart, skin).

• Gap junctions: Allow ions and small molecules to pass from cell to cell for intercellular communication (important in cardiac and smooth muscle).

Classification of Epithelia

Epithelia are classified based on two criteria:

1. Number of layers:

   • Simple epithelium: one layer

   • Stratified epithelium: more than one layer

2. Cell shape:

   • Squamous: flat and scale-like

   • Cuboidal: box-like, as tall as they are wide

   • Columnar: tall and column-shaped

Stratified epithelia are always named according to the shape of the apical layer of cells.

Overview of Epithelial Tissue Types

Type	Description	Function	Location
Simple squamous	Single layer of flat cells	Allows diffusion and filtration	Air sacs of lungs, lining of heart, blood vessels, lymphatic vessels
Simple cuboidal	Single layer of cube-like cells	Secretion and absorption	Kidney tubules, ducts, small glands
Simple columnar	Single layer of tall cells	Absorption, secretion of mucus/enzymes	Digestive tract, gallbladder, excretory ducts
Pseudostratified columnar	Single layer of cells of differing heights	Secretion, propulsion of mucus by cilia	Trachea, upper respiratory tract
Stratified squamous	Thick membrane, several layers	Protects underlying tissues	Esophagus, mouth, skin epidermis
Stratified cuboidal	Typically two layers of cube-like cells	Protection	Some sweat and mammary glands (rare)
Stratified columnar	Limited distribution, usually at transition areas	Protection and secretion	Pharynx, male urethra, some glandular ducts
Transitional	Resembles both stratified squamous and cuboidal	Stretches readily	Lines ureters, bladder, part of urethra

Specialized Epithelia

• Endothelium: Lining of lymphatic vessels, blood vessels, and heart

• Mesothelium: Epithelium of serous membranes in the ventral body cavity

Glandular Epithelia

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

1. Number of cells: unicellular (e.g., goblet cell) or multicellular

2. Site of product release: endocrine (ductless, secrete hormones into interstitial fluid) or exocrine (secrete products onto surfaces or into ducts)

3. Mode of secretion: merocrine, apocrine, or holocrine

Endocrine Glands

• Ductless glands that secrete hormones, which diffuse into blood or lymph to reach target organs

Exocrine Glands

• More numerous than endocrine glands

• Secrete products (mucus, sweat, oil, etc.) into ducts that release onto body surfaces or into body cavities

• Classified by duct structure (simple or compound) and secretory unit structure (tubular, alveolar, or tubuloalveolar)

Mode of Secretion	Description	Example
Merocrine	Products secreted by exocytosis	Pancreas, sweat, salivary glands
Apocrine	Products accumulate, apex pinches off	Mammary glands (in some species)
Holocrine	Products accumulate, cell ruptures	Sebaceous (oil) glands

#2 Connective Tissue

Overview

Connective tissue is the most abundant and widely distributed of the primary tissues. It supports, binds, and protects other tissues and organs. There are four main classes:

1. Connective tissue proper

2. Cartilage

3. Bone

4. Blood

Major Functions of Connective Tissue

• Binding and support

• Protection

• Insulation

• Storing reserve fuel

• Transporting substances (blood)

Characteristics of Connective Tissue

1. All originate from mesenchyme (embryonic tissue)

2. Varying degrees of vascularity (blood supply)

3. Composed of cells separated by an extracellular matrix (fibers and ground substance)

Extracellular Matrix Components

• Ground substance: Unstructured material that fills space between cells; medium for diffusion of nutrients and waste. Contains interstitial fluid, cell adhesion proteins, and proteoglycans (trap water, affect viscosity).

• Fibers:

  • Collagen fibers: Strongest, most abundant, high tensile strength (e.g., tendons, ligaments)

  • Elastic fibers: Contain elastin, allow stretch and recoil (e.g., elastic ligaments of vertebrae)

  • Reticular fibers: Short, fine, highly branched; form networks (stroma) that support cells (e.g., sheaths around organs)

Cells of Connective Tissue

• Mitotically active and secretory cells end in "-blasts" (e.g., fibroblasts, chondroblasts, osteoblasts)

• Mature cells end in "-cytes" (e.g., fibrocytes, chondrocytes, osteocytes)

• Other cells: adipocytes (fat cells), white blood cells, mast cells, macrophages, mesenchymal stem cells, melanocytes

Connective Tissue Cell Populations

• Fibroblasts: Most abundant, secrete fibers and ground substance

• Fibrocytes: Maintain connective tissue fibers

• Adipocytes: Store fat

• Mesenchymal cells: Stem cells that differentiate in response to injury

• Macrophages: Phagocytic, eat pathogens and debris

• Mast cells: Release histamine and heparin, stimulate inflammation

• Lymphocytes: Immune response, may become plasma cells (antibody production)

• Microphages: Phagocytic blood cells (e.g., neutrophils, eosinophils)

• Melanocytes: Synthesize and store melanin pigment

Classification of Connective Tissue Proper

• All connective tissues except bone, cartilage, and blood

• Two subclasses:

  • Loose connective tissue: More ground substance, fewer fibers (areolar, adipose, reticular)

  • Dense connective tissue (fibrous): More fibers, less ground substance (dense regular, dense irregular, elastic)

*Additional info: The notes continue with more detail on cartilage, bone, blood, and other connective tissue types, as well as muscle and nervous tissue, which are covered in subsequent sections of the chapter.*