Chapter 4: Histology – The Study of Tissues

Histology: The Study of Tissues

Introduction to Tissues

Histology is the study of the normal structure of tissues, which are groups of structurally and functionally related cells and their external environment that together perform common functions. All tissues share two basic components:

• Cells: A discrete population of cells related in structure and function.

• Extracellular Matrix (ECM): The surrounding material, which differs in composition among tissue types.

Types of Tissues

There are four primary tissue types in the human body:

• Epithelial Tissues: Sheets of tightly packed cells with little ECM; cover and line body surfaces and cavities, and form parts of glands.

• Connective Tissues: Connect all other tissues together; cells are scattered through the ECM; bind, support, protect, and allow transport of substances.

• Muscular Tissues: Cells contract and generate force; contain little ECM.

• Nervous Tissues: Cells (neurons) generate, send, and receive messages; include supporting cells with a unique ECM.

Extracellular Matrix (ECM)

Functions and Components

The ECM is composed of substances surrounding the cells in a tissue and functions to:

• Provide strength to resist tensile (stretching) and compressive forces

• Direct cells to their proper places within a tissue

• Regulate development, mitotic activity, and survival of cells

• Hold cells in their proper positions

ECM has two main components: Ground Substance and Protein Fibers.

Ground Substance

• Gel-like substance containing extracellular fluid (ECF) with water, ions, nutrients, and other solutes.

• Contains three families of macromolecules:

  • Glycosaminoglycans (GAGs): Negatively charged polysaccharide chains (e.g., chondroitin sulfate, hyaluronic acid) that attract positively charged ions and draw water into the ECM.

  • Proteoglycans: GAGs bonded to a protein core; aggregate to make the ECM firmer and act as a barrier to diffusion.

  • Glycoproteins: Also called cell-adhesion molecules (CAMs); bind cell surface proteins and protein fibers, maintaining tissue architecture.

Protein Fibers

• Collagen Fibers: At least 20 types; make up 20-25% of all protein in the body; very resistant to tension and pressure.

• Elastic Fibers: Made of elastin protein; can stretch to 1.5 times their resting length (distensibility) and return to original length (elasticity).

• Reticular Fibers: Thinner, shorter collagen fibers; form scaffolds to support cells and ground substance, especially in organs.

Cell Junctions

Types of Cell Junctions

• Tight (Occluding) Junctions: Integral "locking" proteins prevent passage of macromolecules; some are leaky.

• Desmosomes: Integral "linker" proteins distribute mechanical stress; similar to buttons.

• Gap Junctions: Protein channels allow small substances to pass freely between cells.

Epithelial Tissues

Functions of Epithelial Tissue

• Protection: Shields underlying tissue from injury; produces keratin; rapid mitosis.

• Immune Defenses: Contains immune cells.

• Secretion: Forms glands that produce substances (e.g., sweat, oil, hormones).

• Transport: Selectively permeable barriers for passive or active transport.

• Sensation: Supplied with nerves; some specialized for sensation.

Components and Classification of Epithelia

• Basement Membrane: Anchors epithelial tissue to underlying connective tissue; consists of:

  • Basal Lamina: ECM of epithelial tissue (collagen fibers, ground substance).

  • Reticular Lamina: Produced by underlying connective tissue (reticular fibers, ground substance).

• Cell Surfaces: Apical (faces extracellular space), Basal (contacts basal lamina), Lateral (contacts adjacent cells).

Classification by Layers and Shape

• Number of Layers:

  • Simple: Single layer

  • Stratified: Multiple layers

  • Pseudostratified: Appears multilayered but is a single layer

• Cell Shape:

  • Squamous: Flattened

  • Cuboidal: Cube-shaped

  • Columnar: Tall and elongated

Covering and Lining Epithelia

• Simple Epithelia: One cell layer; lines hollow organs and surfaces for diffusion/transport.

  • Simple Squamous: Flat cells; rapid diffusion (lungs, blood vessels).

  • Simple Cuboidal: Cube-shaped; secretion and absorption (kidney tubules, glands).

  • Simple Columnar: Tall cells; may have microvilli or cilia; secretion/absorption (intestine, uterine tube).

  • Pseudostratified Columnar: Appears stratified; often ciliated; contains goblet cells (respiratory tract).

• Stratified Epithelia: Multiple layers; protection in high-stress areas.

  • Stratified Squamous: Keratinized (skin) or nonkeratinized (mouth, esophagus).

  • Stratified Cuboidal: Rare; ducts of sweat glands.

  • Stratified Columnar: Rare; ducts of salivary glands, male urethra.

  • Transitional: Dome-shaped apical cells; stretches (urinary bladder).

Transport Across Simple Epithelia

• Paracellular Transport: Substances leak between cells.

• Transcellular Transport: Substances move through cells:

  1. Enter through the phospholipid bilayer

  2. Diffuse through cytosol

  3. Exit through the opposite surface