BIO 201 (chapter 4)

The Tissue Level of Organization

  • A tissue is defined as a group of similar cells that are specialized for a particular function.

  • Both the cells and the extracellular fluid in tissues contribute to determining the function of the tissue.

Introduction to Cell Junctions

  • Tissues consist of cells working in coordination; thus, they require mechanisms to hold cells together.

Cell Junctions

  • Cell junctions are points of contact between adjacent plasma membranes.

  • Various types of cell junctions exist, and their structure dictates their functions.

  • Types of Cell Junctions:

    • Tight Junctions

    • Adherens Junctions

    • Gap Junctions

    • Desmosomes

    • Hemidesmosomes

Detailed Analysis of Tight Junctions

  • Characteristics:

    • Form watertight seals between cells.

    • Plasma membranes are fused with a strip of proteins.

    • Commonly found in cells lining the gastrointestinal (GI) tract and bladder, where leakage must be prevented.

Epithelial Cells

  • These cells create a protective barrier over body surfaces and line hollow organs.

  • They are closely packed with minimal extracellular material.

  • The structure allows them to absorb nutrients effectively; for instance, in the intestines, epithelial cells absorb molecules and subsequently transfer them to blood vessels.

Functions of Epithelial Cells

  • Nutrient Absorption:

    • Molecules enter through specific transporters located only on the apical surface of the epithelial cell membrane facing the lumen, ensuring a selective absorption process akin to a filter.

Polarization of Epithelial Cells

  • Epithelial cells exhibit polarization:

    • The apical side is lined with microvilli, increasing surface area for absorption.

    • The basal side faces the basement membrane, anchoring the cells.

Challenges of Cell Membrane Proteins

  • First Difficulty:

    • If proteins float freely in the membrane, how are transporters maintained at the apical side without dispersing?

  • Second Difficulty:

    • How does the epithelial structure prevent leakage between cells?

Tight Junctions and Their Components

  • Tight junctions are located near the apical membrane of cells.

    • Common proteins in tight junctions include occludin and claudin, which share structural similarities with 4 intermembrane helical domains and 2 external loop structures.

    • The extracellular loops differ in size between occludin and claudin.

Functionality of Tight Junctions

  • Tight junction proteins arrange in rows, “stitching” adjacent cells together.

  • This arrangement makes it difficult for even small molecules to pass through the junction.

Physiology of Tight Junctions

  • Tight junctions serve to prevent leakage between cells and anchor the cell membrane to the cytoskeleton.

  • This connection allows epithelial cells to withstand mechanical stress and return to their original shape.

Adherens Junctions

  • Adherens junctions support epithelial cells by connecting them mechanically.

    • Structural components include a plaque (dense layer of proteins) inside the cell membrane connected to microfilaments (actin) that extend into the cytoplasm.

Additional Junctions

  • Gap Junctions:

    • Consist of tiny spaces between the plasma membranes of two cells, fluid-filled tunnels created by connexons for cell communication with ions and small molecules.

  • Desmosomes:

    • Similar in structure to adherens junctions, but connect to intermediate filaments within cells.

    • Common in tissues undergoing mechanical stress, including cardiac muscle and epithelial cells.

  • Hemidesmosomes:

    • Connect epithelial cells to the basement membrane using integrin, a transmembrane glycoprotein.

Types of Tissues

  • There are four principal types of tissues based on function and structure:

    1. Epithelial Tissue: Covers, lines, and forms glands.

    2. Connective Tissue: Protects, supports, binds tissues, stores energy, and provides immunity.

    3. Muscle Tissue: Responsible for movement and generation of force.

    4. Nervous Tissue: Facilitates communication and control within the body.

General Features of Epithelial Tissue

  • Serves as a protective barrier, lining both external and internal body surfaces.

  • Composed of densely packed cells with minimal extracellular material.

  • Exhibits polarity with distinct apical and basal surfaces.

  • Avascular: Nutrients and waste are exchanged via diffusion rather than direct blood vessels.

  • Contains a good nerve supply and a high rate of cell division (mitotic rate).

Basement Membrane Structure

  • Composed of two layers:

    • Basal Lamina: Synthesized by epithelial cells, containing Type VII collagen fibers.

    • Reticular Lamina: Secreted by connective tissue cells, containing Type III collagen fibers.

Functions of the Basement Membrane

  • Anchors epithelium to loose connective tissue.

Types of Epithelium

  1. Covering and Lining Epithelium:

    • Found in the epidermis of the skin, lining blood vessels, and various tracts (respiratory, reproductive, urinary, and GI).

  2. Glandular Epithelium:

    • Involved in secretion, grouped into gland structures (e.g., thyroid, adrenal, sweat).

Classifying Types of Epithelium

  • Based on arrangement of layers and cell shape:

    • Simple: One layer of cells;

    • Squamous: Thin and flat (e.g., alveoli);

    • Cuboidal: Cube-shaped (e.g., kidney tubules);

    • Columnar: Rectangular (e.g., GI tract lining, often with microvilli).

    • Stratified: Multiple layers of cells;

    • Stratified Squamous: Many layers, flat on the surface (e.g., skin);

    • Transitional: Varying surface cell shapes that adapt to stretching (e.g., urinary bladder).

Glandular Epithelium

  • Exocrine Glands:

    • Secrete substances (sweat, saliva) onto body surfaces via ducts.

  • Endocrine Glands:

    • Secrete hormones into the bloodstream to regulate homeostasis.

Structural Classification of Exocrine Glands

  • Unicellular Glands:

    • Example: Goblet cells.

  • Multicellular Glands:

    • Can be unbranched (simple) or branched (compound); may have tubular or acinar (flask-like) shapes.

Functional Classification of Exocrine Glands

  1. Merocrine Glands:

    • Form and secrete products without damaging the cell; e.g., salivary glands.

  2. Apocrine Glands:

    • Secretions involve a portion of the cell being shed with products; e.g., earwax.

  3. Holocrine Glands:

    • Cell contents are released by rupture, resulting in cell death; e.g., oil glands (sebaceous).

Connective Tissue Overview

  • Cells in connective tissue are separated by an extracellular matrix composed of fibers and ground substance.

  • Connective tissue has a good nerve and blood supply, with exceptions like cartilage and tendons.

Extracellular Matrix Components

  • Collagen Fibers: Tough, resistant to stretching, made of collagen.

  • Elastic Fibers: Provide stretch and resilience, made of elastin.

  • Reticular Fibers: Form a supporting network, made of collagen and glycoprotein.

Types of Mature Connective Tissue

  1. Connective Tissue Proper:

    • Loose connective tissue (areolar, adipose, reticular).

    • Dense connective tissue (dense regular, dense irregular, elastic).

  2. Cartilage: Types include hyaline, elastic, and fibrocartilage.

  3. Bone Tissue: Comprised of compact and spongy bone.

  4. Blood and Lymph: Unique tissues with liquid matrices.

Loose Connective Tissue Types

  • Areolar Connective Tissue:

    • Contains collagen and elastic fibers, acting as a filler tissue.

  • Adipose Tissue:

    • Composed mostly of fat cells; provides cushioning and insulation.

  • Reticular Connective Tissue:

    • Supports organs with a network of fibers, found in organs like the liver and spleen.

Dense Connective Tissue

  • Dense Regular Tissue:

    • Contains parallel bundles of collagen fibers; found in tendons and ligaments.

  • Dense Irregular Tissue:

    • Collagen fibers arranged irregularly; provides strength in various directions, found in the dermis of the skin.

Cartilage Characteristics

  • Cartilage is a dense network of collagen and elastic fibers embedded in chondroitin sulfate, produced by chondrocytes.

  • Lacks blood vessels and nerves except in the perichondrium, a dense connective tissue layer around cartilage.

Types of Cartilage

  1. Hyaline Cartilage:

    • Smooth, translucent; found at joint surfaces, ribs, and respiratory tract.

  2. Elastic Cartilage:

    • Contains elastic fibers; maintains shape (e.g., in the ear).

  3. Fibrocartilage:

    • Rich in collagen; found in intervertebral discs; provides rigidity and strength.

Growth of Cartilage

  • Appositional Growth:

    • Addition of new cells to the tissue surface, increasing width.

  • Interstitial Growth:

    • Growth occurs from within as chondrocytes divide, increasing the tissue's size.

Bone Tissue Features

  • Protects organs, facilitates movement, stores minerals, and is the site of blood cell formation.

  • Comprised of a matrix containing mineral salts and collagen, with cells known as osteocytes.

  • Structured into spongy and compact bone.

Blood as a Unique Connective Tissue

  • Blood features a liquid matrix (plasma) containing formed elements (e.g., red blood cells, white blood cells, and platelets).

  • Lymph is a variation of interstitial fluid, flowing through lymphatic vessels and contains less protein than plasma, aiding in cellular transport.

Membranes Overview

  • Membranes are sheets of tissue covering or lining parts of the body.

  • Types of epithelial membranes include mucous, serous, and cutaneous membranes (skin).

  • Synovial membranes line joints and consist solely of connective tissue.

Mucous Membranes

  • Line cavities that open to the exterior (e.g., the digestive tract).

Serous Membranes

  • Line body cavities that do not open to the outside, secrete serous fluid for lubrication.

Cutaneous Membranes

  • Comprise the skin, protecting underlying tissues.

Synovial Membranes

  • Line joint cavities of mobile joints, secrete synovial fluid for lubrication.

Muscle Tissue Overview

  • Muscle tissue consists of cells modified for contraction facilitating movement, posture maintenance, and heat generation.

  • Comprised of three types:

    1. Skeletal Muscle

    2. Cardiac Muscle

    3. Smooth Muscle

Skeletal Muscle

  • Consists of long cylindrical cells with striations; under voluntary (conscious) control.

Cardiac Muscle

  • Made of branched cells with a central nucleus; involuntary and striated, featuring intercalated discs for cell connection.

Smooth Muscle

  • Composed of spindle-shaped cells with a single nucleus; involved in involuntary actions within hollow organs (e.g., blood vessels, GI tract).

Nervous Tissue Characteristics

  • Comprises neurons (nerve cells) and neuroglia (supporting cells).

  • Neurons have a structure consisting of a cell body, dendrites (receive signals), and axons (send signals).

The Tissue Level of Organization

  • Histology is the branch of science that studies tissues. A tissue is defined as a group of similar cells that are specialized for a particular function.

  • Both the cellular components and the extracellular matrix (ECM) contribute to determining the function of the tissue. The ECM is composed of ground substance and protein fibers.

Detailed Classification of Cell Junctions

Cell junctions are points of contact between adjacent plasma membranes that allow for communication and structural integrity.

  1. Tight Junctions

    • Composition: Web-like strands of transmembrane proteins (primarily occludin and claudin) that fuse the outer surfaces of adjacent plasma membranes.

    • Structure: These proteins possess 4 intermembrane helical domains and 2 external loops of varying sizes.

    • Function: Form watertight seals to prevent paracellular transport of molecules (leakage).

    • Location: Found in epithelial tissues of the stomach, intestines, and urinary bladder.

  2. Adherens Junctions

    • Composition: Contains plaque, a dense layer of proteins on the inside of the plasma membrane.

    • Mechanism: Transmembrane glycoproteins called cadherins join the cells. The plaque attaches to both membrane proteins and to microfilaments (actin) of the cytoskeleton.

    • Function: Form "adhesion belts" that help epithelial surfaces resist separation during various contractile activities, such as food moving through the intestines.

  3. Desmosomes

    • Structure: Similar to adherens junctions (use plaque and cadherins), but the plaque attaches to intermediate filaments composed of the protein keratin.

    • Function: Act as "spot welds" to provide stability to tissues under mechanical stress.

    • Location: Common in the epidermis (outer layer of skin) and the cardiac muscle cells of the heart.

  4. Hemidesmosomes

    • Structure: Resemble half of a desmosome but do not link adjacent cells. Instead, they use transmembrane glycoproteins called integrins.

    • Function: Anchor the basal surface of epithelial cells to the basement membrane.

  5. Gap Junctions

    • Structure: Membrane proteins called connexins form tiny fluid-filled tunnels called connexons that connect neighboring cells.

    • Function: Facilitate chemical and electrical communication by allowing ions and small molecules (like glucose) to diffuse from the cytosol of one cell to another.

    • Location: Critical for electrical signaling in the heart and smooth muscle of the gastrointestinal tract.

Epithelial Tissue Analysis

Epithelial cells create a protective barrier and are characterized by high cellularity and polarity.

Structural Polarity

  • Apical (Free) Surface: Faces the body surface, a body cavity, the lumen of an internal organ, or a tubular duct. May contain microvilli or cilia.

  • Lateral Surfaces: Face adjacent cells on either side; contain cell junctions.

  • Basal Surface: Adheres to the basement membrane via hemidesmosomes.

The Basement Membrane

  1. Basal Lamina: Closest to the epithelium; contains laminin and ext{Type VII} collagen fibers secreted by epithelial cells.

  2. Reticular Lamina: Closer to the underlying connective tissue; contains ext{Type III} collagen fibers secreted by connective tissue cells (fibroblasts).

Classification of Epithelium

  • Simple Epithelium: A single layer involved in diffusion, osmosis, filtration, secretion, or absorption.

    • Simple Squamous: Lines the cardiovascular and lymphatic system (endothelium) and forms the epithelial layer of serous membranes (mesothelium).

    • Simple Cuboidal: Found in kidney tubules and smaller ducts of many glands.

    • Simple Columnar: Can be nonciliated (with microvilli) or ciliated.

  • Stratified Epithelium: Two or more layers; protects underlying tissues in areas of wear and tear.

    • Stratified Squamous: Keratinized (dry surfaces like skin) or non-keratinized (moist surfaces like the mouth).

    • Transitional (Urothelium): Lines the bladder; cells change shape (from cuboidal to squamous) as the organ stretches.

Glandular Epithelium and Secretory Mechanisms

  1. Exocrine Glands: Secrete products into ducts that empty onto the surface of a covering and lining epithelium.

    • Merocrine Glands: Secretions are synthesized on ribosomes, processed in the Golgi, and released via exocytosis (e.g., salivary glands).

    • Apocrine Glands: Accumulate secretory products at the apical surface; then, that portion of the cell pinches off (e.g., mammary glands, though mostly merocrine, follow this pattern for milk fats).

    • Holocrine Glands: Accumulate products in the cytosol; the cell matures, ruptures, and becomes the secretory product itself (e.g., sebaceous glands).

  2. Endocrine Glands: Ductless glands that secrete hormones directly into the interstitial fluid and bloodstream.

Connective Tissue Components

Connective tissue (CT) consists of cells and an extracellular matrix.

  • Ground Substance: The component of the CT between cells and fibers. Contains water and large organic molecules like glycosaminoglycans (GAGs) (e.g., hyaluronic acid, chondroitin sulfate) and proteoglycans.

  • Fibers:

    • Collagen Fibers: Very strong and resist pulling forces (tension), but not stiff.

    • Elastic Fibers: Branch and join together to form a fibrous network; consist of elastin and fibrillin.

    • Reticular Fibers: Fine bundles of collagen coated with glycoprotein; provide support in the walls of blood vessels and form a network around fat cells, nerve fibers, and skeletal/smooth muscle cells.

Growth and Repair of Tissues

  • Cartilage Growth:

    • Interstitial Growth: Occurs within the tissue; chondrocytes divide and secrete new matrix. Resulting in an increase in length.

    • Appositional Growth: Occurs at the outer surface; cells in the perichondrium differentiate into chondroblasts, which secrete matrix onto the surface. Resulting in an increase in width.

  • Repair Factors: Epithelial tissues have a high capacity for renewal (mitosis), whereas nervous and muscle tissues have a very limited capacity for repair.