2005 Cytology 6

Cytology Overview

  • Cell Surface Structures

    • Cilia and Flagella (microtubules)

    • Microvilli and Stereocilia (actin filaments)

    • Lamellipodia and Filiopodia (actin filaments)

  • Intercellular Junctions

    • Tight junctions

    • Cytoskeleton-linked junctions

      • Adherens junctions

      • Desmosome

      • Hemidesmosome

    • Communicating junctions

Cell Surface Structures Function

  • Key Roles:

    • Movement of cells

    • Movement of extracellular materials

    • Detection of movement in extracellular materials

    • Enlargement of cell surface area

    • Improvement of cell-to-cell contact

  • Association with Cytoskeleton:

    • Cell surface processes are linked to cytoskeletal dynamics.

Cilia and Flagella

  • Structure:

    • Supported by stabilized microtubules

    • Hair-like (cilia) and whip-like (flagella) projections

  • Function:

    • Capable of movement due to dynein and ATP

    • Cilia create currents in extracellular fluid

    • Flagella facilitate movement of the cell

Microtubule Arrangement in Cilia and Flagella

  • Growth and Arrangement:

    • Microtubules grow from the basal body

    • Arranged in a 9+2 formation (9 outer doublets + 2 central microtubules)

      • 9 outer doublets contain complete (13 protofilaments) and incomplete (10 protofilaments) microtubules

  • Dynein Mechanism:

    • Dynein facilitates the sliding of microtubules leading to bending.

Cilia

  • Characteristics:

    • Diameter: 0.25 µm, Length: 5-15 µm

    • Several hundred cilia per cell

    • Undulating movement creates waves across the epithelium.

  • Functions:

    • In respiratory tract: movement of mucus

    • In reproductive tract: propulsion of spermatozoa/oocytes

    • Present in central canal of spinal cord, ventricles of the brain, and surfaces of peritoneal and pleural cavities

Flagella

  • Characteristics:

    • Long structure (up to several 100 µm)

    • Rapid successive waves of bending facilitate movement

    • Each spermatozoon has a single flagellum about 70 µm long

Microvilli

  • Varying Structures:

    • Size ranges from small irregular projections to tightly packed uniform ones

    • Not motile, they increase surface area for absorption

    • Diameter: < 0.1 µm, Length: ~5 µm

  • Structure:

    • Supported by 20-30 parallel actin filaments for rigidity

    • Actin filaments anchor to the plasma membrane and interact with a terminal web

  • Location:

    • Apical surface of epithelial cells in the small intestine and gallbladder

Filopodia and Lamellipodia

  • Functionality:

    • Irregular protrusions for cell crawling, e.g., migrating white blood cells, growing axons

  • Mechanism:

    • Actin polymerization forms sheetlike (lamellipodia) or thin (filopodia) protrusions

    • Protrusions adhere to surfaces via integrins, dragging the cell body forward

Leukocyte Adhesion Deficiency (LAD)

  • Condition:

    • Deficiency of β-2 integrin subunit (CD18)

    • Results in recurrent bacterial infections due to impaired neutrophil adhesion and chemotaxis

    • Observed in humans, Holstein cattle (BLAD), and Irish Setter dogs (CLAD)

Intercellular Junctions in Epithelia

  • Role:

    • Create selectively permeable barriers

    • Control transport and respond to external signals

    • Separate compartments within epithelial sheets

  • Structure:

    • Polarized sheets with an apical (exposed) and basal (attached to tissue) surface

    • Connect via various junctions (tight, adherens, desmosomes, gap junctions)

Specific Junction Types

  • Tight Junctions:

    • Located only in epithelial cells to prevent leakage

    • Formed by transmembrane proteins (claudin and occludin)

  • Cytoskeleton-Linked Junctions:

    • Includes adherens junctions and desmosomes

    • Facilitates intercellular connections through cytoskeletal linkages

  • Adherens Junctions:

    • Belt-like structures using cadherins linked to actin filaments

    • Important in embryonic development and tissue organization

  • Desmosomes:

    • Disc-like junctions linking cells through cadherins to intermediate filaments

    • Provides tensile strength and integrity to tissues

  • Hemidesmosomes:

    • Connects cells to the extracellular matrix (basal lamina) via integrins

Junctional Epidermolysis Bullosa (JEB)

  • Condition:

    • Inherited defect in Laminin 5 affecting hemidesmosome formation

    • Foals develop blisters and severe infection resulting in high mortality

Communicating Junctions

  • Gap Junctions:

    • Allow direct passage of ions and small molecules for cell communication

    • Formed by connexons from adjacent cells

Summary of Intercellular Junctions

  • Types and Functions:

    • Tight Junction: Seals cells to prevent leakage

    • Adherens Junction: Joins actin bundles between adjacent cells

    • Desmosome: Connects intermediate filaments for structural integrity

    • Gap Junction: Allows passage of small water-soluble molecules

    • Hemidesmosome: Anchors intermediate filaments to basal lamina

Acknowledgements

  • Sources include Alberts et al., Essential Cell Biology, and various practical class resources and microscopic slides.