Week 5 - Epithelium & Connective Tissue Series 3: Epithelial Specialisations

Cell Surface Domains:

Not all cell surfaces are the same. Different cell surfaces are in contact with differing extracellular environments.

Cell Surfaces:

  • Apical Surface touching lumen.

  • Lateral Surface touching other epithelial cells.

  • Basal Surface touching basement membrane and usually underlying CT.

Pathways across Epithelia:

Paracellular Pathway = Between cells

Transcellular pathway = Through cell cytoplasm

Whatever pathway taken is influenced by the membrane domain (proteins and channels present) and the presence/absence of surface specialisations.

Cell Surface Specialisations:

Apical Surface Specialisations:

Microvilli - Increase surface area for absorption.

  • Approximately 1 micrometer long

  • Central dense core of actin filaments

  • Finger-like protrusions found on apical surface of cells

Function: Increase surface area of membrane and number of enzymes, carrier proteins by maximising absorption

Cilia (+ Flagella) - Make current over surface for movement

  • Motile processes on cell surface covered with cell membrane

  • Taller than microvilli, 2-10 micrometers in length

  • Beat in unison and create unidirectional current along cell surface

  • Core has microtubules. Arranged as 9 double pairings of microtubules as a circle on the outside surrounding 2 central microtubules (9+2)

Function: Move contaminants away from lungs

Stereocilia - Bulk Absorption + Sensory Function

  • Long branched microvilli

  • Similar structure to microvilli, but longer + branched.

  • Thinner and twice the height of cilia

Very limited distribution in body. Mainly found in epidermis layer

Function: Re-absorb fluids to concentrate them.

Lateral Surface Specialisations:

Lateral surfaces are the surface between adjacent epithelial cells.

Made up of intercellular junctions, which are important for barrier integrity and coordinated cellular activity

Tight Junction - Seal intercellular space

Structure:

  • Transmembrane proteins

  • Junctional adhesion molecule (JAM)

  • Occludin

  • Claudin

  • Has 20 associated proteins in cell cytoplasm which interact with actin cytoskeleton filaments.

    Function: Forms barrier between lumen and deeper tissues

Desmosome - Protein fibres holding adjacent cells, but intercellular space is not sealed

Structure:

  • Local spot-like junction

  • Concentration of different proteins form attachment plaque on cytoplasmic side of cell membrane. Plaque are attachment sites for intermediate filaments

  • Transmembrane proteins = Cadherins

  • Wide intercellular space between adjacent cell membranes

    Function: Provide strong attachment over several points of epithelial surface to hold adjacent cells together

    • Numerous in epithelia subjected to abrasion + tearing stresses. Particulary abundant in deep layers of epidermis.

Gap Junction - Tunnels allowing direct transmission between cells.

Structure:

  • Connexin Proteins make up gap junctions. They align to form connexons tubules. These pair up with another connexon in adjacnet cell to form a channel across the intercellular space.

  • Tunnels made up of proteins that align to form pores.

    Function: Allows for movement of molecules from cell to neighbouring cell. Cell can communicate, ionic signals can be transmitted. Ionic connections.

Basal Surface Specialisations:

Hemidesmosomes - Attach membrane of epithelial cells to basement membrane. Prevents tissue from pulling away from CT.

  • Provides immobility to epithelium layers + enables strong attachment to CT

Structure:

  • Plaques attach to intermediate filaments in cytoplasm.

  • Similar to half-desmosomes

  • Attachment between epithelium, basal lamina and underlying CT.

Junctional Complexes:

Order of junctions from apical surface to basal surface.

  • Can go from extensive strands of tight junctions (zonula occludens) + modified desmosomes/adherent junctions (zonula adherens) encircling bands around apical ends of cells.

  • Deeper layers have demosomes and gap junctions.

Functional Implications of Gap Junctions:

  • Allows direct passage of small molecules from one cell to next

  • Chemical + Electrical coupling of cellular activites, enabling cells to act as a single unit.

Functional Implications of Tight Junctions:

  • All passage across epithelium occurs through cytoplasm (selective passage)

  • No intercellular seepage

  • Ions can be transported against concentration gradient

  • Membrane proteins remain localised to correct domain

Functional Implications of Zonula Adherens:

  • Adhesion of cellular sheets for membrane integrity

  • Attach to actin filaments that can change cell shape

Functional Implications of Desmosomes

  • Attach to intermediate filaments to provide support and tissue integrity.

Learning Objectives:

  • Identify & describe epithelial cell surface domains, their specific characteristics/specialisations & functional significance

Cell Surfaces:

  • Apical Surface touching lumen.

  • Lateral Surface touching other epithelial cells. Made up of intercellular junctions, which are important for barrier integrity and coordinated cellular activity

  • Basal Surface touching basement membrane and usually underlying CT.

Apical Surface Specialisations:

Microvilli - Increase surface area for absorption.

Cilia - Make current over surface for movement

Stereocilia - Bulk Absorption + Sensory Function

Lateral Surface Specialisations:

Tight Junction - Seal intercellular space

Desmosome - Protein fibres holding adjacent cells, but intercellular space is not sealed

Gap Junction - Tunnels allowing direct transmission between cells.

Basal Surface Specialisations:

Hemidesmosomes - Attach membrane of epithelial cells to basement membrane. Prevents tissue from pulling away from CT.

Provides immobility to epithelium layers + enables strong attachment to CT

  • Correlate individual structural features of epithelial cell surfaces with their specific functions, & the overall function of the tissue of which they are a part

  • Appreciate that epithelium is a dynamic tissue that can constantly renew & respond to changes within its environment, including by metaplasia

Epithelium can undergo cell turnover.

  • Has varying turnover rates which are location specific.

  • Most frequent in location subject to most stress.

  • I.E. GIT - Rapid Turnover

  • Skin - Slower Turnover

  • Large Glands - Very Slow

Metaplasia adapt to injurious agents in a changed environment, and replaces a cell type with another that is not normally present in tissue.