Cell and Tissue Biology

Epithelial Cells: Surface Domains & Junctions

Kimberly Brown January 12, 2004

652 Preston Research Building

936-2137

kim.brown@vanderbilt.edu

Epithelial cells have 3 surface domains:

1. Apical
2. Lateral
3. Basal

I. Apical domain: free surface always directed toward exterior or lumen of the enclosed body cavity. May have surface modifications depending upon specific function:

i. Microvilli:

§ Closely packed, finger-like projections of cytoplasm that increase surface area of the cell.

§ Number and shape on cell surface correlate with absorptive capacity.

§ Can be seen under LM (“brush border” or “striated border”).

§ Contain a core of Actin filaments, which are anchored to Villin in tip.

§ Actin also extends downward into apical cytoplasm where attaches to terminal web:

a. Horizontal network of Actin filaments lying just below base of microvillus.

b. These Actin filaments are stabilized by Spectrin. Spectrin anchors terminal web to apical membrane of cell.

§ Also contains myosin II and tropomyosin filaments, which allows microvillus to contract.

§ Usually present on surface of microvilli is an amorphous cell coat of glycoprotein “glycocalyx”.

§ Examples of where found: kidney and intestine (fluid and metabolites actively transported and absorbed).

From: Ross, Histology: A Text and Atlas, 4^th ed.

ii. Stereocilia:

§ Long microvilli.

§ Apical cytoplasmic protrusions, with intermingling thin and thick regions. Cytoplasmic bridges interconnect thick regions.

§ Actin filament bundles that are cross-linked by fimbrin support them.

§ Actin bundles in stem portion are anchored to network of µ-actinin present in cross-bridges and apical cytoplasm of cell.

§ Erzin, a plasma membrane molecule, anchors the actin bundles to plasma membrane.

§ Villin is not found in tip of stereocilium.

§ Can be seen under LM

§ Where found: limited to the male reproductive tract (epididymis and ductus deferens) and the receptor hair cells in the ear.

From: Ross, Histology: A Text and Atlas, 4^th ed.

iii. Cilia:

§ Motile cytoplasmic structures capable of actively propelling particles along cell surface.

§ Beat in a synchronous pattern and utilize ATP for movement.

§ Arranged into orderly rows (respiratory tract) OR occur as single structure (rete testis or vestibular cells of ear).

§ By LM, cilia appear as hair-like structures from apical surface. They are anchored into apical cytoplasm by a basal body:

a. Modified centriole that contains nine microtubule triplets in its core forming a ring structure.

b. Under LM, basal bodies appear as a thin, dark-staining band at cilia base.

§ Each cilium contains an inner core of microtubules arranged in a 9+2 pattern. There are nine pairs or doublets of microtubules that are circularly arranged around 2 central microtubules.

a. The microtubules composing each doublet are designated as the “A microtubule” and the “B microtubule”.

b. “A” microtubule: 13 tubulin dimers arranged in a side-by-side manner.

c. “B” microtubule: 10 tubulin dimers and does not form a complete circle. Instead it “shares” a portion of its wall with “A” microtubule.

d. Each doublet contains a “pair of arms” that extends off the “A” microtubule to form cross-bridges with the “B” microtubule in the adjacent doublet. These arms contain “dynein”.

e. The protein “nexin” links the doublets together.

f. Radial spokes extend from the nine outer doublets to the central doublet.

§ Where found: respiratory epithelium-nasal cavity, trachea, bronchus, oviducts.

From: Ross, Histology: A Text and Atlas, 4^th ed.

II. Lateral domain: route of communication between adjacent cells within epithelium. Contains special attachments or “contact” sites that adhere one cell to another.

• Some cells have lateral surface folds that increase surface area of the cell.

§ Characterized by proteins that form adhesion molecules to anchor cells together.

§ Terminal Bars: attachment sites (intercellular cement) between epithelial cells in the apical-lateral margins. Also a barrier to the passage of substances across epithelium.

o Can appear as a dense bar or line between cells or as a dot-like structure depending upon plane of section.

o Difficult to identify by LM without immunohistochemical techniques.

o Continuous around entire circumference of each cell.

o Composed of junctional complexes responsible for joining individual cells.

i. Occluding junctions (tight junctions or zonula occludentes):

a. Located apically within lateral domain

b. Encircle the cell, separating luminal space from intercellular space and the CT

c. Narrow region where the plasma membranes of adjacent cells come in contact to seal off intercellular space, forming an impermeable diffusion barrier between cells.

d. Not a continuous seal but rather a series of focal fusions of the outer leaflets of plasma membrane that encircle the cell.

e. Focal fusions are created when transmembrane junctional proteins of adjacent cells traverse the plasma membrane and come in contact to seal off intercellular space.

f. Prevent migration of proteins between apical and lateral surfaces.

From: Sawada, N. et al. (2003). Med Electron Microsc. 36: 147-156.

ii. Anchoring junctions (adherens junctions):

§ Maintain cell-cell adherence by linking transmembrane proteins on adjacent cells, resulting in a fusion of adjoining cell membranes. Two types of anchoring cell junctions:

1. Zonula adherens: interacts with actin filaments inside the cell.

o Composed of the transmembrane adhesion molecule E-cadherin.

o Intracellular tail of E-cadherin binds to b-catenin on the cytoplasmic side of the plasma membrane forming a cadherin-catenin complex.

o This complex binds to a-catenin and the actin cytoskeleton.

o The extracellular components of E-cadherin molecules bind to Ca²⁺ making morphologic and functional integrity of zonula adherens calcium dependent.

o Resistance to mechanical stress is limited.

o Continuous band or belt that completely encircles the cell.

o 15-20 nm space between opposing membranes and is electron-lucent.

2. Macula adherens (desmosomes): interact with intermediate filaments.

o Disk-shaped structure (spot welds) at surface of one cell that is matched with an identical structure on surface of adjacent cell.

o Circular plaque (attachment plaque) of material made of ~12 proteins on cytoplasmic side of plasma membrane.

o Wide intercellular space containing dense medial band of desmoglein.

o Intermediate keratin filaments are inserted into attachment plaque and loop back towards cytoplasm. Provide firm adhesion between cells.

iii. Communication junctions (gap junctions or nexus): coordinate activities between cells by permitting movement of ions or signaling molecules between cells.

§ Accumulation of transmembrane channels or pores in a tightly packed array.

§ Pores in one cell membrane line up with corresponding pores on opposite membrane. Permit passage of small molecules, nutrients, and charged ions and signaling agents between adjacent cells.

§ Each pore consists of minute tubular structure (connexon) which traverses gap.

o Connexon consists of six transmembrane proteins called connexins that are arranged in a circular pattern.

o Diameter of channel is regulated by reversible changes in confirmation of individual connexins.

From: Ross, Histology: A Text and Atlas, 4^th ed.

iv. Hemidesmosomes

§ Attach basal plasma membrane to the basal lamina.

§ Form attachment plaques on cytoplasmic side of plasma membrane.

§ Found where abrasion and mechanical shearing forces would tend to separate epithelium from underlying CT.

From: Junqueira and Carneiro, Basic Histology: A Text and Atlas.

III. Basal Domain: rests on “basal lamina” or “basement membrane” and anchors epithelium to underlying CT.

• Layer of variable thickness beneath basal surface of each cell comprising the epithelium.
• Not easily distinguished by H&E.
• Easily seen after staining with Periodic Acid-Schiff (PAS)-appears as magenta layer between epithelium and CT. PAS reacts with sugar moieties of proteoglycans produced by the epithelial cells and accumulates under the basal layer.
• Using EM, it appears as a dense layer – the “lamina densa”.

o Electron-dense matrix ~50-100 nm thick consisting of 3-4 nm filaments.

o Structural attachment site for anchoring fibrils that extend to the reticular fibers in the CT layer beneath the epithelium.

§ There may be electron-lucent layers on either side of the densa (“lamina rarae or lucida”). Contains extracellular portions of cell adhesion molecules from integrin family of transmembrane proteins.

§ Main components of basal lamina:

i. Type IV collagen

a. Contains short filaments that provide structural integrity.

b. Secreted by epithelial cells.

ii. Laminin

a. Cross-shaped glycoprotein molecule secreted by epithelial cells.

b. Contains domains that bind Type IV collagen, heparin sulfate, & integrins.

c. Bridges lamina lucida and lamina densa to plasma membrane.

iii. Entactin and fibronectin

a. Glycoproteins that act as adhesive substance.

b. Bind plasma membrane to proteoglycans.

iv. Proteoglycans

a. Consists of hydrated molecules that form bulk volume of lamina.

b. Highly negative charge so regulate passage of ions.

§ Layer of reticular fibers (anchoring fibrils) that consist of Type III collagen underlies basal lamina (basement membrane) and helps anchor epithelium to CT.

§ Anchoring fibrils consisting of Type VII collagen extend from basal lamina matrix to connect the reticular fibers. They are usually associated with hemidesmosomes located in basal domain of the epithelial cell.

§ Functions of Basal Laminae:

i. Compartmentalization: Separates CT from epithelia, nerve or muscle tissues.

ii. Filtration: Regulates movement of substances to and from CT (mainly by ionic charges).

iii. Polarity induction: Basal lamina attributes specific properties to basal membrane surface.

iv. Tissue scaffolding: Basal lamina serves as guide or scaffold during regeneration of epithelium.

From: Junqueira and Carneiro, Basic Histology: A Text and Atlas.