Occluding junctions in a cultured transporting epithelium: structural and functional heterogeneity

Area of Science:

• Cell Biology
• Epithelial Physiology
• Biophysics

Background:

• Madin-Darby canine kidney (MDCK) cells, derived from renal epithelia, mimic transporting epithelia.
• These cells form polarized monolayers with functional occluding junctions, crucial for barrier function.

Purpose of the Study:

• To investigate the structural and functional heterogeneity of occluding junctions in MDCK cell monolayers.
• To correlate morphological features with electrical properties of the epithelial barrier.

Main Methods:

• Scanning electric field analysis to map current flow across MDCK monolayers on permeable supports.
• Freeze fracture electron microscopy to visualize the ultrastructure of occluding junctional strands.

Main Results:

• Electric field scanning revealed heterogeneous current flow, indicating regions of high resistance and high conductance.
• Freeze fracture analysis showed variations in the number of junctional strands, from 8-10 down to 1-2.
• High conductance sites likely correspond to regions with fewer junctional strands.

Conclusions:

• MDCK cell occluding junctions are functionally and morphologically heterogeneous.
• These junctions comprise a mixture of 'tight' (high resistance) and 'leaky' (high conductance) regions.