Morphological and cytochemical aspects of capillary permeability

Area of Science:

• Cardiovascular Physiology
• Cell Biology
• Microcirculation

Background:

• Capillary wall transport of plasma constituents is crucial for cardiovascular physiology.
• Physiological studies suggest two pore sizes for protein and small solute transport, but morphological evidence remains debated.
• Endothelial cell structures are investigated as potential pathways for transcapillary exchange.

Purpose of the Study:

• To review proposed morphological pathways for plasma constituent transport across endothelial cells.
• To correlate proposed pathways with physiological definitions of capillary pores.
• To present new morpho-cytochemical evidence for intercellular clefts in small molecule diffusion.

Main Methods:

• Review of existing literature on endothelial transport mechanisms.
• Presentation of data from morpho-cytochemical studies.
• Analysis of proposed transendothelial pathways including vesicular system, tubulo-vesicular system, intercellular clefts, and fenestrae.

Main Results:

• The vesicular system's role in transport is supported by some data but remains controversial.
• A tubulo-vesicular system, potentially forming transendothelial channels, is proposed as a pathway for large molecules.
• Morpho-cytochemical evidence identifies intercellular clefts as diffusion sites for small molecules (<3,000 Da).
• Fenestrae are confirmed as transport sites in fenestrated capillaries, which also retain other transport pathways.

Conclusions:

• Multiple pathways, including intercellular clefts, tubulo-vesicular systems, and fenestrae, facilitate transcapillary transport.
• The precise function and regulation of these pathways, particularly the intracellular tubulo-vesicular system in non-fenestrated capillaries, require further investigation.
• Understanding the distinct roles of these pathways in different capillary types is essential for a comprehensive view of microcirculation.