Evidence that Folliculo-Stellate Cells do not Impede the Permeability of Intercellular Spaces to Molecular Diffusion in Three-Dimensional Aggregate Cell Cultures of Rat Anterior Pituitary*

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Abstract
The permeability of intercellular spaces within the anterior pituitary (AP) and the influence of folliculo-stellate (FS) cells on compartmentalization within this tissue, has become a matter of debate. In reaggreated pituitary cell cultures as well as in the AP in situ the intercellular gaps and follicle-like structures remain accessible to molecular diffusion, whereas in some studies FS cells were reported to form tight epithelia that impede macromolecular transport through the spaces between the epithelial cells. In the present study the permeability of AP cell reaggregates was examined using fluorescent BSA as a tracer. Using confocal scanning laser microscopy a direct visualization of the permeation process was achieved. Quantitative estimation of the effective diffusion coefficient (Deff) for fluorescein-BSA within the aggregates was obtained using the fluorescence photobleaching recovery technique. Deff was 1.33 .+-. 0.31 .times. 10-7 cm2/sec (mean .+-. SD) in aggregates from 14-day-old female rats and 2.45 .+-. 0.55 .times. 10-7 cm2/sec in aggregates from adult female rats. These values are about three times lower than in free solution. Calculation of the time-dependent concentration distribution inside the aggregate for a Deff = 2 .times. 10-7 cm2/sec revealed that the concentration of the fluorescent tracer in the center of the aggregate reaches 90% of the concentration outside the aggregate after 0.5 min for aggregates with a radius of 50 .mu.m and 6 min for aggregates with a radius of 150 .mu.m. Aggregates enriched in FS cells, in which we previously showed a sustained inhibition of secretory response to stimulatory and inhibitory agents as compared to total population aggregates, showed a diffusion coefficient (Deff = 1.85 .+-. 0.77 .times. 10-7 cm2/sec) which was not significantly different from that in the total population aggregates. The present study shows that AP cell aggregates are fully permeable to diffusing molecules within minutes and that in a three-dimensional tissue configuration FS cells, which were reported to tonically inhibit AP hormone release in response to various secretagogues, do not impede molecular diffusion to an extent which would account for sustained inhibition of hormone release.