Light microscopical observations on luminally administered dyes, dextrans, nanospheres and microspheres in the pre-epithelial mucus gel layer of the rat distal colon

The permeability in vivo of the pre-epithelial mucus gel layer (PML) of the rat distal colon to several intraluminally administered dyes, dextrans, nano- and microspheres was studied by transmission and fluorescence light microscopy. Methylene Blue (MW 356) passed through the colonic PML in less than 1 min and stained mucosal as well as propria cells. Light Green SF yellowish (MW 792.9) was found in all sublayers of the PML and had stained the superficial cells of the colon mucosa 5 min after luminal administration. Alcian Blue (MW 1300), Evans Blue (MW 960.8) and Trypan Blue (MW 960.8) seemed to be trapped within the PML when intraluminally administered as 1% solution. Dextran 10 000 was transferred through all of the PML-sublayers within 1 min of its application into the gut lumen and it was found in the nuclei of the superficial cells of the distal colonic mucosa. Dextran 40 000 and Dextran 70 000 also entered the PML, but they were not taken up into the epithelial cells. Small polymeric nanosphere particles reached the apical membranes of the surface epithelium cells less than 2 min after luminal administration and were stuck to them. Midsize nanospheres (415 nm diameter) were also found to be adhered to the apical membranes of the surface epithelium cells 30 min after dosage, Instead of 'mucoadhesive' properties, these latex nanospheres were found rather to possess 'mucosa-adhesive' or 'cell-adhesive properties. In contrast, there was no accumulation of large microspheres (1.09 mu m diameter) on the surface cell membranes within the observation period of 30 min following administration. They were found within the PML, but away from the mucosa. The PML of the rat distal colon represents a diffusion barrier for solutions of the cationic dyes Alcian Blue, Evans Blue and Trypan Blue and for large molecules. On the other hand, surprisingly the PML does not represent a barrier for Dextran 10 000. Furthermore, Dextran molecules of MW 40 kDa and higher pass the PML sublayers slowly in the direction of the superficial cells of the gut epithelium. The PML is not a diffusion barrier for virus size latex particles (nanospheres), nor even an absolute barrier for near bacterium-size latex particles (microspheres), although these do seem to be slowed down in their diffusion through the PML due to their large size.