Alterations in the intermediate layer of goldfish meninges during adaptation to darkness.

The morphological changes in the intermediate endomeningeal layer of the goldfish brain during light and dark adaptation were studied by freeze-fracture electron microscopy. During the different stages of adaptation no significant changes were found in the density of intramembrane particles and nuclear pores in these cells. The density of plasmalemmal vesicles in the meningocyte surface increased in the groups maintained in the dark for 48 and 72 h (maximum) and then decreased in the group maintained for 96 h in the dark to a basal level. There were also morphological changes in the junctional complexes. At the upper cell membranes (in contact with the outer layer) of meningocytes in the group maintained in the dark for 48 h, we found an increase in the surface occupied by gap junctions. In addition, gap junctions were absent in the lateral membranes of meningocytes from animals maintained in the dark for 72 h. The morphology of gap junctions in the group maintained in the dark for 96 h was similar to that of the control group. These results suggest that the cells of the teleost intermediate endomeningeal layer undergo important changes in activity during adaptative experiments.