To what extent are the retinal capillaries ensheathed by Muller cells? A stereological study in the tree shrew Tupaia belangeri

The cellular ensheathment of capillaries in the 3 outer capillary layers of the central retina of the adult tree shrew Tupaia belangeri was studied qu antitatively by transmission electron microscopy. Using a stereological app roach, the relative surface of capillary basal lamina ensheathed by Muller cells and by nonmacroglial cells (collectively termed non-Muller cells) was estimated in 5 animals. The participation of Muller cells was distinctly d ifferent in the 3 capillary layers studied. In the outermost capillary laye r 1, the mean (standard deviation) percentage surface coverage by non-Mulle r cell processes was 46.8 (15.3)%. Much less of the capillary basal lamina was ensheathed by non-Muller cells in capillary layers 2 and 3 (3.0 (2.1)% and 0.3 (0.3)% respectively). The observed total variation of the stereolog ical estimates for the surface fraction of Muller cells (expressed as the b etween-subject coefficient of variation) was significantly higher in capill ary layer 1 (28.8%) compared with capillary layers 2 (2.2%) and 3 (0.3%). I n capillary layer 1, the high observed total variation was due to a high bi ological variation among animals for the fractions of both Muller cell and non-Muller cell ensheathment. The rare occurrence of direct contacts betwee n the capillary basal lamina and the perikarya of either microglial cells ( capillary layer 3) or amacrine cells (capillary layer 2) corresponded well to the low stereological values obtained for the relative capillary surface ensheathed by non-Muller cells in these capillary layers. Previously, exte nsive and frequent contacts between the basal lamina of capillaries belongi ng to capillary layer 1 and horizontal cells had been observed in single se ctions. The present study quantitatively demonstrates a marked paucity of m acroglial investment of capillaries located in capillary layer 1 of Tupaia. It can be concluded that horizontal cells ensheath most of the capillary s urface not invested by Muller cells.