Capillary-contacting horizontal cells in the retina of the tree shrew Tupaia belangeri belong to the mammalian type A

Previously, ultrastructural evidence has been presented that, in the retina of adult Tupaia belangeri, the perikarya and processes of horizontal cells extensively ensheath the basal lamina of capillary cross sections located between the inner nuclear layer and the outer plexiform layer The present study tests whether these horizontal cells can be further characterized by applying a polyclonal antibody against glial fibrillary acidic protein (GFAP). GFAP-immunoreactivity was noted in the astrocytic plexus ensheathing retinofugal axons in the nerve fiber layer. The vitreal endfeet and parts of the trunks of Muller cells were also labelled. Moreover, a large subpopulation of vessel-contacting horizontal cells was strongly GFAP-immunoreactive. Immunoreactivity was found in the perinuclear cytoplasm and in the sturdy primary dendrites of these cells. The somata of GFAP-immunoreactive horizontal cells were unevenly distributed. These cells had three to seven primary dendrites that showed considerable overlap with the dendrites of neighbouring horizontal cells. For these reasons, GFAP-immunoreactive horizontal cells were classified as belonging to the mammalian type A. Whether the simultaneous occurrence of two glial features, viz. extensive ensheathment of retinal capillaries and immunoreactivity for a polyclonal antibody towards GFAP, supports the view that retinal horizontal cells represent a cell type intermediate between neurons and glial cells is discussed.