SER-rich Müller cell end-feet contact astrocytes, microglia, endothelia, and neurons in the mouse retina. A, Vertical section of the proximal, electron-dense end-foot of a Müller cell (MC) adjacent to the ocular vitreous (*), with abundant end-foot mitochondria (m). MCs are immediately adjacent to neurons such as ganglion cells (GC; blue), large bundles of axon fascicles (F), astrocytes (AsC; green), and endothelial cells (Endo) of capillaries (C). B, A horizontal section of six Müller cell proximal end-feet (MC1–6) approaching the vitreal border and surrounding an astrocyte. MC4 exemplifies the high MC SER tubule density with many 15–20 nm tubules running parallel to the plasmalemma next to the astrocyte. Mitochondria are embedded in dense SER networks. C, A horizontal section of six Müller cell proximal end-feet surrounding the initial ascending dendrite of a ganglion cell, and contacting both endothelial and microglial cells. D, High-resolution view of Müller cell end-foot MC4 from C demonstrating the high density of SER tubules near the ganglion cell. Large SER tubules tend to be dentate in shape (vertical arrows) and branch into very tiny tubules ∼15 nm in diameter near the membrane (slanted arrows). E, A horizontal section of a dozen SER-rich Müller cell end-feet with two (MC3, MC4, single arrows) forming extensive apposition to the basement membrane (yellow) of capillary endothelia. An astrocyte forms a small basement membrane apposition. F, High-resolution view of Müller cell end-foot MC1 from E demonstrating the high density of fine SER tubules near the astrocyte (vertical arrows) and cytoskeletal elements in the two cells. Actin filaments (10 nm) are positioned in bundles along the Müller cell membrane. Finer intermediate filaments (5–6 nm) form large axial bundles throughout the astrocyte. Scale markers on all panels, 1000 nm.