MORPHOLOGICAL BASES FOR A NURSING ROLE OF GLIA IN THE TOAD RETINA. ELECTRON MICROSCOPE OBSERVATIONS

Mfiller cells are glial elements that constitute the sponge-like framework in which retinal neurons are embedded (1, 12). They have their nuclei at the inner nuclear layer and the cell bodies extend across all the retinal layers in between the two limiting membranes. Miiller cell processes are present everywhere in the retina, interposed between the other cells, and consequently little, if any, extracellular space is left in this tissue. As a result of this architecture, metabolites reaching retinal neurons probably pass through the cytoplasma of Mfiller cells, as seems also to be the case for glial cells in the central nervous system (5). It is therefore possible that Mfiller cells are not simply supporting elements and that they might have a nursing role, as postulated for other glial cells (6, 9, 10). The present investigations were undertaken to determine whether there are any morphological indications, at the level of the electron microscope, of such a hypothetical nursing function of Mfiller cells. Retinas of light-adapted toads (Bufo arenarum Hensel) were fixed for 2 hours at 4°C in a solution containing 1 per cent OsO4, polyvinylpyrrolidone and balanced ions (15). Small pieces of retina were embedded, properly oriented for perpendicular sectioning, in a mixture of 10 per cent methyl methacrylate in n-butyl methacrylate. Observations were accomplished in an R C A E M U 2 E microscope. When a section of toad retina is inspected, it is seen that the retinal neurons are apparently embedded in a clear matrix which occupies all available spaces in between cells. Bounding the cellular profiles and intervening between them and this low-density substance, there are two dense membranes separated by a clear space about 100 A wide (Fig. 2). At first sight it seems as if retinal cells have a double plasma membrane, but actually one of the membranes is the plasma membrane of the Mfiller cell, the clear matrix in between retinal neurons being the ground cytoplasm of Miiller cell processes. At the outer nuclear layer where nuclei, perikarya and fibers of the photoreceptor cells are located, Mfiller cell processes are very thick and show a high concentration of cytoplasmic organelles (Fig. 1). These consist of vacuoles of the endoplasmic reticulum (Fig. 2), an impressive array of long and very tortuous double membranes (Figs. 1 and 2), and ovoid or elongated bodies about 0.5 # wide and 1 t~ long which are bounded by two membranes and have an amorphous matrix of higher density than the surrounding cytoplasm (Fig. 1). These bodies sometimes have in their matrix a few crista-like structures (Fig. 1) which in some instances show continuity with the inner of the two membranes. It seems, therefore, reasonable to assume that they are mitochondria, a conclusion supported by the fact that no other structures resembling mitochondria are found in the Miiller cells of the toad retina. These atypical mitochondria can be found also at the level of the inner nuclear and ganglion cell layers, but they are more numerous at the outer nuclear layer, in the thick Mfiller cell processes interspersed between the photoreceptor cells? In a survey of this area, it first came to our attention that there is occasionally a close relationship between the mitochondria and the plasma membrane of the Mfiller cell. When systematic observations were made, it was found that some mitochondria appear to adhere to the plasma membrane of the Miiller cell and are thus in close proximity to the plasma membrane of the photoreceptor cell (Figs. 2 to 4).