It is currently believed that in each vertebrate species Muller cells in the central retina constitutes a fairly homogeneous population from the morphologic point of view and that particularly the chick Miller cell attains full shape differentiation at prenatal stages. However, in this study of the chick retina, from day 1 to day 55 of life, we show that there is a large variety of Muller cell shapes and that many of them complete shape differentiation postnatally. We used a cell dissociation method that preserves the whole shape of the Muller cells. Unstained living and unstained fixed cells were studied by phase-contrast microscopy; and fixed cells immunostained for intermediate filaments of the cytoskeleton were studied by fluorescence microscopy. Our results show that (1) Muller cell shapes vary in the origination of the hair of vitread processes, in the shape of the ventricular (outer or apical) process, in the presence or absence of an accessory process, as well as in the number and shape of processes leaving from the ventricular process at the level of the outer nuclear and outer plexiform layers (ONL/OPL); (2) during the first month of life, many Muller cells differentiate the portion of the ventricular process that traverses the ONL, most Muller cells differentiate the ONL/OPL processes, and all Muller cells differentiate the thin short lateral processes leaving from the vitread hair processes at the level of the inner plexiform layer (IPL). The number of cells differing in the shape of the ventricular process and that of cells with and without accessory process were estimated. The spatial relationship between the outer portion of the ventricular process of the Muller cell and the photoreceptor cells was also studied. Our results show that the branching of the ventricular process and the refinement of Muller cell shape is achieved without apparent participation of growth cones. We give a schematic view of how the branching of the ventricular process might take place and propose the size increase of photoreceptor soma as a factor responsible for this branching.
