Ultrastructure of Müller cells in the developing human retina.

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The posterior retina of human embryos from 4 to 200 mm of crown-rump length was studied by electron microscopy. At 20 mm dense inner Müller-cell processes near ganglion cells contained rough endoplasmic reticulum, free ribosomes, small matrix particles, and some intermediate filaments. These processes soon had smooth endoplasmic reticulum. By 71 mm many of these inner processes were lucent and contained many intermediate filaments and glycogen particles. Müller-cell nuclei and outer processes were observed between differentiating cone cells at 66 mm, and these outer radial-cell processes soon contained many dense matrix particles and glycogen particles. As neurons in the inner nuclear layer differentiated by 100 mm, Müller-cell cytoplasm in the mid-retina was identified by its intermediate filaments and glycogen particles. Müller cells have composite glial features that appear in the horizontal retinal layers concomitant with neuronal differentiation and maturation in each layer.