Differentiation of chick embryo neuroretina cells in monolayer cultures. An ultrastructural study. I. Seven-day retina.

Neuroretinas from 6--7 day-old chick embryos were cultivated after trypsin dissociation as monolayer cultures in Petri dishes, and examined after various intervals of time with the electron microscope. Soon after plating, cells begin to reaggregate in small clumps, and typical rosettes are formed. During the first week in vitro, cells appear to differentiate as neuroblasts and presumed Müller cells; the latter form a continous sheet on the substrate, upon which neuroblasts migrate and grow their neurites. Differentiated ribbon synapses are found after 8 days in vitro, the time at which they normally appear in situ. After 15 and 21 days in vitro, synapses are still found in large numbers, mimicking their "in vivo" counterparts. Photoreceptor cells were identified on the basis of the presence of typical ribbons in their cytoplasm, but no outer segment was found. It appears then that synaptogenesis in the retina is programmed independently of the tissue environment, which is markedly disturbed in the monolayer culture.