Müller cell differentiation in the zebrafish neural retina: evidence of distinct early and late stages in cell maturation.

The vertebrate neural retina is mainly composed of cells of neuroectodermal origin. The primary cell types found in all vertebrate retinas are several categories of neurons and the archetypical retina glial cell the Müller cell. Although the neurons and the single glial cell type of the retina are specialized for very distinct functions, they all have a common developmental origin within the tissue. How the distinctions between cell types, in particular between neurons and glia, arise during embryonic development remains a central issue in neurobiology. In this report, we examine the genesis of Müller glial cells during zebrafish (Danio rerio) eye development. Particular emphasis is placed on the expression of the Müller cell maturation markers carbonic anhydrase and glutamine synthetase. In addition, we report that the HNK-1 monoclonal antibody, which identifies a particular glycoconjugate frequently found on cell surface recognition molecules, also identifies zebrafish retina Müller cells early in development. The expression patterns of these three markers clearly show that the Müller cells mature in stages: HNK-1 labeling and glutamine synthetase arise earlier than carbonic anhydrase expression. In addition, the embryonic zebrafish neural retina is characterized by the presence of amoeboid, carbonic anhydrase-positive microglial cells even before the genesis of retinal neuroectodermal glia. The stepwise maturation of the glia is likely to be indicative of an overall retinal maturational program in which cell differentiation and the expression of certain phenotype-defining gene products may be separately regulated.