The transcription factor cSox2 and Neuropeptide Y define a novel subgroup of amacrine cells in the retina.

The retina has been extensively used as a model to study the mechanisms responsible for the production of different neural cell phenotypes. The importance of both extrinsic and intrinsic cues in these processes is now appreciated and numerous transcription factors have been identified which are required for both neuronal determination and cell differentiation. In this study we have analysed the expression of the transcription factor Sox2 during development of the chick retina. Expression was found in the proliferating cells of the retina during development and was down regulated by nearly all cell types as they started to differentiate and migrate to the different layers of the retina. In one cell type, however, Sox2 expression was retained after the cells have ceased division and migrated to their adult location. These cells formed two rows located on either side of the inner plexiform layer and were also positive for Neuropeptide Y, characteristics which indicate that they were a subpopulation of amacrine cells. The expression of Sox2 by only this population of post-mitotic neurones makes it possible to follow these cells as they migrate to their adult location and shows that they initially form a single row of cells which subsequently divides to form the double row seen in the adult tissue. We suggest that retained expression of Sox2 is involved in directing the differentiation of these cells and is an early marker of this cell type.

cSox3 expression and neurogenesis in the epibranchial placodes.

Epibranchial placodes are local thickenings of the surface ectoderm, which give rise to sensory neurons of the distal cranial ganglia. The development of these placodes has remained unclear due to the lack of any definitive marker for these structures. We show here that the chick transcription factor, cSox3, is expressed in four lateral patches at the rostral edge of the epibranchial arches and that these mark the epibranchial placodes. These patches of cSox3 expression arise by gradual thinning from broader areas of cSox3 expression with concomitant loss of cSox3 in nonplacodal regions. Cells leaving the epithelial placodes as they initiate neurogenesis, lose cSox3 expression and sequentially express Ngn1, NeuroD, NeuroM, and Phox2a, but do not express Ngn2. This is in contrast to studies in the mouse where it is Ngn2, rather than Ngn1, that is predominantly expressed in epibranchial-derived neuroblasts. Overexpression of cSox3 interferes with normal neuroblast migration and results in changes in ectodermal morphology. Thus, cSox3 provides a useful tool for the study of placode formation, and loss of cSox3 expression appears to be a necessary event in normal neurogenesis from the epibranchial placodes.