We are interested in the mechanisms of glial cell development in the vertebrate central nervous system. We have identified genes that can direct the formation of glia in the retina. rax, a homeobox gene, Hes1, a basic helix-loop-helix gene, and notch1, a transmembrane receptor gene, are expressed in retinal progenitor cells, downregulated in differentiated neurons, and expressed in Müller glia. Retroviral transduction of any of these genes resulted in expression of glial markers. In contrast, misexpression of a dominant-negative Hes1 gene reduced the number of glia. Cotransfection of rax with reporter constructs containing the Hes1 or notch1 regulatory regions led to the upregulation of reporter transcription. These data suggest a regulatory heirarchy that controls the formation of glia at the expense of neurons.

In this study we challenge the generally accepted view that cardiac chambers form from an array of segmental primordia arranged along the anteroposterior axis of the linear and looping heart tube. We traced the spatial pattern of expression of genes encoding atrial natriuretic factor, sarcoplasmic reticulum calcium ATPase, Chisel, Irx5, Irx4, myosin light chain 2v, and beta-myosin heavy chain and related these to morphogenesis. Based on the patterns we propose a two-step model for chamber formation in the embryonic heart. First, a linear heart forms, which is composed of "primary" myocardium that nonetheless shows polarity in phenotype and gene expression along its anteroposterior and dorsoventral axes. Second, specialized ventricular chamber myocardium is specified at the ventral surface of the linear heart tube, while distinct left and right atrial myocardium forms more caudally on laterodorsal surfaces. The process of looping aligns these primordial chambers such that they face the outer curvature. Myocardium of the inner curvature, as well as that of inflow tract, atrioventricular canal, and outflow tract, retains the molecular signature originally found in linear heart tube myocardium. Evidence for distinct transcriptional programs which govern compartmentalization in the forming heart is seen in the patterns of expression of Hand1 for the dorsoventral axis, Irx4 and Tbx5 for the anteroposterior axis, and Irx5 for the distinction between primary and chamber myocardium.

The Notch gene plays a role in the development of disparate tissues in multiple organisms. Because the vertebrate eye is an excellent model system for both patterning and cell fate determination, two processes that can involve Notch, we examined the expression patterns of Notch 1 and Notch 2, and their ligands Delta and Jagged, in the developing rat eye. Notch 1 and Delta were found to be expressed in the neural retina during the period of cell fate determination and differentiation. Notch 2 was found to be expressed in the non-neuronal derivatives of the optic cup, including the pigment epithelium, optic stalk, and ciliary body. Jagged was expressed in distinct regions within the optic vesicle, ciliary body, and lens, with patterns that changed over time. The potential function of Notch 1 in cell-type specification and differentiation was examined by introducing a constitutively active form of Notch 1 in vivo using a replication-incompetent retrovirus. This form of Notch 1 was found to cause abnormal growth and interfere with the differentiation of multiple retinal cell types.