Color vision is facilitated by distinct populations of cone photoreceptors in the retina. In rodents, cones expressing different opsin photopigments are sensitive to middle (M, 'green') and short (S, 'blue') wavelengths, and are differentially distributed across the retina. The mechanisms that control which opsin is expressed in a particular cone are poorly understood, but previous in vitro studies implicated thyroid hormone in cone differentiation. Thyroid hormone receptor beta 2 (TR beta 2) is a ligand-activated transcription factor that is expressed in the outer nuclear layer of the embryonic retina. Here we delete Thrb (encoding Tr beta 2) in mice, causing the selective loss of M-cones and a concomitant increase in S-opsin immunoreactive cones. Moreover, the gradient of cone distribution is disturbed, with S-cones becoming widespread across the retina. The results indicate that cone photoreceptors throughout the retina have the potential to follow a default S-cone pathway and reveal an essential role for Tr beta 2 in the commitment to an M-cone identity. Our findings raise the possibility that Thrb mutations may be associated with human cone disorders.

We have reported previously that neurotoxic damage to the chicken retina causes Müller glia to dedifferentiate, proliferate, express transcription factors common to retinal progenitors, and generate new neurons and glia, whereas the majority of newly produced cells remain undifferentiated (Fischer and Reh, 2001). Because damaged retinal cells have been shown to produce increased levels of insulin-related factors and FGFs, in the current study we tested whether intraocular injections of growth factors stimulate Müller glia to proliferate and produce new neurons. We injected growth factors and bromodeoxyuridine into the vitreous chamber of the eyes of chickens and assayed for changes in glial phenotype and proliferation within the retina. Although insulin or FGF2 alone had no effect, the combination of insulin and FGF2 caused Müller glia to coexpress transcription factors common to retinal progenitors (Pax6 and Chx10) and initiated a wave of proliferation in Müller cells that began at the retinal margin and spread into peripheral regions of the retina. Most of the newly formed cells remain undifferentiated, expressing Pax6 and Chx10, whereas some differentiate into Müller glia, and a few differentiate into neurons that express the neuronal markers Hu or calretinin. There was no evidence of retinal damage in eyes treated with insulin and FGF2. We conclude that the combination of insulin and FGF2 stimulated Müller glia to dedifferentiate, proliferate, and generate new neurons. These findings imply that exogenous growth factors might be used to stimulate endogenous glial cells to regenerate neurons in the CNS.

The ciliary epithelium of the ciliary body is derived from the anterior rim of the developing optic cup. Several recent studies have found that developmental abnormalities in this tissue can underlie congenital glaucoma. However, there is little known about the development of the ciliary epithelium. To better understand the developmental events responsible for the specification of this domain of the optic cup, we used a subtractive library, differential screening approach along with the construction of cDNA arrays to identify genes expressed in the ciliary epithelium of the chicken. We identified many genes specifically expressed in the ciliary epithelium, including a number that had been described previously as enriched in the ciliary epithelium of other species. By analyzing the expression of these genes during eye development, we were able to correlate the onset of ciliary epithelial gene expression with a reduction in mitotic activity in this region. We propose that the mechanisms that regulate the expression of ciliary epithelial genes are linked to the reduction in proliferation that results in the epithelial monolayer in this region. Developmental Dynamics 229:529 -540, 2004.

Neural progenitors at the retinal margin of the post-hatch chicken normally produce amacrine and bipolar cells, but not photoreceptor or ganglion cells. The purpose of this study was to test whether exogenous growth factors influence the types of cells produced by progenitors at the retinal margin. We injected insulin, FGF2 or a combination of insulin and FGF2 into the vitreous chamber of post-hatch chickens. To assay for growth factor-induced changes at the retinal margin, we used in situ hybridization and immunocytochemistry on cryosections. One day after the final injection, we found that insulin alone stimulated the addition of cells to the retinal margin, but this was not further increased when FGF2 was applied with insulin. Insulin alone increased the number of cells in the progenitor zone that expressed neurofilament, and this was further increased when FGF2 was applied with insulin. These neurofilament-expressing cells in the progenitor zone included differentiating neurons that expressed Islet1 or Hu. Four days after the final dose of growth factor, we found that the production of ganglion cells was induced by co-injection of insulin and FGF2, but not by either insulin or FGF2 alone. We conclude that the types of cells produced by progenitors at the retinal margin can be altered by exogenous growth factors and that normally the microenvironment imposes limitations on the types of neurons produced.