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Ectodermal Placodes and the Epidermis
Like the cerebral and cerebellar cortices, the neural retina develops into a layered array of different neuronal types (Figure 1A). In mammals, these layers include the light- and color-sensitive photoreceptor cells (rods and cones); the cell bodies of the ganglion cells; and bipolar neurons that transmit electric stimuli from the rods and cones to the ganglion cells (Figure 1B). In addition, the retina contains numerous Müller glial cells that maintain its integrity, amacrine neurons (which lack large axons), and horizontal neurons that transmit electric impulses in the plane of the retina.
The neuroblasts of the retina appear to be competent to generate all the retinal cell types (Turner and Cepko 1987; Yang 2004). In amphibians, the type of neuron produced from a multipotent retinal stem cell appears to depend on the timing of gene translation. Photoreceptor neurons, for instance, are specified through expression of the Xotx5b gene, while Xotx2 and Xvsx1 expression is critical for specifying the bipolar neurons. Interestingly, these three genes are transcribed in all retinal cells, but they are translated differently. Those Xenopus neurons whose birthday is at stage 30 translate Xotx5b mRNA and become photoreceptors, while those neurons forming later (birthdays at stage 35) translate the Xotx2 and Xvsx1 messages and become bipolar interneurons (Decembrini et al. 2006, 2009). This time-dependent regulation of translation is mediated by microRNAs.
Not all the cells of the optic cup become neural tissue. In addition to the pigmented epithelium that forms from the outer layer of the optic cup, the tips of the optic cup on either side of the lens develop into a pigmented ring of muscular tissue called the iris. The iris muscles control the size of the pupil (and give an individual his or her characteristic eye color). At the junction between the neural retina and the iris, the optic cup forms the ciliary body. This tissue secretes the aqueous humor, a fluid needed for the nutrition of the lens and for forming the pressure needed to stabilize the curvature of the eye and the constant distance between the lens and the cornea.
Literature Cited
Decembrini, S. and 9 others. 2009. MiRNAs couple cell fate and developmental timing in retinal histogenesis. Proc. Natl. Acad. Sci. USA 106: 21179–21184.
Decembrini, S., M. Andreazzoli, R. Vignali, G. Barsacchi and F. Cremisi. 2006. Timing the generation of distinct retinal cells by homeobox proteins. PLoS Biol. 4: e272.
Turner, D. L and C. L. Cepko. 1987. A common progenitor for neurons and glia persists in rat retina late in development. Nature 328: 131-136.
Yang, X.-J. 2004. Roles of cell-extrinsic growth factors in vertebrate eye pattern formation and retinogenesis. Sem. Cell Dev. Biol. 15: 91–103.