Development

Control of retinal ganglion cell growth: a new role for Sonic hedgehog. Trousse, F. et al. Development 128, 3927–3936 (2001) [PubMed]

Trousse et al. report on a new role for Sonic hedgehog (Shh), a signalling molecule already known to be required for many aspects of neural ontogeny, in the developing visual system. In vitro, they show that Shh inhibits the growth of retinal ganglion cell axons. In vivo, they suggest that it is important for the initial positioning of the optic chiasm, and that its repulsive effect on ganglion cell axons might facilitate their guidance through the chiasm at later developmental stages.

Sensory systems

Auditory detection and discrimination in deaf cats: psychophysical and neural thresholds for intracochlear electrical signals. Vollmer, M. et al. J. Neurophysiol. 86, 2330–2343 (2001) [PubMed]

Cochlear implants work by providing direct electrical stimulation to the auditory nerve, bypassing the non-functional sensory cells of a deaf cochlea. How are these electrical stimuli processed and represented in the central auditory system? The authors used a deaf animal model to address this question, directly comparing behavioural and neurophysiological thresholds to intracochlear electrical signals. This model might assist in the design and calibration of cochlear implants and other speech-processing strategies.

Sensory systems

Induction of photoreceptor-specific phenotypes in adult mammalian iris tissue. Haruta, M. et al. Nature Neurosci. 12 November 2001 (10.1038/nn762) [PubMed]

The gene Crx is crucial for photoreceptor differentiation. Haruta et al. show that transfecting Crx into cells of the iris leads to the expression of photoreceptor-specific antigens. This finding reveals an unexpected plasticity of the iris, and raises the possibility that this structure could be used as an autologous source of tissue for retinal transplants. It remains to be determined whether Crx is sufficient to drive the cells of the iris through the complete photoreceptor differentiation programme.

Neurogenetics

Chromosomal variation in neurons of the developing and adult mammalian nervous system Rehen, S. K. et al. Proc. Natl Acad. Sci. USA 98, 13361–13366 (2001) [PubMed]

Do all neurons in a given brain have identical genomes? Apparently not. Rehen et al. show that as many as one third of cortical neuroblasts might be aneuploid. They observed lagging chromosomes during neuroblast mitosis and found all kinds of related alterations — hyperploidy, monosomy, trisomy. Moreover, when looking at differentiated neurons, the authors found a subpopulation of cortical cells that were aneuploid. It remains to be seen whether this observation has a functional correlate.