Coarse and Fine Tuning

Abstract

DEVELOPMENTAL BIOLOGY Deletion of two myogenic transcription factors, Myf5 and MyoD, is sufficient to abrogate the skeletal muscle lineage in mice. The absence of muscle leads to degeneration of neuronal components. Spinal and brainstem motor neurons and proprioceptive sensory neurons begin to develop but are then eliminated by apoptosis. Competing theories about brain cortical development disgaree over the developmental causation of cortical specializations. What defines, for example, the olfactory cortex or, of more interest in these experiments, the specialized region known as the motor cortex? Regions of the cortex might be specified by preexisting intrinsic programs, or they might be specified in reaction to signals coming from outside sources, such as the thalamus. Kablar and Rudnicki find that, in mice lacking skeletal muscle, the large-scale organization of the neocortex is normal, favoring early intrinsic patterning mechanisms. However, the cortical neuronal cell populations that would subserve motor activity show a spectrum of deficits, suggesting that fine elaboration of neuronal structures depends on extrinsic signals. — PJH Int. J. Dev. Neurosci . 20 , 573 (2002).