The specification of the motor neuron subtypes that innervate digits is not well understood. Limb-innervating motor neuron specification is influenced by retinoic acid, the synthesis of which is regulated by specific expression patterns of homeobox (HOX) transcription factors and their cofactor FOXP1 (forkhead box protein P1). The authors found that, unlike limb-innervating neurons, digit-innervating motor neurons express a distinct set of Hox genes and, unexpectedly, are insensitive to retinoic acid. This distinction might reflect a specialization that occurred during the evolutionary emergence of digits and their role in skilled motor behaviour.
References
Mendelsohn, A. I., Dasen, J. S. & Jessell, T. M. Divergent Hox coding and evasion of retinoid signaling specifies motor neurons innervating digit muscles. Neuron http://dx.doi.org/10.1016/j.neuron.2017.01.017 (2017)
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Lewis, S. Digit development. Nat Rev Neurosci 18, 194 (2017). https://doi.org/10.1038/nrn.2017.32
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DOI: https://doi.org/10.1038/nrn.2017.32