Involvement of transcription factors in early motor neuron cell death in cervical segments of avian embryos

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We previously reported that cell death of cervical motor neurons in early avian embryo occurs only in a subpopulation of motoneurons(MNs) that do not express Lim3, a member of LIM-HD transcription factors. In the present study, to elucidate mechanisms that determine cells to die, we clarified development of subtypes of MNs in the cervical and thoracic spinal cord. We examined correlations between birthdates of MNs and their final locations, detailed migration patterns, and change of expression patterns of LIM-HD transcription factors (Isl1, Isl2 and Lim3) and MNR2 protein in the chick embryo.Detailed analysis of expression patterns of Isl1, Isl2 and Lim3 and MNR2 revealed that these four marker proteins were once expressed by all motoneurons in both cervical and thoracic segments. However, Lim3 expression was lost in subpopulations of MNs by E4. In thoracic segments this Lim3-negative population became MNs in the lateral division of the medial motor column that innervate intercostal muscles, whereas in the cervical spinal cord this population disappeared by E5 because of cell death. Both populations became postmitotic in similar developmental stages and are located in similar regions of the motoneurons column. This consistency suggests that two populations are analogous neuronal groups. Difference of rostrocaudal differentiation of the spinal cord, which is determined by expression of Hox transcription factors, may determine different fates of the two populations.Although we pursued this possibility, we could not obtain decisive results within the period of this research project.