Fig. 1. Restricted expression of MNR2 and HB9 by postmitotic spinal motor neurons. Transverse sections through stage 29 chick spinal cord reveal the columnar restriction of MNR2 (A,C,and E) and HB9 (B,D,F) expression within motor neurons. (A) At brachial levels of the spinal cord, MNR2 is expressed in the medial MMC and in a lateral population of motor neurons, located within the confines of the LMC. (B) At brachial levels, HB9 is expressed by medial MMC and a subset of lateral LMC neurons. Isl1/2 is expressed in all medial MMC and LMC neurons. A dorsal population of interneurons also expresses Isl1. (C) At thoracic levels, MNR2 is expressed in all medial (m) MMC neurons but by only a few lateral (l) MMC neurons and not by CT neurons. (D) At thoracic levels, HB9 is expressed in all MMC neurons but not in CT neurons. (E) At lumbar levels, MNR2 is expressed in medial MMC neurons but not in LMC neurons. (F) At lumbar levels, HB9 is expressed in medial MMC and lateral LMC neurons. INT, interneuron.

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