Abstract
Heterogeneous classes of neurons are present in the spinal cord and are essential for its function. Expression patterns of transcription factors in neural progenitor cells determine neuron subtypes during development. Nkx2.2 is expressed in the progenitor cell pool located just ventrally to the Olig2-positive pool and is indispensable for V3 interneuron generation in the spinal cord and also for visceral motoneuron generation in the hindbrain. However, whether Nkx2.2-positive progenitor cells generate diverse classes of neuron is not fully understood. Using a chick lineage tracing method in a genetically-defined manner, we found that Nkx2.2-expressing progenitor cells differentiate into general visceral motoneurons as well as sim1-positive V3 interneurons. Surprisingly, we further observed that Nkx2.2-expressing progenitors differentiate into somatic motoneuron. Our findings suggest that the different classes of motoneurons are derived from more complex sources than were previously expected in the chick spinal cord.
Highlights
The spinal motor circuitry that generates motor output consists of several types of motoneurons and interneurons
Motoneurons form the medial motor column (MMC) which regulates trunk muscles and the lateral motor column (LMC) which regulates limb muscles, whereas the sympathetic preganglionic motor column and MMC are formed in the thoracic spinal cord
Progenitor cells of somatic motoneurons are located in the ventral neural tube and express Olig2, a basic helix-loop-helix transcription factor, forming the pMN domain
Summary
The spinal motor circuitry that generates motor output consists of several types of motoneurons and interneurons. Progenitor cells of somatic motoneurons are located in the ventral neural tube and express Olig, a basic helix-loop-helix transcription factor, forming the pMN domain. Nkx2.2 is a homeodomain transcription factor and is expressed just ventrally to the pMN domain, demarcating the p3 domain, and is required for V3 interneuron development [5]. It was suggested that Nkx2.2-lineage cells contribute to visceral motoneurons in the spinal cord [10]. These reports raised the possibility that various types of neurons were generated from Nkx2.2 positive progenitors. Whether a population of motoneurons derives from Nkx2.2-expressing progenitors is not fully understood in the chick spinal cord
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