Factors influencing the spinal motoneurons in development

Abstract

The development of the spinal cord needs a concerted interaction of transcription factors activating diverse genes and signals from outside acting on the specification of the different cells. Signals have to act on the segments of the embryo as well as on the cranial-caudal axis and the dorso-ventral axis. Additionally the axons of the motoneurons have to cross the central nervous system barrier to connect to the periphery. Intensive anatomical studies have been followed by molecular characterization of the different subsets of transcription factors that are expressed by cells of the developing spinal cord. Here, intensive studies for the most important appearing cells, the motoneurons, have resulted in a good knowledge on the expression patterns of these proteins. Nonetheless motoneurons are by far not the only important cells and the concert activity of all cells besides them is necessary for the correct function and integrity of motoneurons within the spinal cord. This article will briefly summarize the different aspects on spinal cord development and focuses on the differentiation as well as the functionalization of motoneurons.

Keywords: axon; extracellular matrix; neurite; synapse muscle; transcription factors.

Figure 1

Segmentation and motoneuron connection during spinal cord development in mice. Motoneurons are positioned in motoneuron pools within the segments of the spinal cord from rostral to caudal. Eight cervical segments (C₁ to C₈) followed by 12 thoracic (T₁ to T₁₂ segments and 7 lumbal segments (L₁ to L₇). The expression of the homeobox protein Hoxc8 marks the area of motoneurons necessary for forelimb prehension efficiency (Tiret et al., 1998). The more rostrally positioned motor columns innervate the forelimbs while the motoneurons of the thoracic segments innervate the sympathetic ganglia, the dermomyotome and the peripheral trunk muscles. The different motor columns along the rostro-caudal axis are characterized by expression of different homeobox transcription factors: Isl-1 and Isl- 2, (Islet-1 and -2), Lim-1, Lim-3 (Lim Homeobox transcription factor-1 and -3), Lhx4 (Lim homeobox transcription factor 4).