Mlf mlf

Figure 7 An example of hindbrain patterning in the transverse plane. Expression of the transcription factor Pax7 by progenitor cells within the proliferative zone (immediately subjacent to the fourth ventricle) defines two zones with a sharp boundary at a specific site along the floor plate-roof plate axis. The hindbrain has been opened dorsally and laid flat prior to sectioning, such that the floor plate-roof plate axis runs medial to lateral on each side. Two different hindbrain neuron groups are retrogradely labeled from their axons in the right-side medial longitudinal fascicle (mlf). The group on the left side derives from the Pax7-negative progenitors, whereas the group on the right side derives from the Pax7-expressing progenitors.

systematic patterning along the floor plate-roof plate axis. This is not as obvious from the morphological standpoint because there are few related structural landmarks aside from the sulcus limitans. The organization of the cranial nerve nuclei into longitudinal columns, however, gives an immediate reflection of this transverse element of patterning. Transcription factors also figure prominently in setting up longitudinal domains within the hindbrain. Expression of Pax and Nkx genes within the progenitor cell population provides a good example. These genes, like the Hox genes, encode transcription factors known to regulate the regional differentiation of cells in a variety of tissues and species. They are expressed in overlapping longitudinal bands along much of the neur-axis, including the hindbrain, with very sharp boundaries of expression at specific levels along the floor plate-roof plate axis (Fig. 7). They can exhibit dynamic changes prior to boundary formation, and they can also exhibit modulations in expression intensity within the expression domain that becomes established. Thus, these genes provide a sequentially ordered combinatorial scheme of gene expression along the floor plate-roof plate axis in much the same way that the Hox genes pattern the longitudinal axis.

How is the normal pattern of gene expression in the transverse plane generated? This has been studied primarily in the developing spinal cord, but the emerging picture appears to be generally applicable to the hindbrain as well. Diffusible signals are released at or near the ventral and dorsal poles of the neural tube. Ventral signals derive from the notochord, a mesodermal structure that lies immediately ventral to the neural tube, as well as from the floor plate. Dorsal signals derive from the roof plate and overlying surface ectoderm. These signals establish opposing gradients that dictate which genes are expressed at different positions along the floor plate-roof plate axis. The way these signals work and the way the genes whose expression they regulate interact are complex issues that remain the subject of active research. The end result, however, is the establishment of overlapping, longitudinal bands of differential gene expression that, like the zones of differential Hox gene expression, can be correlated with the differentiation of specific neuron groups (Fig. 7).



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