Developmental Biology Select

The Hox genes encode homeodomain transcription factors that are crucial for body patterning during development. Hox genes exist in clusters and their chromosomal order mimics their spatial and temporal order of expression during embryonic development. This issue's Developmental Biology Select examines recent studies that shed further light on the key roles of Hox genes and their relatives during development and evolution. The mouse trigeminal nerve collects sensory information from different parts of the face—such as the whiskers and lower jaw. These different branches of the trigeminal nerve provide inputs to nuclei in the brain stem from which the information is relayed to the thalamus and, ultimately, the somatosensory cortex. Oury et al. (2006) now report that the early segmentation of the mouse embryonic hindbrain correlates with the final topographical map of the neurons in the principal sensory (PrV) nucleus that receive input from the trigeminal nerve. During development, the vertebrate hindbrain is divided into segments called rhombomeres along the anterior/posterior axis. By genetically labeling specific rhombomeres to trace their fate, the authors found that certain regions in the PrV nucleus are derived from specific rhombomeres. For example, rhombomere 3 sets up the network of neurons in the PrV nucleus that receive sensory input from the whiskers. Given that Hox genes determine rhombomere identity, do these genes also contribute to the formation of this intricate pattern of neurons in the PrV nucleus? The authors homed in on Hoxa2, which is expressed in the second and third rhombomere and highly expressed in the region of PrV derived from rhombomere 3 during early development. Absence of Hoxa2 resulted in a defect in the ability of the trigeminal ganglion neurons to find their way to the PrV, and instead they made aberrant projections into the cerebellum. During later stages of development, Hoxa2 is required for arborization of ''whisker'' axons that project from the PrV nucleus to the thalamus. Lack of arborization of ''whisker'' axons in Hoxa2-deficient animals may be due to reduced expression in the PrV of Eph receptors, which are known to be important in axon guidance. Furthermore, during prenatal development, axons from the PrV nucleus in Hox2a-deficient mice project into the VPM nucleus but form aberrant patterns. The authors conclude that the correct ''whisker map'' is set up by Hoxa2 expression in rhombomere 3. These results suggest that the complex neuronal circuitry in the face is established early in development and can …