Rhombomere

In the vertebrate embryo, a rhombomere is a transiently divided segment of the developing neural tube, within the hindbrain region (a neuromere) in the area that will eventually become the rhombencephalon. The rhombomeres appear as a series of slightly constricted swellings in the neural tube, caudal to the cephalic flexure. In human embryonic development, the rhombomeres are present by day 29. In the vertebrate embryo, a rhombomere is a transiently divided segment of the developing neural tube, within the hindbrain region (a neuromere) in the area that will eventually become the rhombencephalon. The rhombomeres appear as a series of slightly constricted swellings in the neural tube, caudal to the cephalic flexure. In human embryonic development, the rhombomeres are present by day 29. In the early developmental stages of the neural tube, segmentation of the neuroepithelium occurs. This segmentation turns into a series of neuromeres. Each segment is called a rhombomere. Every rhombomere develops its own set of ganglia and nerves. Later on in development, rhombomeres form the rhombocephalon, which forms the hindbrain in vertebrates. Each rhombomere expresses its own unique set of genes, which has been shown to influence postnatal rhythmic behaviors, such as respiration, mastication, and walking. In mice, it was shown that the patterning of the neural tube into rhombomeric segments may regulate spatial and temporal appearance of the central pattern generator. Rhombomeres are considered self-governing developmental units, with certain aspects of the rhombomere phenotype being determined at the time of formation. Each rhombomere expresses a unique combination of transcription factors, and so each rhombomeric domain has its own distinct molecular cues that could, theoretically, establish rhombomere-specific patterns of neuronal differentiation. Some of these neuronal populations have been identified in some species. Many of the mature hindbrain nuclei can occupy either one or several rhombomere-derived regions. Vestibular nuclei have been shown to span all the rhombomeres, some correlating with the boundaries of the rhombomeres. Using phosphorylated retrograde labeling, it has been shown that vestibulospinal groups correspond mostly to single rhombomeres rather than over several rhombomeric regions. It has also been shown that the vestibule-ocular groups can either relate to single or multiple rhombomeres, as long as the rhombomeres are closely related. The conclusion drawn was that segmentation of the hindbrain contributes to the way axons project within the vestibular complex. Finally, vestibulospinal neurons have been shown to differentiate in three neighboring rhombomeres, specifically r4, r5, and r6. While vestibule-ocular neurons differentiate across seven, the least differentiated in f4. The method of this differentiate is still unknown, with many types of proteins involved in both the migration, expression of proteins, and for neuron growth and apoptosis. The types of receptors, as well, can vary their activity to be cell-specific. Rhombomeres determine the pattern of the following maturation of the rhombencephalon into its final parts. The final parts are defined as the pons, cerebellum and medulla. Cells that form the boundaries of the rhombomere bulges proliferate much faster than those in the middle. It is very difficult for cells to cross from one rhombomere to another, so cells tend to stay within one rhombomere. Each rhombomere eventually gives rise to one or more vestibular neuron types. However, it is not necessarily dependent on segmentation. The motor nerves form depending on rhombomeric patterns, but each nerve can come from either one rhombomere or a pair of rhombomeres that are neighbors. Furthermore, the correct development of the various pharyngeal arches is believed to depend on interactions with specific rhombomeres. With these mechanisms, neural crest cells, for instance, from each rhombomere give rise to different ganglia, or clusters of neurons. Many of these rhombomeres have been mapped to an extent in species other than human. For example, r2 has been shown to give rise to the trigeminal ganglion, while r4 has been shown to give rise to the geniculate ganglion as well as spiral and Scarpa's ganglia. r5 and r6 gives rise to the abducens nerve, and the lower part of r6 and the upper part of r7 gives rise to the petrosal ganglion. Finally, the border of r7 that is not in contact with r6 gives rise to the jugular/nodose ganglia. These mappings, however, cannot be applied cross species.