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.
Each rhombomere expresses its own unique set of genes, which has been shown to influence postnatal rhythmic behaviors, such as respiration, mastication, and walking.
The conclusion drawn was that segmentation of the hindbrain contributes to the way axons project within the vestibular complex.
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.
For example, rhombomeres are detectable by light microscopy in Swiss/Webster strain of mice for up to embryonic day (E)10.5, but they disappear at E11.5.
Gli2 and Gli3 had overlapping functions that concerned the patterning of the ventral spinal cord which was important for correct organization and formation of the motoneurons.
Mutations in the Gli2 protein causes more severe damage in the ventral patterning of the hindbrain than that of the spinal cord.
Gli2 and Gli3 in a developing hindbrain also have distinct functions in the Shh (sonic hedgehog) signal transduction.
The Hox gene also has been shown to play a part in the formation of the cranial motor nerves.
Another possibility for this lack of correlation is that most studies were based on in situ hybridization, which only maps the location of transcripts rather than proteins.
The Hox gene was expressed rostrocaudally in the same sequence that was physically within the chromosome and its transcription was regulated by retinoic acid.
It was found in one study that reticular neurons in the lamprey hindbrain, which included isthmic, bulbar, and Mauthner cells, developed in conserved rhombomere-specific positions, similar to those in the zebrafish.
The trigeminal and facial motor nuclei were also shown to not correlate well with the rhombomere boundaries in the lamprey.
Several studies have shown that the fibroblast growth factor (FGF) is secreted from the midbrain-rhombomere 1 boundary.
Studies have shown that the FGF receptors, or FGFRs act partially redundantly to support cell survival in the dorsal ectoderm, promote r1 tissue identity, and regulate the production of ventral neuronal populations, including the midbrain dopaminergic neurons.
Rhombomeres also lead to the creation of vestibular projection neurons, which are probably produced relatively early in the developmental stages of the brain stem.
In the rhombomeres, members of the T-box transcription factor family, have been linked to the proper development of migrating cells.
However, they have a unique migration pattern, where their cell bodies cross the midline to the contralateral side.
From this one can conclude that multiple subclasses of motor neurons are produced in specific locations in the developing hindbrain.