Nuclear derivatives and axonal projections originating from rhombomere 4 in the mouse hindbrain.

The r4-derived territory is located in the pontine region of the brainstem, forming a wedge-shaped slice that broadens from the choroidal roof to the ventral midline. R4-derived neuronal populations migrate radially inside and tangentially outside this rhombomere, forming nuclei of the sensorimotor auditory, vestibular, trigeminal and reticular systems. R4-derived fibre tracts contribute to the lateral lemniscus, the trigeminothalamic tracts, the medial tegmental tract and the medial forebrain bundle, which variously project to the midbrain, thalamus, hypothalamus and telencephalon. Other tracts such as the trigeminocerebellar and vestibulocerebellar tracts reach the cerebellum, while the medial and lateral vestibulospinal tracts, and the reticulospinal and trigeminal oro-spinal tracts extend into the spinal cord. Many r4-derived fibres are crossed; they decussate to the contralateral side traversing the midline through the cerebellar, collicular and intercollicular commissures, as well as the supraoptic decussation. Moreover, some fibres enter into the posterior and anterior commissures and some terminals reach the septum. Overall, this study provides an overview of all r4 neuronal populations and axonal tracts from their embryonic origin to the adult final location and target.