Development of the rat phrenic nucleus and its connections with brainstem respiratory nuclei

The development of phrenic motoneurons and descending bulbospinal projectio ns to the cervical spinal cord have been examined in prenatal and early pos tnatal rats with the aid of the carbocyanine dyes DiI and DiA. Phrenic moto neurons could be identified by retrograde labelling as early as E13, while aggregation of phrenic motoneurons into a column and the formation of dendr itic bundles became apparent from E16. The initial phrenic motoneuron dendr itic bundles were oriented in the dorsolateral and ventromedial directions, while ventrolaterally directed bundles entering the marginal zone appeared by E16, and rostrocaudal bundles were clearly visible by E21. The column o f phrenic motoneurons extended rostrocaudally from C2 to C6 at E13 and E14, but this became confined to the C3-5 segments by E21. Two-way tracing of c onnections between putative brainstem respiratory centres and cervical spin al cord with the carbocyanine dyes, DiI and DiA, indicated that brainstem b ulbospinal neurons in the position of the adult ventral respiratory group ( VRG) and medial parabrachial (MPB) nuclei appeared to project to the cervic al cord white matter as early as E15 and may contribute axons to the grey m atter of the cervical cord as early as E17 These findings are consistent wi th electrophysiological studies of respiratory function development in the fetal rat, which found relatively regular rhythmic phrenic discharge by E20 to 21. In summary, our findings indicate that the structural differentiati on of phrenic motoneurons is well-advanced prior to birth and that the desc ending pathways involved in the control of respiratory function are in plac e several days before birth.