Quantitative morphological changes in phrenic and intercostal motor columns and their respective spinal cord segments during postnatal development in the kitten.

In newborn kittens, the nervous control of breathing appears more mature than that of motricity which follows a cephalo-caudal evolution. In order to determine if the different postnatal evolutions of the respiratory and the motor function have an anatomical support at the spinal cord level, we made morphometric comparisons of the postnatal development of the spinal segments including motor columns sustaining both limb and respiratory movements (cervical and thoracic segments), with the postnatal development of segments containing only motoneurones involved in locomotion (lumbar segments). Furthermore, we used horseradish peroxidase to label cervical and thoracic groups of inspiratory motoneurones, i.e. the phrenic and the intercartilaginous motor nuclei at several postnatal ages. The present study suggests that the development of the white matter is the same at every spinal level and that it is delayed compared to the maturation of the grey matter. Overall evaluations of grey matter areas showed that the thoracic grey matter is more mature at birth but, further, has a slower rate of growth than the cervical and lumbar ones. This observation may be related to the maturity of the respiratory phasic activities within the early postnatal life. The phrenic and intercartilaginous motor nuclei have different patterns of development. These results suggest that the spinal postnatal functional maturation is not strictly related to its quantitative macroscopic changes.