An HRP study of the cat's spinal respiratory motoneurones during postnatal development.

The localization and morphology of spinal respiratory motoneurones (phrenic and intercostal) were studied in the cat by retrograde labelling using Horseradish Peroxidase (HRP), at different stages of postnatal development. At birth, the distribution of the phrenic and intercostal motoneurones in the cervical and thoracic ventral horn, respectively, is similar to that observed in adult animals. At birth, the phrenic and intercartilaginous motoneurone somata have respectively 60% and 40% of their adult volume, appearing much more developed than the motoneurones involved in the motor control of limbs. During postnatal development, the phrenic and intercartilaginous motoneurones undergo a characteristic sagittal elongation without evident modification along their transverse axes. From birth, the ratio of the somatic volume to that of its corresponding motor column markedly decreases inside of the phrenic column compared to the data obtained in the limb's muscle motor columns by other authors. Similar determinations in intercostal motor columns give intermediate values between those obtained from the phrenic column and from the motor system. These results indicate that the motoneurones innervating the respiratory muscles have some specific features of development.