Development of motoneurons and primary sensory afferents in the thoracic and lumbar spinal cord of the south American opossum Monodelphis domestica

The postnatal development of the primary sensory afferent projection to the thoracic (T4) and lumbar (L4) spinal cord of the marsupial species Monodel phis domestica was studied by using anterograde and retrograde neuronal tra cers. Large numbers of primary afferents and motoneurons were labelled by a pplication of the carbocyanine dye DiI into individual dorsal root ganglia (DRG) afferents in short-term organ cultures. Dorsal root axons had entered the cord at birth, but most primary afferent innervation of the grey matte r and the establishment of cytoarchitectural lamination occurs postnatally. In addition to ipsilateral projections, some primary afferents that projec ted to the dorsal horn extended across the midline into the equivalent cont ralateral regions of the grey matter. Similarly, motoneuron dendrites occas ionally extended across midline and into the contralateral grey matter. The first fibres innervating the spinal cord project to the ventral horn and f ormed increasingly complex terminal arbours in the motor columns between P1 and P7. After P5 many afferents were seen projecting to the dorsal horn, w ith the superficial dorsal horn being the last region of the spinal grey to be innervated. Histochemical labelling with the lectin Griffonia simplicif olia indicated that C fibre primary afferents had arborised in the superfic ial dorsal horn by P14. The sequence of primary afferent innervation is thu s similar to that described in the rat, but this sequence occurs over a per iod of several weeks in Monodelphis, compared with several days in the rat. (C) 1999 Wiley-Liss, Inc.