Development of raphe-spinal connections in the North American opossum.

The Falck-Hillarp technique, serotonin (5-HT) immunohistochemistry and the retrograde transport of horseradish peroxidase (HRP) were utilized to investigate the development of raphe-spinal connections in the pouch-young opossum. The brainstem raphe and adjacent reticular formation contain 5-HT immunoreactive neurons in the newborn opossum (12 days after conception) and processes from these cells can be visualized in the marginal zone of the spinal cord. Between eight and 15 days after birth 5-HT immunoreactive varicosities begin to grow into the presumptive deep layers of the dorsal horn, the intermediolateral cell column and the ventral horn. In the latter region some of them approximate presumed motor neurons. Between 40-50 days after birth 5-HT immunoreactive varicosities appear in presumptive laminae I and II of the dorsal horn.