The early development of subcortical projections to presumptive somatic sensory-motor areas of neocortex in the North American opossum

Somatosensory lateralization in the newborn brain

Since the onset and early postnatal development of hemispheric lateralization in the human brain are unknown, we studied cortical activation induced by passive extension and flexion of the hand in neonates using functional magnetic resonance imaging (fMRI). In contrast to that seen in older age groups, somatosensory areas in the pre- and postcentral gyri of the neonate showed no significant hemispheric lateralization at term. Instead, our findings from independent left- and right-hand experiments suggest the presence of an emerging trend of contralateral lateralization of the somatosensory system at around term.

Serotonergic neurons in the brainstem of the wallaby, Macropus eugenii

The organisation and cytoarchitecture of the serotonergic neurons in a diprotodont marsupial were examined by using serial sections of the brainstem processed for serotonin immunohistochemistry and routine histology. The topographic distribution of serotonergic neurons in the brainstem of the adult wallaby (Macropus eugenii) was similar to that of eutherian mammals. Serotonergic neurons were divided into rostral and caudal groups, separated by an oblique boundary through the pontomedullary junction. Approximately 52% of the serotonergic neurons in the wallaby brainstem were located in the rostral midline nuclei (caudal linear nucleus, dorsal, median, and pontine raphe nuclei and the interpeduncular nucleus), whereas 21% were found in the caudal midline region (nuclei raphe magnus, obscurus, and pallidus). The remaining serotonergic neurons (27%) were located in more lateral regions such as the pedunculopontine tegmental nuclei, the supralemniscal nuclei (B9 group), and the ventrolateral medulla. The largest serotonergic group, the dorsal raphe, contained one-third of the brainstem serotonergic neurons and showed five subdivisions, similar to that described in other species. In contrast, the median raphe did not show clear subdivisions. The internal complexity of the raphe nuclei and the degree of lateralisation of serotonergic neurons suggest that the wallaby serotonergic system is similar in organisation to that described for the cat and rabbit. This study supports the suggestion that the serotonergic system is evolutionally well conserved and provides baseline data for a quantitative study of serotonergic innervation of the developing cortex in the wallaby.

Brain growth and neocortical development in the opossum

General brain growth and differentiation of the neocortex have been studied in the marsupial, Didelphis virginiana from the 10.5 day embryo through adulthood. Didelphis is born after a short gestation period of about 12.5 days, at a time when the telencephalic wall consists only of two layers and is considered to be at an embryonic stage of development. The cortical plate does not appear until late in the first postnatal week, thus neocortical development is totally a postnatal phenomenon in Didelphis as has been shown in other marsupial species examined to date. The general pattern of development and the establishment of the six-layered adult neocortex in Didelphis is similar to that described in eutherian mammals. Signs of cortical lamination can be seen as early as postnatal day 35 and the cytoarchitecture of a typical mammalian neocortex is well defined by postnatal day 60 in Didelphis prior to the onset of weaning.

The origin of serotoninergic projections to the lumbosacral spinal cord at different stages of development in the North American opossum

We have employed immunohistochemistry and the retrograde transport of Fast blue to study the origin of serotoninergic projections to the lumbosacral spinal cord at different stages of development in the North American opossum. A few serotoninergic axons are present in the lumbosacral cord at birth, 12 days after conception, and serotoninergic neurons are numerous in the brainstem where they are present in most, if not all, of the areas which contain them in the adult animals. A few neurons of the caudal raphe and adjacent reticular formation were labeled by lumbar injections of Fast blue on postnatal day 1, and by postnatal day 3, labeled neurons were numerous within all areas which provide serotoninergic projections to the lumbosacral cord in adult animals. By postnatal day 11, it was possible to combine Fast blue labeling with immunofluorescence to show that some of the labeled neurons were serotoninergic. By postnatal day 24, neurons which provide serotoninergic projections to the lumbosacral cord were especially numerous and some of them were found in areas which do not provide comparable projections in adult animals. In developing and adult animals, few, if any, neurons were labeled in the dorsal raphe or superior central nuclei. We have shown previously that serotoninergic axons do not innervate laminae I and II of the lumbosacral cord until approximately postnatal day 50, although they are present in the marginal zone at birth and have grown into laminae III-X by postnatal day 15. Since serotoninergic axons which project to laminae I and II originate within the raphe magnus and adjacent reticular formation, and those areas provide serotoninergic projections to the spinal cord well before postnatal day 50, it is possible that serotoninergic innervation of laminae I and II is provided by late growth of collaterals from axons that have been present in the marginal zone for some time.