Neonatal host astrocyte migration into xenogeneic cerebral cortical grafts

The Therapeutic Effectiveness of Delayed Fetal Spinal Cord Tissue Transplantation on Respiratory Function Following Mid-Cervical Spinal Cord Injury

Respiratory impairment due to damage of the spinal respiratory motoneurons and interruption of the descending drives from brainstem premotor neurons to spinal respiratory motoneurons is the leading cause of morbidity and mortality following cervical spinal cord injury. The present study was designed to evaluate the therapeutic effectiveness of delayed transplantation of fetal spinal cord (FSC) tissue on respiratory function in rats with mid-cervical spinal cord injury. Embryonic day-14 rat FSC tissue was transplanted into a C4 spinal cord hemilesion cavity in adult male rats at 1 week postinjury. The histological results showed that FSC-derived grafts can survive, fill the lesion cavity, and differentiate into neurons and astrocytes at 8 weeks post-transplantation. Some FSC-derived graft neurons exhibited specific neurochemical markers of neurotransmitter (e.g., serotonin, noradrenalin, or acetylcholine). Moreover, a robust expression of glutamatergic and γ-aminobutyric acid-ergic fibers was observed within FSC-derived grafts. Retrograde tracing results indicated that there was a connection between FSC-derived grafts and host phrenic nucleus. Neurophysiological recording of the phrenic nerve demonstrated that phrenic burst amplitude ipsilateral to the lesion was significantly greater in injured animals that received FSC transplantation than in those that received buffer transplantation under high respiratory drives. These results suggest that delayed FSC transplantation may have the potential to repair the injured spinal cord and promote respiratory functional recovery after mid-cervical spinal cord injury.

Putative Inhibitory Extracellular Matrix Molecules Do Not Prevent Dorsal Root Regeneration into Fetal Spinal Cord Transplants

We examined the distribution of several extracellular matrix molecules (ECM) and their relationship to regenerating axons in embryonic day 14 spinal cord transplants 1 to 12 weeks after transplantation into adult rats. We used immunocytochemical tech niques to label chondroitin sulfate proteoglycans (CSPGs) and tenascin-C in adjacent sections. Synthesis of these molecules by astrocytes is thought to be one mechanism by which astrocytes inhibit regeneration in the central nervous system (CNS); glial fibrillary acidic protein antibody was used to label astrocytes and examine their rela tionship to both the ECM molecules and regenerating calcitonin gene-related pep tide (CORP)-contammg dorsal roots. We also compared the expression and distribu tion of these five markers in transplants with normal spinal cord development.

Control of astrocyte migration in the developing cerebral cortex

Development of the vertebrate central nervous system is characterized by significant long distance cell migration. While the radial migration of neuronal precursors to their final location is well established the migration of glial cells and their precursors is less well understood. To define the pathways of migration and dissect the cell and molecular mechanisms mediating such migration requires the development of appropriate models. Here we show that purified neonatal astrocytes injected into organotypic slice cultures of developing cerebral cortex migrate in defined patterns depending on where they are placed within the tissue. Injection into gray matter resulted in radially oriented migration either towards the pial or ventricular surface. By contrast injection into developing white matter resulted in largely longitudinal migration along developing axon tracts. While the cytoarchitecture of the tissue influenced the pattern of migration, the extent of migration appeared to be regulated primarily by the age of the host tissue. Homochronic injections performed prior to postnatal day 4 resulted in extensive migration while after day 7 migration was relatively limited. Heterochronic injections indicated that while astrocytes within the 1st postnatal week retained the capacity to migrate extensively, older tissue failed to support extensive migration of either young or old astrocytes. These data suggest the existence of distinct migrational cues in the CNS and that environmental, not cell intrinsic properties primarily regulate astrocyte migration through the developing cortex.

Differences in host serotonin innervation of intrastriatal grafts are not determined by a glial scar or chondroitin sulfate proteoglycans

Serotoninergic (5-HT) neurons of adult recipients provide a much denser innervation of striatal than ventral mesencephalic grafts implanted into the neostriatum of the rat. Moreover, grafts from both brain regions are more innervated by host 5-HT axons after implantation in neonatal than adult hosts. To test the hypothesis that differences in glial scarring or expression of the growth inhibitory molecules, chondroitin sulfate proteoglycans (CSPG), be responsible for these differences in 5-HT innervation of neural grafts, we examined the 5-HT innervation, the astroglial reaction and the expression of CSPG in ventral mesencephalic grafts implanted into newborn (1-5 days old), juvenile (15 days old), or adult rats and in striatal grafts implanted in adult rats, using immunohistochemistry against 5-HT, glial fibrillary acidic protein (GFAP) and CSPG. Immunostaining for GFAP showed a stronger initial gliosis (1-10 days after grafting) in neonatal than adult recipients of mesencephalic grafts, but this gliosis subsided gradually at later time points. Nevertheless, a glial scar formed at the graft-host interface in both neonatal and adult recipients, 5-10 days after transplantation, although it decreased over a longer time course--up to 60 days--in adults. Immunostained astrocytes appeared first in the host brain tissue around the graft and then immunoreactive processes and perikarya gradually invaded the graft. Immunoreactivity for CSPG was similar in neonatal and adult hosts: it was strongly expressed inside the graft early after transplantation, and almost completely down-regulated at 60 days. The reaction of adult hosts to striatal and mesencephalic grafts was similar, although GFAP was more heterogeneously distributed and CSPG immunoreactivity remained in patches inside striatal grafts, even after 60 days. The 5-HT innervation of mesencephalic grafts was much denser after implantation in newborns than in adults. It was also stronger in striatal than in mesencephalic grafts implanted in adults. Thus, the presence of a glial scar or the expression of CSPG cannot totally account for the different degrees of 5-HT innervation in the various types of neural grafts.

The migration of astrocytes after implantation to immature brains

Using a species-specific marker, we have found that astrocytes, taken from donors of varying ages from fetal to adult, migrate in highly stereotypic patterns in immature host brains. Migration is primarily within and towards cell layers, although some cells are seen to migrate along fibre bundles. This contrasts with studies using the same approach in mature hosts, where migration is predominantly within fibre layers, largely excluding cellular regions.