Regenerative Effects and Development Patterns of Solid Neural Tissue Grafts Located in Gelatin Hydrogel Conduit for Treatment of Peripheral Nerve Injury

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Background:

The regeneration of the peripheral nerves after injuries is still a challenging fundamental and clinical problem. The cell therapy and nerve guide conduit construction are promising modern approaches. Nowadays, different sources of cells for transplantation are available. But it is little known about the interaction between fetal central nervous system cells and peripheral nerve tissue. In this study, we analyzed the development of the fetal neocortex and spinal cord solid grafts injected into the gelatin hydrogel conduits and their effects on sciatic nerve regeneration after cut injury.

Methods:

Frontal neocortex tissue was obtained from E19.5 and spinal cord tissue was obtained from E14.5 fetuses harvested from transgenic EGFP mice. The grafts were injected into the hydrogel conduits which were connected to the nerve stumps after cut injury. The recovery of motor function was estimated with walking track analysis at 2, 5, and 8 weeks after surgery. Then immunohistochemical study was performed.

Results:

The histological examination showed that only fetal neocortex solid graft cells had survived after implantation. Immunostaining revealed that some of the transplanted cells expressed neural markers such as neurofilament protein and NeuN. But the cells mostly differentiated in glial lineage, which was confirmed with immunostaining for GFAP and S100β. The walking-track analysis has shown that 8 weeks after surgery bioengineered conduit differed significantly from the control.

Conclusions:

We revealed that the hydrogel conduit is suitable for nerve re-growth and that the fetal neocortex grafted cells can survive and differentiate. Bioengineered conduit can stimulate functional recovery after the nerve injury.

Differentiation of Cholinergic Neurons in Rat Spinal Cord Under Conditions of Allotransplantation into a Peripheral Nerve and In Situ Development

The method of ectopic transplantation of embryonic anlages of CNS allows studying histoblastic potencies of progenitor cells developing under conditions of changed microenvironment. Some progenitor cells in the transplants of rat embryonic spinal cord retained their ability to express choline acetyltransferase after transplantation into the sciatic nerve of adult animals. Comparative analysis of cholinergic neurons in the neurotransplants and neurons formed in rat spinal cord during normal ontogeny showed that choline acetyltransferase-positive cells after transplantation into the nerve reached morphological differentiation of motor neurons at later terms than cells developing in situ. They were scattered one by one and did not form nuclear nerve centers. We did not fi nd structures similar to presynaptic cholinergic buds typical of intact spinal cord near these cells throughout the observation period. Solitary cholinergic neurons survived in the transplants for 19 months.

Effect of allotransplants containing dissociated cells of rat embryonic spinal cord on nerve fiber regeneration in a recipient

Regeneration of nerve fibers in rat sciatic nerve was quantitatively assessed after injury (ligation) and injection of dissociated cells derived from embryonic spinal cord. A suspension of dissociated spinal cord cells from rat embryos was transplanted under the perineurium of a nerve trunk. After transplantation, bromodeoxyuridine-labeled precursor cells survived and retained the label for more than 2 months; some of these cells differentiated into NeuNpositive neurons. Analysis of semithin sections of the distal nerve segment from the recipient taken at a distance of 0.5 cm from the site of injury showed that transplantation of dissociated cells of embryonic spinal cord led to an increase in the number of myelinated nerve fibers in the recipient nerve.

Development of rat embryonic spinal ganglion cells in damaged nerve

The development of dissociated cells from rat embryonic spinal ganglion after transplantation to damaged nerve of adult animals was studied using immunohistochemical differentiation markers of neural and glial cells. The cell suspension obtained after dissociation of rat embryonic spinal ganglia (embryonic day 15) was injected into the proximal segment of crushed sciatic nerve. The nerve was damaged by ligation for 40 sec. Progenitor cells were labeled with 5-bromo-2'-deoxyuridine (BrdU) before transplantation. BrdU-immunopositive cells were detected in the nerve trunks of recipients on days 1, 21, and 28 after transplantation. Dissociated cells of rat embryonic spinal ganglion (embryonic day 15) survived for at least 4 weeks after transplantation to the nerve and differentiate into NeuN-immunopositive neurons with morphological properties of sensory neurons and satellite cells containing S100 protein.

Differentiation of dissociated rat embryonic brain after allotransplantation into damaged nerve

Fragments of the dorsolateral wall of the anterior brain vesicle from rat embryos on embryonic day 15 were dissociated and the resultant suspension containing single cells and cell aggregates was injected into the proximal segment of crushed sciatic nerve of adult animals for evaluation of their engrafting and differentiation under conditions of changed microenvironment. On days 1 and 21 postoperation, Msi-1, GFAP, NeuN, vimentin, and PCNA were detected by immunohistochemical methods. Small clusters of Msi-1-immunopositive cells were detected in the nerve trunks on the next day after transplantation. On day 21 after surgery, these precursors differentiate into nerve cells, astrocytes, and primarily ependymocytes.

Astrogliogenesis in heterotopic allotransplants of rat embryonic neocortex

We studied gliogenesis in transplants of rat embryonic neocortex (E14-15) in 3, 7, 15, and 30 days and 12-13 months after transplantation into the sciatic nerve of adult animals. Immunogistochemical reactions to intermediate filament proteins nestin, vimentin, glial fibrillary acidic protein were used. In transplants, vimentin- and nestin-positive precursor cells differentiate into astrocytes earlier that in the developing rat neocortex (in situ). One year after transplantation, some astrocytes start to express nestin and vimentin, which attests to the development of reactive gliosis in the transplants.

The role of embryonic motoneuron transplants to restore the lost motor function of the injured spinal cord

Spinal cord injury or disease result in the loss of critical numbers of spinal motoneurons and consequentially, in severe functional impairment. The most successful way to replace missing motoneurons is the use of embryonic postmitotic motoneuron grafts. This method may also at least partially restore integrity of the injured spinal cord. It has been shown that grafted motoneurons survive, differentiate and integrate into the host cord and many of them are able to reinnervate the denervated muscles. If grafted motoneurons are provided with a conduit (e.g. reimplanted ventral root) the grafted cells are able to extend their axons along the entire length of the peripheral nerves and reach the hind or forelimb muscles and to restore limb locomotion patterns. Grafted motoneurons show excellent survival in motoneuron-depleted adult host cords, but the developing spinal cord appears to provide an unfavourable environment for these motoneurons as they do not survive in immature cords. The long term survival and maturation of the grafted neurons depend on the availability of a nerve conduit and one or more target muscles, independently of whether these are ectopic nerve-muscle implants or limb muscles in their original site. Thus, grafted and host motoneurons induce functional recovery in the denervated limb muscles when their axons can grow into an avulsed and reimplanted ventral root and then reach the limb muscles. Following segmental loss of motoneurons induced by partial spinal cord injury, motoneuron-enriched embryonic grafts can be placed into the gap-like hemisection cavity in the cervical spinal cord. Such transplants induce the regeneration of great numbers of host motoneurons possibly by the bridging effect of the grafts. In this case, the regenerating host motoneurons reinnervate their original target muscles while the small graft plays a minimal role in the reinnervation of muscles. These results suggest that reconstruction of the injured spinal cord using an embryonic motoneuron-enriched spinal cord graft is a feasible way to achieve improvement after severe functional motor deficits of the spinal cord.