Fetal brain grafts promote axon regeneration and survival of adult rat retinal ganglion cells

Transplants of immature astrocytes promote axonal regeneration in the adult rat brain

To study beneficial effects of immature astrocytes on axonal regeneration in the injured adult mammalian brain, we have stereotactically implanted cultured astrocytes from embryonic (E 14-16) rat cerebral cortex into the lesion site following transection of the postcommissural fornix. The spatio-temporal pattern of axonal degeneration and regrowth in the proximal fornix stump was investigated using wheat germ agglutinin-horseradish peroxidase tracing techniques and quantitative analysis of myelinated axon profiles. Transection of the postcommissural fornix tract caused disintegration of the axons in the distal stump as well as rapid and pronounced retrograde axonal degeneration up to 800-1,200 microns proximal to the lesion site. While a small bundle of subicular fibers spontaneously extended to the lesion site within 4 weeks after injury, axonal regeneration was markedly stimulated in those animals that had received an astroglial implant. Following the former pathway, regenerating axons sprouted towards the implant but did not penetrate the graft. Instead, the axons elongated over the surface of the transplant, avoiding growth into the surrounding neuropil or into the distal fornix segment. In grafted animals we further observed a substantial increase in the number of myelinated axons of approximately 31.5% (at the level of 800 microns) and approximately 40% (at the 400 microns level) compared with the injured tract lacking a transplant. Our results indicate the capacity of juvenile astrocytes to stimulate axonal regeneration after injury of the post-commissural fornix tract in the adult rat brain. We further demonstrate myelination of the regenerated axons.

Fetal neocortical transplants into the medial forebrain bundle attract ingrowth of catecholaminergic fibers in adult rat brain

The hypothesis that fetal tissue grafts may exert a trophic influence on damaged catecholaminergic fibers was examined. Ascending dopamine and norepinephrine axons normally innervate frontal cortex targets in the intact rat brain. These and other ascending catecholaminergic fibers were disrupted with stereotaxic injections of 6-hydroxydopamine into the medial forebrain bundle (mfb), followed after 1 or 14 days by grafts of fetal neocortical tissue placed into the injection site, or by sham grafts. Glyoxylic acid histofluorescence techniques were then used to examine catecholaminergic fiber distribution. When such lesions were made without subsequent grafting, virtually no growth of catecholaminergic fibers occurred beyond the injection site and frontal cortex norepinephrine levels were depleted to 15% of control levels. However, when grafts of fetal neocortical tissue were made into the lesion site and animals examined 3 months later, catecholaminergic fibers grew through the lesion site to ramify within the graft tissue. Catecholaminergic fibers were seen in all portions of most grafts, though they were most dense on the caudal and ventral edges of the graft, close to the path of the mfb. Similar densities of graft innervation were seen 3 months after animals received grafts placed into the same site without prior lesioning of catecholaminergic fibers. Fetal neocortical grafts thus induce collateral sprouting from intact host catecholaminergic axons and may also promote regenerative sprouting when such fibers are otherwise irreparably damaged.