Astrocytes, oligodendroglia, extracellular space volume and geometry in rat fetal brain grafts

Abstract

Fetal neocortex or tectum transplanted to the midbrain or cortex of newborn rats develops various degrees of gliosis, i.e. increased numbers of hypertrophied, glial fibrillary acidic protein-positive astrocytes. In addition, there were patches or bundles of myelinated fibres positive for the oligodendrocyte and central myelin marker Rip, and increased levels of extracellular matrix molecules. Three diffusion parameters—extracellular space volume fraction α (α=extracellular volume/total tissue volume), tortuosity λ (λ= D/ADC , where D is the free and ADC is the apparent tetramethylammonium diffusion coefficient) and non-specific uptake k′—were determined in vivo from extracellular concentration–time profiles of tetramethylammonium. Grafts were subsequently processed immunohistochemically to compare diffusion measurements with graft morphology. Comparisons were made between the diffusion parameters of host cortex and corpus callosum, fetal cortical or tectal tissue transplanted to host midbrain (“C- and T-grafts”) and fetal cortical tissue transplanted to host cortex (“cortex-to-cortex” or C-C-grafts). In host cortex, α ranged from 0.20±0.01 (layer V) to 0.21±0.01 (layers III, IV and VI) and λ from 1.59±0.03 (layer VI) to 1.64±0.02 (layer III) (mean±S.E.M., n=15). Much higher values were found in “young” C -grafts (81–150 days post-transplantation), where α=0.34±0.01 and λ=1.78±0.03 ( n=13), as well as in T -grafts, where α=0.29±0.02 and λ=1.85±0.04 ( n=7). Further analysis revealed that diffusion in grafts was anisotropic and more hindered than in host cortex. The heterogeneity of diffusion parameters correlated with the structural heterogeneity of the neuropil, with the highest values of α in gray matter and the highest values of λ in white matter bundles. Compared to “young” C-grafts, in “old” C -grafts (one year post-transplantation) both α and λ were significantly lower, and there was a clear decrease in glial fibrillary acidic protein immunoreactivity throughout the grafted tissue. In C-C-grafts, α and λ varied with the degree of graft incorporation into host tissue, but on average they were significantly lower (α=0.24±0.01 and λ=1.66±0.02, n=8) than in young C- and T-grafts. Well-incorporated grafts revealed less astrogliosis, and α and λ values were not significantly higher than those in normal host cortex. The observed changes in extracellular space diffusion parameters could affect the movement and accumulation of neuroactive substances and thus impact upon neuron–glia communication, synaptic and extrasynaptic transmission in the grafts. The potential relevance of these observations to human neuropathological conditions associated with acute or chronic astrogliosis is considered.