Internal Composition of Striatal Grafts: Light and Electron Microscopy

Abstract

Grafts of fetal rat striatal primordium implanted into a host rat striatum, previously lesioned using the excitotoxins kainic, ibotenic or quisqualic acids, develop features reminiscent of adult neostriatum with respect to cell types, transmitters and receptors (Isacson et al., 1986; Clarke et al., 1988; DiFiglia et al., 1988; Graybiel et al., 1989; Helm et al., 1990; Liu et al., 1990; Zhou, 1990). Evidence is also accruing for the formation of afferent and efferent graft-host connections (Pritzel et al., 1986; Clarke et al., 1988; Wictorin et al., 1989, 1990a and b, 1991; Xu et al., 1990). A striking feature of the grafts is, however, the clearly heterogeneous distribution of many of the markers, with areas or “patches” containing high concentrations of eg. AChE or dopaminergic afferents separated by “non-patch” regions with low concentrations of these markers. Simple explanations for this heterogeneity such as the intermingling of neurons of both graft and host origin or the presence of large areas of non-neuronal tissue, have been ruled out using tritiated thymidine pre-labelling of either the donor tissue or recipient animals (Liu et al., 1990) and cross-species grafting followed by identification of the graft tissue with a species-specific neuronal marker (Wictorin et al., 1991). Furthermore, it seems unlikely that this “patch/non-patch” arrangement could be explained by the well-known organization of the mature striatum of striosomes and matrix (Graybiel, 1990), since in the normal rat striatum this patterning is not as easily identifiable using the marker AChE or staining for dopaminergic afferents as in the striatal grafts and, moreover, markers of both the adult striosome and matrix compartments have been located together within the grafts’ “patch” regions (Graybiel et al., 1989).