Laminar boundaries persist in the hippocampal dentate molecular layer of the mutant Shaking Rat Kawasaki despite aberrant granule cell migration

The present report provides the first detailed description of the hippocamp us in the Shaking Rat Kawasaki (SRK) mutant by using a panel of antibody ma rkers to delineate its laminar organization. The mutant was characterised a t postnatal day 21 by severe malformations of both neuronal position and or ientation, the most striking of which was the presence of a rounded central granule cell mass in the dentate gyrus rather than the normal V-shaped gra nule cell layer. Despite this finding, the SRK dentate gyrus not only retai ned a cell-sparse molecular layer (thinner but similar in gross appearance to that of control littermates), but the sharp laminar boundary between its inner and outer parts was as clearly marked by IM1 and OM4 antibody staini ng as it was in the normal dentate gyrus. These immunocytochemical data sug gest that the entorhinal terminal field of the dentate gyrus may be relativ ely normal in the mutant, despite entorhinal afferents appearing to take an abnormal trajectory after they fail to cross the hippocampal fissure. Lami nar malformations included disruption of the SRK pyramidal cell layer, with spreading of the CA3 mossy fibre projection to an ectopic infrapyramidal p osition, radial displacement of CA1 pyramids, and transposition of a hither to unremarked longitudinal fibre bundle immunoreactive for calretinin from its normal position in the stratum lacunosum-moleculare of field CA2 to an alvear position in SRK. The SRK malformations were very like but not identi cal to those seen in the mouse reeler mutant, suggesting similar underlying developmental mechanisms. (C) 1999 Wiley-Liss, Inc.