Retrohippocampal, hippocampal and related structures of the forebrain in the reeler mutant mouse

Retrohippocampal, hippocampal and some related forebrain regions of the reeler mutant mouse were examined in cresyl violet and myelin stained preparations. All cortical structures are abnormal. The entorhinal cortex, parasubiculum, presubiculum, subiculum, and anterior hippocampal cortex share an anomaly of common quality. All lack an external plexiform layer and their neurons are not arranged in the usual laminae. The positions of neurons are not random, however. Cells of smaller average size and of polymorphic shape are grouped in an outer relatively narrow zone external to a broad zone of larger cells. The histological features of these two cell zones in the mutant are similar to the deep polymorphic cell stratum and the superjacent pyramidal field respectively in the normal animal, but their relative depths in the cortex are reversed. The histological pattern of the mutant hippocampus is less disturbed. Its external plexiform layer is well developed and, although the positions of some neurons are abnormal, there is an explicit laminar pattern and the cytological features of the individual hippocampal cytoarchitectonic fields are readily recognized. It appears from these observations that in a variety of cortical structures, even those which are most disturbed, the positions of neurons are systematically regulated, yet are controlled according to some rule that differs in normal and mutant. The basic developmental event which dictates eventual neuron position probably occurs early and its effects may depend on critical timing or precise location of interacting cells as influenced by the reeler locus.