Distribution and morphology of corticospinal tract neurons in reeler mouse cortex by the retrograde HRP method

Reeler, a recessive mutation in mice, causes cytoarchitectonic abnormalities to the cerebral and cerebellar cortices. In normal controls, corticospinal (CS) tract neurons retrogradely labelled after HRP injection into the lumbar cord were situated only in layer V (the inner pyramidal layer). In the reeler, by contrast, the labelled CS neurons were scattered diffusely throughout all levels of the corresponding cortical area. In addition to the malpositioning of the somata, the labelled CS neurons in the cortex of the reeler could be divided into two major classes according to their dendritic pattern: typical pyramidal neurons and atypical ones. The typical pyramidal neuron had an apical dendrite projecting from the superior pole of the soma and ascending toward the pia mater and several basal and basolateral dendrites projecting from the inferior pole of the soma. The atypical neurons consisted of six types: (1) inverted, (2) tumbled, (3) bipolar, (4) V-shaped, (5) hook-shaped, and (6) superficial polymorphic. The typical pyramidal neurons in the reeler tended to be situated relatively deep in the cortex and the atypical neurons tended to lie relatively superficially in the cortex. The axons of both the typical pyramidal neurons and the atypical ones in the reeler usually extended from the lower surface of the soma or one of the descending dendrites as in the normal control.