Abstract:
Lesions of the hippocampal formation or transec- tions of the fornix are followed by shrinkage of the medial mammillary nucleus (MMN). We determined whether the shrinkage of this nucleus was due to loss and/or shrinkage of neurons in addition to the loss of neuropil. We examined the MMN in a patient (KB) with an infarct that led to marked atrophy of the left hippocampus and subiculum, leaving the right MMN intact. Unbiased, stereological measure- ment techniques were used to compare the total cell number and individual neuronal cross-sectional ar- eas in both left and right MMN in this patient and in two control human brains. We also analyzed the MMN in four macaque monkeys that underwent ex- perimental unilateral transections of the fornix. The volume of the MMN on the lesioned side in KB was 55% of the unlesioned side (2.8 mm 3 vs 5.1 mm 3 ); the MMN in the monkey cases were reduced to 47–58% of the volume of the nonlesioned side. Neurons in the deafferented MMN of KB and of the monkey subjects were decreased in cross-sectional area (16 –20%, P < 0.0001). There was a trend toward decreased cell numbers (11–15%) on the lesioned side in all cases. We have estimated that the loss in cell number and shrinkage of remaining cells contribute negligibly to the 45% reduction in MMN volume. Therefore, the loss of neuropil (dendrites and afferent and efferent axons) appears to be the major contributor to the change in MMN volume.