A study of retrograde cell degeneration in the lateral mammillary nucleus of the cat, with special reference to the role of axonal branching in the preservation of the cell

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Abstract
The significance of axonal branches in maintaining the integrity of the neuronal soma after axon section has been studied in the lateral mammillary nucleus (LMN) of the cat. Earlier studies with the Golgi and reduced silver methods have shown that the axons of most, if not all, of the cells of this nucleus which enter the principal mammillary tract (PMT) bifurcate with one branch entering the mammillo‐thalamic tract (MThT) and the other the mammillo‐tegmental tract (MTgT). Some of the fibers in the MThT may again bifurcate before ending bilaterally in the anterodorsal nucleus of the thalamus (AD).Unilateral destruction of the AD results in no detectable retrograde cell degeneration in the LMN. Lesions of the MThT (below the interanteromedial nucleus) cause a slight (∼ 15%) cell loss in the LMN, while isolated lesions of the MTgT have no detectable effect upon the cells of the nucleus. Combined lesions of the MThT and the MTgT cause severe retrograde degeneration in the LMN: approximately 60% of the cells disappear and many of the surviving neurons are frankly atrophic. Lesions of the PMT involving the parent axons of the cells in the LMN result in about the same degree of cell loss as combined lesions of the MThT and MTgT, but the persisting neurons are more severely shrunken and pyknotic.The implications of these findings for the organization of the connections of the LMN and for the interpretation of retrograde cell degeneration are discussed.