The abnormal development of the gracile nucleus in the neurological mutant rat mf

A morphological and quantitative study was carried out of the prenatal and early postnatal development of the gracile nucleus in the mutant rat 'mutilated foot' (mf), which is affected by a sensory neuropathy inherited by autosomal recessive transmission. Microscopic examination showed that the nucleus of both normal and mf rats become morphologically identifiable at embryonic day 19 and that its appearance was comparable in the two groups of rats up to postnatal day 2. Subsequently the nucleus in the mutants appeared smaller than in control rats and the number of nerve cells in it decreased dramatically. Glomeruli, the type of synapses known to be formed between primary sensory ascending fibres and dendrites of gracile nerve cells, were observed in the mf rat, but were greatly reduced in number and in size compared with those in normal littermates. Reconstruction of nerve cell volumes showed that, in mf rats, volumes increased considerably less than in controls. These results suggest that the abnormalities observed in the gracile nucleus of mf rats are secondary to the decreased number of afferent fibres originating from the dorsal root ganglia and represent a form of 'anterograde transneuronal degeneration'. In the mutant this is particularly severe and occurs at a rapid pace since it takes place in immature organisms, known to be particularly vulnerable to this type of lesions.