Development of the human magnocellular red nucleus: a morphological study

The development of the human magnocellular red nucleus (RNm) was studied in 20 fetuses at 12-39 weeks of gestation (WG). With microscopic observation on serial sections of the brain, we measured the profile area of a neuronal cell body. At 12WG, several islands of immature cells of the RNm appeared dorsal to the parvocellular red nucleus (RNp). At 16WG, the RNm was detected ventral to the RNp as a cluster of semilunar shape, consisting of basophilic neurons of various sizes. During 18-23WG, the neurons were dispersed dorsal to the RNp. They were isolated or aggregated as small clusters among the myelinated oculomotor nerve roots. Twenty-eight WG onwards, the neurons were widely distributed ventrolateral to the superior cerebellar peduncle and around the caudal pole of the RNp. Measurement of the profile area revealed that the average size of overall neurons increased almost linearly with the gestational age, and that two populations (large and small neurons) were clearly distinguished on the histogram from 33WG onwards. The relative position of the RNm to the RNp may vary among the individuals, especially in earlier fetal stage. This study suggests that the differentiation and maturation of neuronal cytoarchitecture of the RNm may gradually and monotonously progress during the later half of gestation.