Development of the human parvocellular red nucleus. A morphological study.

Morphology of the human parvocellular red nucleus (RNp) was investigated in 14 fetuses aged from 12 to 39 weeks of gestation (WG). The brains were processed into celloidin-embedded serial sections. At 12 WG, the anlage of RNp was observed as an ovoid mass of immature neurons clustering into some groups. Lobular appearance in cross-sectional images was conspicuous during the early stages (12-23 WG), particularly at rostral levels. The fasciculus retroflexus of Meynert was seen as a prominent bundle of fibers surrounded by the most rostral part of RNp. Two types of neurons were identified: large and small neurons. Large neurons were earlier observed at 16 WG, and had a polygonal or multipolar perikaryon with abundant Nissl bodies from 28 WG onwards. Small neurons later appeared among large neurons at 21 WG, and had a triangular or ovoid perikaryon with scanty Nissl bodies. The volume of RNp showed an exponential increase with age during 20-39 WG. The mean of neuronal perikaryonal areas showed a linear increase with age in both types during 16-39 WG, although the degree of change was much greater in large neurons than small neurons. The current study has clearly demonstrated the presence of two neuronal populations and their differential growth in developing human RNp. 2008 S. Karger AG, Basel.