DEVELOPMENT OF THE HUMAN GIGANTOCELLULAR RETICULAR NUCLEUS - A MORPHOMETRIC STUDY

The cytoarchitectonic development of the human gigantocellular reticular nucleus (GRN) was studied quantitatively in 15 fetuses (16-39 weeks of gestation: WG) and in 2 adults (16 and 85 years old). With microscopic observation on serial celloidin sections of the brain, we measured them to obtain the following morphometric parameters: numerical density (ND), profile area (PA), and perimeter (PL) of the GRN neurons. GRN appeared as early as 16-18 WG, but most neurons were still immature and the cell nucleus was relatively large (nucleocytoplasmic ratio was high), although a few large neurons containing fine Nissl bodies were observed. Typical, coarse Nissl bodies of large multipolar neurons were first recognized at 21 WG. The earliest myelination was noticed at 22-23 WG, but myelinated nerve fibers were not prominent until the late fetal period. ND was largest at 16 WG and decreased rapidly with gestational age (coefficient of correlation, r=-0.85): it was reduced to about 10% of that at 16 WG by the end of the fetal period. ND was smaller in the adults than in the fetuses. Average PA increased monotonously with gestational age (r=0.79). The coefficient of variation (CV) of PA was high early in the fetal period (47-48%: 16-18 WG) and increased gradually with age (r=0.40). The average circularity ratio (CR=4 pi PA/PL(2)) decreased monotonously with gestational age (r=-0.85), while the CV of CR increased (r=0.76). In conclusion, this study suggests that immature GRN neurons may appear by 16 WG after migration and that the subsequent differentiation and maturation may progress gradually and monotonously during the later half of gestation.