NUMBER OF NUCLEOLI AND COILED BODIES AND DISTRIBUTION OF FIBRILLAR CENTERS IN DIFFERENTIATING PURKINJE NEURONS OF CHICK AND RAT CEREBELLUM

We used differentiating chick and rat Purkinje cells to investigate in homologous neurons the influence of the number of nucleolar organizer regions (two in the chick and six in the rat) on the behaviour of the nucleolus and coiled bodies. We employed specific silver-staining met hods on smear preparations and on semithin and ultrathin sections. In chick Purkinje cells the number of nucleolar silver-staining granules increased from 15.7+/-3 (mean+/-SD) at embryonic day 13 to 23.8+/-3 at post-hatching day 7. These nucleolar granules were unevenly distribut ed between the two nucleoli of binucleolated cells. Electron-microscop ic cytochemistry showed that nucleolar granules are equivalent to the fibrillar centres with their associated shell of dense fibrillar compo nent. A reduction in the number of nucleoli was found during the diffe rentiation of both chick and rat Purkinje cells, although in mature ce lls the average number of nucleoli per cell was higher in the chick (1 .60) than in the rat (1.07). The number of coiled bodies decreased fro m 1.33 in newborn rats to 0.47 at postnatal day 90 in the rat. Coiled bodies were not observed in homologous chick Purkinje cells. The dynam ic behaviour of nucleoli and coiled bodies during neuronal differentia tion and the relationship of these two nuclear organelles with the num ber of nucleolar organizer regions is discussed.