Correlative data are presented here on the developmental history, dynamics, histochemistry, and fine structure of intranuclear rodlets in chicken sympathetic neurons from in vivo material and long-term organized tissue cultures. The rodlets consist of bundles of ∼70 ± 10 A proteinaceous filaments closely associated with ∼0.4–0.8 µ spheroidal, granulofibrillar (gf) bodies of a related nature. These bodies are already present in the developing embryo a week or more in advance of the rodlets. In early formative stages rodlets consist of small clusters of aligned filaments contiguous with the gf-bodies. As neuronal differentiation progresses these filaments increase in number and become organized into well-ordered polyhedral arrays. Time-lapse cinemicrography reveals transient changes in rodlet contour associated with intrinsic factors, changes in form and position of the nucleolus with respect to the rodlet, and activity of the gf-bodies. With the electron microscope filaments may be seen extending between the nucleolus, gf-bodies, and rodlets; nucleoli display circumscribed regions with fine structural features and staining reactions reminiscent of those of gf-bodies, We suggest that the latter may be derivatives of the nucleolus and that the two may act together in the assemblage and functional dynamics of the rodlet. The egress of rodlet filaments into the cytoplasm raises the possibility that these might represent a source of the cell's filamentous constituents.

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