Abstract
When cell cycle withdrawal accompanies terminal differentiation, biosynthesis and cellular growth are likely to change also. In this study, nucleolus size was monitored during cell fate specification in the Drosophila eye imaginal disc using fibrillarin antibody labeling. Nucleolus size is an indicator of ribosome biogenesis and can correlate with cellular growth rate. Nucleolar size was reduced significantly during cell fate specification and differentiation, predominantly as eye disc cells entered a cell cycle arrest that preceded cell fate specification. This reduction in nucleolus size required Dpp and Hh signaling. A transient enlargement of the nucleolus accompanied cell division in the Second Mitotic Wave. Nucleoli continued to diminish in postmitotic cells following fate specification. These results suggest that cellular growth is regulated early in the transition from proliferating progenitor cells to terminal cell fate specification, contemporary with regulation of the cell cycle, and requiring the same extracellular signals.
Highlights
The differentiation of multipotent progenitor cells into terminal cell types must involve a number of coordinated changes
Zone 6 corresponded to the peak of the Second Mitotic Wave cell divisions that follows immediately posterior to the morphogenetic furrow
Since postmitotic cells do not need to grow in order to maintain cell size, it is to be expected that cellular growth rate might decline when terminal cell fates are specified and differentiation begins
Summary
The differentiation of multipotent progenitor cells into terminal cell types must involve a number of coordinated changes. The end of proliferation is likely to be coordinated with changes in cellular growth, since doubling of cell components is necessary for mitotic cell populations to maintain cell size, but is not necessary for non-dividing cells. The Drosophila eye imaginal disc, like many other developing tissues, grows rapidly and becomes largely post-mitotic on terminal differentiation [1]. The changes in the cell cycle that accompany eye differentiation, and their regulation, have been studied previously [1,2,3,4]. The changes and regulation of nucleolar size are addressed
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