Fig. 8. Extrinsic and intrinsic signals on the process of generating an epithelial sheet de novo: LEC replacement by histoblasts. During larval stages, histoblasts are arrested in G2 and increase their size. LECs endoreduplicate, become polyploid and secrete the larval cuticle. At the onset of metamorphosis, the histoblasts undergo a series of G1-less synchronous cell divisions and reduce their size. Histoblast nests do not expand and remain confined to their original territories. LECs undergo apolysis, detach from the old larval cuticle and secrete the pupal cuticle. Images show the increment in number and the reduction in size of histoblasts from a ventral nest during pupariation. Histoblasts express GFP under the control of the Esg-Gal4 driver. The cell cytoplasm is marked in red with Propidium Iodide. In pupal stages, histoblasts undertake stochastic cell divisions and nests expand to replace LECs. These extrude from the epithelia, die and are cleared by the action of circulating haemocytes. In the images, histoblasts and LECs can be distinguished by their size (nuclear DAPI staining). Images show in false colour the spreading of nests in the period between 18 and 30 hours APF. Colour coding is as in Fig. 1 (i.e. green, anterior dorsal; red, posterior dorsal; yellow, spiracular; blue, ventral nest). The proliferation and expansion of histoblasts and the death of LECs are very precisely triggered by external (hormonal) inputs. An early Ecdysone peak of expression activates the synchronous divisions of histoblasts in prepupae. A late peak of Ecdysone correlates with histoblast loss of synchrony and it is essential for the initiation of cell replacement. Nonetheless, intrinsic interactive mechanisms involved in the coordination of histoblast proliferation and LEC death are also in place. LECs do not die in the absence of histoblast proliferation; conversely, histoblasts do not expand when LEC death is blocked. Mutual exchange of distinct signals thus appears to be necessary, beyond hormonal triggering events, to implement and harmonize the behaviour of histoblasts and LECs during abdominal epithelial morphogenesis