Expression of cyclin A can induce S phase. Embryos were pulse-labeled with BrdU for 30 min to visualize DNA synthesis. Embryos were collected for 1 h, aged for 7.5 h and heat-shocked (HS) for the times indicated. After a recovery period of 20 min, embryos were labeled with BrdU for 30 min, then fixed. Incorporated BrdU was detected with indirect immunofluorescence and analyzed on a confocal microscope. Optical section are shown from (a,c,e) the epidermal cell layer and from (b,d,f) the internal cell layers of the developing organs. (a,b) Wild-type control embryos show normal BrdU incorporation in the central nervous system including the brain (Br), the anterior and posterior midgut (AMG and PMG, marked with arrows), the hindgut and malphigian tubules. In the central midgut (CMG, marked with a star) some cells finish S phase 17. No S phase can be seen in the epidermis. (c,d) Embryos carrying two copies of a HS-DmcycA transgene were heat-shocked for 15 min. Only a few cells in the epidermis were induced to go into S phase. In the internal tissues, the normal spatial and temporal program of S phases is disturbed and in all cells of the midgut and the salivary gland (marked with a triangle) S-phase induction can be seen. (e,f) A heat-shock of 30 min induces S-phase in cells of the epidermis. Cells of the ventral epidermis show a slightly stronger label than cells of the dorsal epidermis. In the internal tissues, a homogeneous S phase is induced. (g,h) Schematic drawings of the embryos in (e,f) Abbreviations: Br, brain; DE, dorsal epidermis; VE, ventral epidermis; SG: salivary gland; AMG, anterior midgut; CMG, central midgut; PMG, posterior midgut.