Fig. 1. Dorsal views of the amnioserosa showing morphogenesis of this tissue during dorsal closure. Anterior is to the left in this and subsequent figures. Dashed yellow lines demarcate clusters of apically constricted cells. (A,C,E,G) Embryos at progressively later stages of dorsal closure stained with anti-phosphotyrosine antibodies. (B,D) Embyros stained with anti-nonmuscle myosin heavy chain antibodies (Kiehart and Feghali, 1986). (F,H) Embryos stained with anti-ß-galactosidase antibodies. (A) A stage 13 embryo prior to commencement of dorsal closure showing amnioserosa as a flat sheet with cells elongated perpendicular to the A-P axis of the embryo. (B) Embryo at similar developmental stage to that in A, showing no enrichment of myosin in the amnioserosa. (C,D) Early stage 14 embryo double-stained to show elevated levels of phosphotyrosine (C) and myosin (D) in cells at the anterior and posterior ends of the amnioserosa. These cells are apically constricted relative to cells in the middle of the amnioserosa (the apical ends of the amnioserosa cells are at the surface of the embryo). The hindgut can be seen as a stained structure under the posterior end of the amnioserosa. (E) Late stage 14 embryo midway through dorsal closure showing apically constricted cells at the ends of the amnioserosa and loss of original elongation of middle amnioserosa cells. (F) UAS-lacZ^1-71;GAL4^332.3 embryo early in dorsal closure with anti-ß-galactosidase staining revealing amnioserosa as an elliptical sheet of cells. Note the lack of expression of the lacZ reporter gene in the epidermis. That no epidermal cells were expressing the lacZ reporter gene was confirmed by double staining UAS-lacZ^1-71;GAL4^332.3 embryos with anti-phosphotyrosine antibodies to show cell boundaries (data not shown). (G) A stage 15 embryo late in dorsal closure showing that the middle amnioserosa cells are both elongated along the A-P axis and apically constricted. (H) A UAS-lacZ^1-71;GAL4^332.3 embryo after the completion of dorsal closure with ß-galactosidase staining revealing amnioserosa as a tubular structure. Note lacZ expression appearing in epidermal cells.