Organ shape in the Drosophila salivary gland is controlled by regulated, sequential internalization of the primordia

During Drosophila development, the salivary primordia are internalized to f orm the salivary gland tubes. By analyzing immune-stained histological sect ions and scanning electron micrographs of multiple stages of salivary gland development, we show that internalization occurs in a defined series of st eps, involves coordinated cell shape changes and begins with the dorsal-pos terior cells of the primordia, The ordered pattern of internalization is cr itical for the final shape of the salivary gland. In embryos mutant for huc kebein (hkb), which encodes a transcription factor, or faint sausage (fas), which encodes a cell adhesion molecule, internalization begins in the cent er of the primordia, and completely aberrant tubes are formed. The sequenti al expression of hkb in selected cells of the primordia presages the sequen ce of cell movements. We propose that hkb dictates the initial site of inte rnalization, the order in which imagination progresses and, consequently, t he final shape of the organ. We propose that fns is required for hkb-depend ent signaling events that coordinate internalization.