Constructing an organ: the Drosophila salivary gland as a model for tube formation

Tube morphogenesis: closure, but many openings remain

“…As discussed in detail by Andrew and colleagues [5], analysis of tube formation during the development of the salivary gland of Drosophila has identified a complex of three transcription factors (Scr, exd, hth) that control tube formation. This complex appears to specify the fate of the salivary gland because a loss of function of any of these transcription factors blocks the formation of the salivary gland.…”

“…One process of forming a simple unbranched tube involves localized invagination or internalization of a small group of cells to form a hollow tube. As discussed by Andrew and colleagues [5], one example is the development of the salivary gland of Drosophila. The gland forms from ~100 cells located in epithelial primordia either side of the ventral midline.…”

“…Although the fate of the primordia of the salivary gland, from which the tube will emerge (see above), is determined by this complex of transcription factors, additional transcription factors (tsh and Abd-B) expressed in the ectoderm restrict the number of cells within the ectoderm that will form the primordia of salivary gland. Andrew and colleagues [5] pointed out that further restriction of the fate of primordia to either salivary-duct cells or secretory cells is determined by factors that work as both local activators and boundary repressors ( Figure 3). For example, epidermal growth factor (EGF) induces the formation of ducts but represses the formation of secretory cells, and fkh ensures the survival of secretory cells but also represses duct-specific gene expression.…”

“…For example, epidermal growth factor (EGF) induces the formation of ducts but represses the formation of secretory cells, and fkh ensures the survival of secretory cells but also represses duct-specific gene expression. Two additional transcription factors, hairy and hkb, regulate proper internalization of cells, and their downstream targets, klar and crb, appear to regulate vesicle trafficking and determine the identity of apical membranes, respectively [5].…”

“…Tube elongation and migration require changes in the dynamics of membrane and cytoskeleton (Figures 2 and 3). Morphological studies of tube outgrowth and extension reveal that tip cells extend filopodia and lamellipodia that form long narrow protrusions into the surrounding extracellular environment [4,5,11]. Although many pathways involving growth-factor receptors participate in the induction of tube elongation and migration (see above), in most cases little is known about their downstream effector pathways.…”