During the elaboration of the body plan cells are committed to specific fates, that is the capacity to differentiate into particular kind of cells. The cell fate commitments have to make sense in regard to the location of the cell, because all organs and tissues are made up of many cells and the entire structure of an organ or tissue requires a cooperative division of labor among the participating cells. Somehow, cell position must be identified and fate assignments must be parcelled out among a cooperating group of cells.
Positional information is generally established through protein signals that emanate from a localized source within a cell or within a developmental field. Cells need positional information to determine their location within a developmental field and to respond by executing the appropriate developmental program. When that positional information has been captured, generally a few intermediate cell types are created within a field. Through further processes of cell division and decision-making, a population of cells with the necessary final diversity of fates will be established. Once a group of cells receives the essential cues that specify the development of a particular structure, the cells must then undergo the necessary shape changes and movements to build the correctly shaped and positioned organ.
Drosophila has emerged as one of the best-known model organisms for studies of the mechanisms by which early patterning events are mediated during development. Although much is known about the processes underlying the establishment of the major body axes and the segmented body plan, the question of how this spatial information is translated into the specification of distinct cells and types of organs is only beginning to be answered. Even less is known about the earliest events of cellular specialization. Studies of organ development over the past several years suggest that Drosophila is also an ideal experimental organism for addressing the questions of how cell fate is determined and implemented. Beside the development of nervous system, the other organs also constitute interesting objects to examine like gut, trachea, salivary glands, and mesoderm derivatives.
With combined genetic, molecular and cellular approaches applied to salivary gland development, organ formation in an intact organism can be studied and a complete understanding of organ specification and morphogenesis can eventually be achieved.