Extracellular Morphogens in Drosophila Embryonic Dorsal-Ventral Patterning

Morphogens are molecules distributed in a concentration gradient that elicit different developmental fates at different concentrations. The graded distribution of a morphogen and a series of concentration-dependent responses of the cells exposed to a morphogen can combine to create a pattern of cell types in a defined spatial array. Several molecules active in early Drosophila development have been identified that fit this definition of a morphogen. The bicoid and hunchback proteins are two clear examples of morphogens that are distributed in gradients and define several different cell fates (Driever and Nüsslein-Volhard 1988; Struhl et al. 1992). The gradients of these morphogens depend on diffusion within the cytoplasm of the syncytial Drosophila embryo. Because other embryos do not have an early syncytial phase, these kinds of intracellular morphogens may not be of general use in development.