Fig. 3.

The hematopoietic system. Hedgehog (Hh) originates from the PSC and is the primary signaling ligand involved in the maintenance of the hematopoietic progenitors through activation of Cubitus interruptus (Ci) in the MZ. Adenosine deaminase-related growth factor (ADGF) expressed downstream of PDGF- and VEGF-receptor related (Pvr) in the CZ cells functions as a retrograde signal also activating Ci. Progenitors are maintained by these signals as well as by Wingless (Wg), which is expressed in the MZ. PSC, posterior signaling center; MZ, medullary zone; CZ, cortical zone; IP, intermediate progenitor; DV, dorsal vessel.