Figure 2.

Two niches control lymph gland homeostasis. (A, B) Schematic representation of third instar larva lymph gland anterior lobes. The medullary zone (MZ) contains three types of progenitors: distal progenitors and core progenitors are in green and hatched green, respectively, and the PH1 is in pink. Intermediate progenitors are in yellow, plasmatocytes and crystal cells in the cortical zone (CZ) are in light and dark blue, respectively. The PSC and the cardiac tube/vascular system are in red and orange, respectively. (A) Differentiated hemocytes result from progenitors’ differentiation (green dashed arrow) In a wildtype (WT) lymph gland, under homeostatic conditions, the PSC regulates the maintenance of a subset of MZ progenitors. Hedgehog (Hh) is required for maintaining distal progenitors. PSC signals required for controlling PH1 remain to be identified, as well as the progenitor subset controlled by Ser expressed in the PSC. Pvf1 secreted by the PSC, controls progenitor maintenance via differentiated hemocytes. (B) The cardiac tube corresponds to a second niche present in the lymph gland. The FGF ligand Branchless (Bnl) activates its receptor Breathless (Btl) in progenitors. Btl-FGF activation regulates intracellular Ca²⁺ levels via PLCγ, and controls the maintenance of core progenitors and in turn the whole progenitor pool. The ligand Slit produced by cardiac cells activates its Robo receptors in the PSC. Robo signaling controls PSC cell clustering and proliferation.