Of blood cells and the nervous system: hematopoiesis in the Drosophila larva

Abstract

Hematopoiesis is well-conserved between Drosophila and vertebrates. Similar as in vertebrates, the sites of hematopoiesis shift during Drosophila development. Blood cells (hemocytes) originate de novo during hematopoietic waves in the embryo and in the Drosophila lymph gland. In contrast, the hematopoietic wave in the larva is based on the colonization of resident hematopoietic sites by differentiated hemocytes that arise in the embryo, much like in vertebrates the colonization of peripheral tissues by primitive macrophages of the yolk sac, or the seeding of fetal liver, spleen and bone marrow by hematopoietic stem and progenitor cells. At the transition to the larval stage, Drosophila embryonic hemocytes retreat to hematopoietic "niches," i.e., segmentally repeated hematopoietic pockets of the larval body wall that are jointly shared with sensory neurons and other cells of the peripheral nervous system (PNS). Hemocytes rely on the PNS for their localization and survival, and are induced to proliferate in these microenvironments, expanding to form the larval hematopoietic system. In this process, differentiated hemocytes from the embryo resume proliferation and self-renew, omitting the need for an undifferentiated prohemocyte progenitor. Larval hematopoiesis is the first Drosophila model for blood cell colonization and niche support by the PNS. It suggests an interface where innocuous or noxious sensory inputs regulate blood cell homeostasis or immune responses. The system adds to the growing concept of nervous system dependence of hematopoietic microenvironments and organ stem cell niches, which is being uncovered across phyla.

Figure 1. Hematopoietic waves in Drosophila. Timeline of hematopoietic waves in the Drosophila embryo and larva. Embryonic and lymph gland hematopoiesis are based on the de novo generation of blood cells, while larval hematopoiesis is founded by embryonic hemocytes that colonize hematopoietic pockets of the larval body wall. Vertical hatching indicates release of hemocytes from hematopoietic sites. Note progressive release of larval hemocytes into circulation over the course of larval development. HM, embryonic head mesoderm; HP, larval hematopoietic pockets; LG, lymph gland.

Figure 2. The PNS as hematopoietic microenvironment. (A) Co-labeling of neurons (21–7-GAL4, UAS-CD8-GFP, green), and hemocytes (Hml∆-DsRed, red), located in the hematopoietic pockets of a filleted 3rd instar larva, anterior left, dorsal up. Two larval abdominal segments showing hemocytes colocalizing with the lateral and dorsal PNS clusters, forming the ‘lateral patch’ and ‘dorsal stripe’. (B) Model of a lateral patch and dorsal stripe. Arrow represents attractive and inductive cues provided by cells of the PNS that support larval hemocytes.