Figure 1.

Embryonic and larval hematopoiesis. (A, B) Embryonic hemocytes (blood cells) originate from the head mesoderm in the embryo and differentiate into plasmatocytes (macrophages, light blue) and a small number of crystal cells (dark blue). Lymph gland progenitors (green) are specified from the thoracic cardiogenic mesoderm in the embryo (A) Anterior (A)/Posterior (P) and Dorsal (D)/Ventral (V) axes are indicated. (B) At the end of embryogenesis, crystal cells remain clustered in the anterior part, whereas plasmatocytes are dispersed throughout the embryo. The lymph gland is composed of one pair of lobes and is localized at the anterior part of the dorsal vessel/cardiac tube. (C) In third instar larvae, plasmatocytes (light blue) and crystal cells (dark blue) of embryonic origin are found in circulation and colonizing local microenvironments, in particular the hematopoietic pockets, where they expand. Close up of a hematopoietic pocket where neurons are in red, oenocytes in grey, and plasmatocytes and crystal cells in light and dark blue, respectively. Activin-β produced by PNS neurons promotes plasmatocyte proliferation and adhesion. The lymph gland (green) is composed of several pairs of lobes aligned along the cardiac tube.