Fig. 1. Morphological features of the developing lymph gland. (A,B) Lymph glands at embryonic stage 16. Cells of the embryonic lymph gland (shown in whole mount) express the hemocyte marker Srp (A, red) and Odd (odd-lacZ expression in B, green; the bluish-green appearance is due to overlap with nuclear marker To-pro-3 in blue). Pericardial cells (B, arrowhead), also express Odd. Cells between the two symmetric lymph gland clusters represent dorsal vessel (DV) cells (B). Scale bar: 10 µm. (C,D) Second instar larval lymph gland. Expression of Srp (C) and odd-lacZ (D) is maintained in the second instar. By this stage, the lymph gland consists of a pair of primary (1°) lobes and smaller, posterior secondary (2°) lobes separated by pericardial cells (arrowhead in D). Scale bar: 10 µm. Expression of odd-lacZ is slightly higher in the region of the posterior signaling center (PSC) of the primary lobes. (E) Multi-lobed structure of the third instar lymph gland. Srp (red) is ubiquitously expressed. Pericardial cells separate lymph gland lobes as in the second instar. Scale bar: 100 µm. (F,G) Expression pattern of Viking-GFP fusion protein (F, green) reveals chamber-like structures within the lymph gland. Similar structures are seen in lymph glands of the uncharacterized GFP-trap line ZCL2867 (G, green). Scale bar: 10 µm. To-pro-3 (blue) shows nuclei. (H,I) Morphological features of the third instar lymph gland. (H) A DIC image of the third instar lymph gland shows two distinct regions within the primary lobe that we call the cortical zone (CZ) and the medullary zone (MZ) (see text for details). Arrows indicate the boundary between the zones. The secondary (2°) lobe is similar in appearance to the medullary zone. (I) Confocal section through a lymph gland from the GFP-trap line G147. Cells in the medullary zone (MZ, outlined) are compactly arranged, whereas cells in the cortical zone (CZ) exhibit voids and gaps. Scale bar: 10 µm.