Expression of S-100 proteins in the embryo. (A and B) Localization of mRNA encoding the S100 proteins MRP8 (A) and MRP14 (B) showing that both are expressed only in the liver of 11.5 dpc embryo. (C and D) Sections through the liver of embryos probed for MRP8 (C) and MRP14 (D) expression. Panel (C) is counterstained with hematoxylin. Note the difference between the two markers. In (C), MRP8-positive cells are small and clustered, whereas in (D), cells expressing MRP14 mRNA appear fewer in number, larger, and often contain inclusions suggesting active involvement in phagocytosis. Bar in (A) and (B) represents 250 μm and in (C) and (D), 20 μm; yolk sac (ys), abdominal wall (abw), and peritoneum (pe).