Histamine (HA) distribution in embryonic stages of the sea urchin S. purpuratus.We used a polyclonal HA antibody for whole mount immunohistochemical (WMIHC) detection of HA in embryonic and post-embryonic (see Figure 4) stages. Panels A-F show late gastrula and early prism stages of the sea urchin S. pupuratus visualized using confocal microscopy. A-C are DIC (Differential Interference Contrast) images of panels D-F. Panels G-I are epiflourescent images of late prism to early pluteus stages. We detected two types of histaminergic cells indicated by white arrows in the panels. The first type are putative skeletal cells (SC) the second type are found at the base of the ciliated band (CBC). Note that we also found several unknown cell types (UC) showing HA immunoreactivity. In the early pluteus stages, cells at base of the ciliated band are clearly visible. We were also able to detect similar cells in the region of the aboral lip and in the ciliated epithelium of the larval arms POA – post oral arms). Early pluteus larvae also clearly show the development of the histaminergic lateral arm cluster (LAC). Blue represents a nuclear stain (DRAQ-5) and green indicates detection of HA antibody. Black arrows in DIC images (A-C) indicate corresponding regions of white arrows. Note that DIC images (A-C) are individual cross sections, while corresponding fluorescent images (D-F) are maximum projections. Therefore cells surface regions are not visible in all DIC images. The ubiquitous green stain on the right (BG) illustrates diffuse background fluorescence also seen in control samples (not shown). Scale bars: A,D - 30 μm; B,E - 35 μm; C,F - 25 μm; G,H,I - 20 μm, Abbreviations: S: Stomach; E: Esophagus; UC: Unkown cell types.