The light organ system of E. scolopes. A. A swimming adult animal. The light organ (internal to the boxed area) is located in the center of the mantle cavity. Bar, 1 cm. B. A confocal image of a light organ (labeled with CellTracker, Molecular Probes) within a diagram of a newly hatched animal. The juvenile organ has a complex, superficial ciliated epithelium (sce) that facilitates colonization by the symbioint. Bar, 200 microns. C. A confocal image of the sites of V. fischeri entry into host tissues (labeled with acridine organ, AO). A set of three pores, into which aggregated V. fischeri cells will migrate, are located at the base of each lateral field of ciliated epithelial cells. In response to interactions with colonizing V. fischeri, host hemocytes (h) migrate into the sce, which will be lost during symbiont-induced light organ morphogenesis. Also visible in this image is the condensed chromatin characteristic of symbiont-induced apoptosis (arrows), which stains vividly with AO. Bar, 20 microns. D. A histological section revealing the path traversed by colonizing symbionts. Once aggregated in mucus outside the light organ, the symbionts enter the pores (p), travel up long ciliated ducts (d) and enter the crypt spaces where they interact with two cell types, the polarized epithelium that lines the crypts (ce) and a transient population of host hemocytes (h). Bar, 30 microns.