Figure 5. Electron microscopy of Riftia trophosome tissue.
(A) Electron micrograph of a cross-section through a Riftia trophosome lobule. Surrounding an efferent central blood vessel (cB), small symbiont cells (SXS) are visible in bacteriocytes (i.e. symbiont-containing host cells) in the central lobule zone. Symbiont cell size increases toward the periphery of the lobule (SL: large symbiont cells). In the outermost bacteriocytes, symbiont cells are digested by host enzymes (S*). Bacteriocytes are interspersed with smaller blood vessels (B), which facilitate blood flow from the lobule periphery to the lobule center (Felbeck and Turner, 1995). The image was assembled from 50 individual transmission electron micrographs of a trophosome section from a Riftia specimen with sulfur-depleted trophosome. The full resolution image is available as Figure 5—figure supplement 1. Contrast and brightness were adapted. (B) Cell division in small Riftia symbionts in the trophosome lobule center of a Riftia specimen with sulfur-rich trophosome. All micrographs show the same dividing Endoriftia cell in three subsequent tissue sections, revealing that both daughter cells are still connected, but are about to be separated (arrow). Scale bar: 1 µm. Despite thorough screening, we did not observe cell division in large Endoriftia cells in any of the TEM sections. This corroborates the idea that small and large symbiont subpopulations are developmental stages of the same Endoriftia strain.
Figure 5—figure supplement 1. High-resolution TEM image of the trophosome lobule section shown in Figure 5.
