Troposome Structure
The troposome houses Riftia’s symbionts. Bacterial endosymbionts are housed within vacuoles, in the basal area of epithelial cells. In a mature troposome bacteria compose at 40% of the bacteriocytes, (cells housing the bacteria) and bacteriocytes make up 41-53% of the area of the troposome. The cytoplasm of the bacteriocytes contains very few organelles, basically just a nucleus and a few mitochondria (Jones and Gardiner 1989).
The troposome is a highly vascularized tissue. No bacterium is separated from blood capillary by more than three bacteria. The overall structure of the troposome is a series of lobules, each of which is supplied by a central artery which feeds into a capillary bed which radiates out to the periphery of the lobule and reconnects with blood vessels on the exterior. Each lobule also contains its own sequence of bacteriocytes (Jones and Gardiner 1989).
In between the bacterial cells in the bacteriocytes lies a complex latticework of fibers resembling pili. These fibers apparently maintain spacing between the individual symbiont cells and help to establish the lobule structure of the troposome. The fibers arise from the symbionts themselves and are a crucial alteration in their physiology to respond to the presence of a host. They may be especially important in the early stages of symbiosis, as the juvenile first acquires symbionts and they become internalized. Proper spacing and the structure of the troposome allow maximum efficiency in the transfer of nutrients and molecules between the host R. pachyptila and its symbiont (Gardiner et al. 1991).
Adapted From Figure 1 (Herve and Minic 2007). Overall Riftia pachyptila Structural Organization.