The filter-house of the larvacean Oikopleura dioica. A complex extracellular architecture: From fiber production to rudimentary state to inflated house.

While cellulose is the most abundant macromolecule in the biosphere, most animals are unable to produce cellulose with the exception of tunicates. Some tunicates have evolved the ability to secrete a complex house containing cellulosic fibers, yet little is known about the early stages of the house building process. Here, we investigate the rudimentary house of Oikopleura dioica for the first time using complementary light and electron microscopic techniques. In addition, we digitally modeled the arrangement of chambers, nets, and filters of the functional, expanded house in three dimensions based on life-video-imaging. Combining 3D-reconstructions based on serial histological semithin-sections, confocal laser scanning microscopy, transmission electron microscopy, scanning electron microscopy (SEM), and focused ion beam (FIB)-SEM, we were able to elucidate the arrangement of structural components, including cellulosic fibers, of the rudimentary house with a focus on the food concentration filter. We developed a model for the arrangement of folded structures in the house rudiment and show it is a precisely preformed structure with identifiable components intricately correlated with specific cells. Moreover, we demonstrate that structural details of the apical surfaces of Nasse cells provide the exact locations and shapes to produce the fibers of the house and interact among each other, with Giant Fol cells, and with the fibers to arrange them in the precise positions necessary for expansion of the house rudiment into the functional state. The presented data and hypotheses advance our knowledge about the interrelation of structure and function on different biological levels and prompt investigations into this astonishing biological object.