The long term goals of this research are to understand the cellular and molecular basis for host defense reactions in selected invertebrate systems. These systems offer relative simplicity and may represent a paradigm for vertebrate blood cells. This proposal deals with defining which of two cell types is involved in the formation of a large multicellular syncytium that functions in a host encapsulation response in echinoderms. Two distinct subsets of coelomocytes will be separated from other coelomic fluid components, the subsets will be marked and their fate during fusion will be followed. The ultrastructure of apparent syncitia will be studied to establish whether cell-cell interaction results in the formation of true synsytia, and cytoskeletal linear elements that may orient cortical flow and/or influence the formation of the cellular aggregate will be identified. The relationship between cytoskeletal motility and cell fusion events will be determined using high resolution, digitally enhanced video microscopy. Cortical motility of cells that do and do not fuse will be compared, and patterns of cortical motility in cells after fusion will be studied. In sea urchins migratory phagocytic cells known as coelomocytes play important roles in the organisms' defense mechanisms such as engulfing foreign micro-organisms, wound repair, and blood clotting. Some aspects of the defense reactions in these animals involve the ability of the phagocytic cells to fuse with each other, which may be related to their ability to encapsulate invading parasitic organisms or to their clot forming ability. Very little research on the cell biology of these important defense reactions in invertebrates has been done taking advantage of modern techniques. The results of this project will provide a detailed analysis of the cellular events underlying the process of coelomocyte fusion and will lay the groundwork for further analysis of the function of defense responses in these organisms.
