AN ELECTRON-MICROSCOPE STUDY OF CONSTITUENTS OF ENCAPSULATING CYSTS IN AMERICAN OYSTER, CRASSOSTREA VIRGINICA, FORMED IN RESPONSE TO TYLOCEPHALUM METACESTODES

Electron-microscope studies of the constituents of the encapsulating cysts in the American oyster, Crassostrea virginica, formed in response to invading Tylocephalum metacestodes have revealed three types of cells and two types of extracellular fibers embedded in a homogeneous matrix of medium electron density. The cellular constituents are fibroblastlike cells, leukocytes, and brown cells. The fibroblastlike cells are primarily concentrated in the innermost region of each cyst. They include mitochondria, rough and smooth endoplasmic reticula (primarily the former) with distended cisternae, cytoplasm of varying densities, and intracytoplasmic fibrils. The leukocytes are scattered throughout the cyst. They include primarily smooth endoplasmic reticula, lysosomelike bodies, dictyosomes, and occasionally, myelinlike bodies. Intracytoplasmic fibrils are also present. The brown cells are situated along the periphery of each cyst. They include globules of different textures and electron densities. Many of the globules of moderate electron density include fibrous material while the denser globules include protein crystals. Rough endoplasmic reticula and mitochondria also occur in the cytoplasm of brown cells. The fine structural similarities between leukocytes and fibroblastlike cells support earlier evidences that the latter have differentiated from leukocytes. The major type of extracellular fibers is of medium electron density and nonperiodic. Those in the vicinity of parasites are found attached to the outer surface of the unit membrane covering the parasites' microvilli. The second type of extracellular fibers is also nonperiodic and is similar to the matrix in electron density. The spatial relationships between the extracellular fibers and the cellular constituents suggest that extracellular fibrillogenesis is influenced by these cells. © 1969.