Phagocytosis and sequestration of foreign agents by immune cells are important and highly conserved functions in the animal kingdom. Amoebocytes, mononuclear phagocytes, or similar cell types are present from the most primitive metazoa through the vertebrates. In mammals, mononuclear phagocytes are present in various tissues as macrophages. In the presence of a foreign antigen, this cell type becomes "activated" and avidly phagocytic. If a highly persistent antigen is present, it may differentiate further into an epithelioid cell, which surrounds and isolates the inciting agent. The latter is so named because in some respects, it resembles histologically an epithelial cell. However, neither avian nor mammalian epithelioid cells possess any hallmarks of true epithelial tissue (i.e., desmosomes, tonofilaments, or cytokeratin) nor have they ever been considered to be related to epithelial cells. In this paper, we report that epithelioid cells from chronic lesions of teleost fishes do possess features of true epithelial cells. These findings may have important implications for the ontogeny of mononuclear phagocytes in the vertebrates and raises questions about the origin of these cells.

Culture Conditions Affecting the In vitro Propagation of Amyloodinium Ocellatum.

Edward J. Noga, Department of Companion Animal and Special Species Medicine, North Carolina State University.

Amyloodinium ocellatum, a common dinoflagellate parasite of marine and brackish water fishes, could be propagated in cell culture using a very simple salt solution. Serum, trace minerals, or other additives were not required. This data that Amyloodinium may derive all its nourishment, trace mineral requirements, from the host cell. it was also found that the salt composition of the medium was important to survival; certain salt compositions were better for continuous growth of the parasites. The simple composition of these culture media may greatly facilitate the study of this parasite because of the culture system's close resemblance to the natural infection cycle.