Dr. Murai Éva - Gubányi András szerk.: Parasitologia Hungarica 27. (Budapest, 1994)

The tissue and organ specificity of myxosporeans Tissue and organ specificity has remained a neglected aspect of myxosporean research to this very day. Although it has long been known that M. cerebralis typically forms plasmodia in the cartilaginous tissue of the skull or that M. musculi forms its pseudocysts in the muscle cells of fish, in most cases the spores' momentary site of occurrence is given as their location, irrespective of the organ or tissue in which they have developed. For these reasons, several technical books (Shulman 1966, Bauer et al. 1981) describe M cyprini, a very common parasite of the common carp, as a species that forms spores in small plasmodia in different organs. In contrast, Molnár and Kovács-Gayer (1985) demonstrated that the given species is a typical muscle parasite, and the formations mistaken for small plasmodia are actually the parasite's macrophage-engulfed spores disseminated all over the body. Therefore, the location shall never be determined merely on the basis of the spores' site of detection. The developmental stages, but at least the plasmodia often referred to as cysts, shall be detected in each case. For species developing in large plasmodia the determination of location usually does not pose a problem if the examination is thorough enough. At the same time, for species whose specific developmental stages take place in different organs the location shall be determined separately for the individual stages. The tissue specificity of myxosporeans The species belonging to Myxosporea are highly tissue-specific parasites. Their development is consistently restricted to a single specific cell type. Myxosporea include species specialized in parasitizing epithelial, muscle, nervous, cartilaginous, bone and connective tissue; moreover, certain species begin to develop, within the basic tissue type, only in a single specific cell type, e.g. gill epithelium, liver cells, compact connective tissue, or perichondrial cells. Although among the myxosporeans Shulman (1966) distinguished species of intracellular, intercellular and coelozoic development, my studies conducted so far allow me to assume that, upon gaining entry into the organism, the Actinosporea sporoplasms reach the site of final colonization and invariably continue their development in an intracellular location. Whether spore formation takes place within the cell or is completed in the intercellular or coelozoic space upon disruption of the host cell seems to depend exclusively on the size of the attacked specific host cell. Species having a specificity for muscle and nerve cells usually have a prolonged intracellular phase of development, as the relatively large host cells provide ample space for the growth of their pseudocysts. These parasites form their spores within the cell, and the spores are released into the intercellular space only after destruction of the host cell (Molnár and Kovács-Gayer 1985, Ferguson et al. 1985). The development of species showing a specificity for endothelial, epithelial, cartilaginous and connective tissue cells differs from the way of development described above. These small cells are destroyed by the growing parasites. However, the trophozoite released from the destroyed cell consistently becomes surrounded by cells of the