Light and electron microscopic studies on the development of Sphaerospora colomani Baska, 1990 and Chloromyxum inexpectatum Baska, 1990.

During a parasitological survey conducted on sterlets (Acipenser ruthenus L.), the developmental stages of two new coelozoic species of Myxosporea were demonstrated in the kidney and in the urinary passages. Sphaerospora colomani develops vegetative stages in the interstitial capillaries of the kidney in winter. The ultrastructure of a large primary cell containing secondary, tertiary and quaternary cells was examined. The vegetative developmental forms were located beside the tubule in the cytoplasm of the endothelial cell. The primary cell developed in the cytoplasm of the endothelial cell, making the cell greatly enlarged. Young, developing and fully vacuolated secondary cells were seen in the cytoplasm. Two tertiary cells developed in each vacuole that contained free nuclei and quaternary cells. Each developmental stage of the parasite bore processes to increase the surface. The pseudoplasmodia of Chloromyxum inexpectatum were found mainly in the lower passages and, less often, in the renal tubules of all fish specimens examined. The vegetative stages were located among the capillaries of the Bowman's capsules. The structure of the Bowman's capsule had changed due to the xenoma developing inside. A thin, shaded area represented the demarcation zone between the xenoma and the host cells. The cytoplasm of the xenoma formed by host and parasitic cells contained nuclei and young, differentiated secondary cells. In the more developed secondary cells, generally two tertiary cells developed and, later on, a quaternary cell appeared in each.