title: Fine structure of the oocyst walls of Isospora serini and Isospora canaria and excystation of Isospora serini from the canary, Serinus canarius L.
abstract: Oocysts of Isospora serini and Isospora canaria, from the canary Serinus canarius, were broken, added to a cell suspension, fixed in Karnovsky's fluid, and studied in the electron microscope. The oocyst wall of each species had an electron-lucent inner layer, a more osmiophilic middle layer and an outer layer of electron-lucent (I. serini) or electron-dense material interspersed with some electron-lucent material (I. canaria). A few, relatively large lipid-like bodies were present in the outer or middle layer of the oocyst wall of I. canaria. As many as 9 membranes were present in the oocyst wall of I. canaria and 3 in that of I. serini. When exposed to a trypsin-sodium taurocholate fluid, sporozoites of I. serini excysted from 5-month-old sporocysts in vitro, but not from sporocysts stored for more than 6 months. No excystation occurred in 15-month-old I. canaria sporocysts. Similarities and differences in excystation between I. serini and other Isospora, Eimeria, and Sarcocystis species are discussed.
mesh_terms: D000818:Animals; D002171:Canaries / Q000469:parasitology*; D003594:Cytoplasmic Granules / Q000648:ultrastructure; D007549:Isospora / Q000254:growth & development / Q000648:ultrastructure*