Fine structure of the oocyst wall and excystation of Eimeria procyonis from the American raccoon (Procyon lotor).

Oocysts of Eimeria procyonis, from the American raccoon (procyon lotor), were broken, added to a cell suspension, fixed in Karnovsky's fluid, and studied with the electron microscope. The oocyst wall has three layers: a thin electron-dense inner layer (8-15 nm), an electron-lucent middle layer (25-35 nm), and a thick outer layer (120-140 nm). The outer layer has an electron-dense inner portion and an electron-lucent outer portion that contains membrane-bound vesicles. When exposed to a trypsin-sodium taurocholate fluid, sporozoites excysted from most sporocysts which were 35-43 months old, but not from sporocysts that looked normal and were 106 months old. Excysted sporozoites measured 13-16 x 3-4 (mean 14.3 x 3.2) micrometer, usually had two refractile bodies, and had a nucleus with a prominent nucleolus.