Scanning and transmission electron microscopy of the oocyst wall of Isospora lacazei.

The oocyst wall of Isospora lacazei from sparrows was studied with scanning (SEM) and transmission (TEM) electron microscopy. In TEM, the oocyst wall consisted of four distinct layers (L1-4). The innermost layer, L1, was moderately electron-lucent and 240--285 nm thick; L2 was electron-dense and 210--240 nm thick; L3 was moderately electron-lucent and 15--150 nm thick; L4, the outer most layer, was discontinuous and consisted of electron-dense discoid bodies which measured 180--220 nm x 320--840 nm. The discoid bodies of L4 as seen by TEM appeared spheroid in shape when observed by SEM. One or two membranes were situated on or between various layers of the oocyst wall. One such membrane occurred on the inner margin of L1, two closely applied membranes were interposed between L1 and L2, one membrane occurred between L2 and L3, and one membrane on the outer margin of L3.