96 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



of the cell under a rather thick pellicle consisting of two mem- 

 branes plus a granular intervening layer. A lateral depression in 

 the pellicle, bordered by a thickened rim, is present in the 

 P. falciparum sporozoite and is termed a micropyle. Garnham and 

 colleagues suggest that after penetration of a cell in the vertebrate 

 host, the sporozoite discards its differentiated pellicle by emerging 

 through the micropyle. 



Duncan and colleagues examined sections of mosquito stomachs 

 at increasing intervals after their infection with P. cathemarium from 

 canaries. Beginning with the third day, the progressive develop- 

 ment of sporozoites within the encysted zygotes (oocysts) could 

 be observed. The oocyst is embedded between epithelial cells of 

 the host's stomach and is surrounded by a thick, homogeneous 

 capsule that is limited internally by the parasite's cell membrane 

 and externally by the membranes of neighboring host epithelial 

 cells. The material of the capsule is indistinguishable from the 

 elastic layer of the stomach wall and may be continuous with it. 

 As development progresses, the capsule becomes thinner and 

 forms abundant protrusions into the oocyst; ultimately it is no 

 longer visible as a continuous layer. 



The early oocyst cytoplasm is extremely dense, so that internal 

 organelles cannot be distinguished. At five days it is somewhat 

 looser in texture, vacuoles appear, and many nuclei, nucleoli, and 

 mitochondria can be distinguished. The size of the oocyst 

 increases greatly during development; the authors consider that 

 the decreasing compactness of the cytoplasm in later stages 

 cannot account for this and that materials for protein synthesis as 

 well as water must enter the parasite via the capsule. By seven or 

 eight days after infection, the oocyst cytoplasm is segmenting into 

 thousands of sporozoites. Mitochondria are long and branched, 

 endoplasmic reticulum is present, and Palade particles are found 

 on the reticulum or scattered in the matrix. By ten days sporozoite 

 development may be complete. Released sporozoites often are 

 embedded in host cells, but separated from their cytoplasm by host 

 cell membrane as well as their own double surface envelope. 

 Their cytoplasm is moderately dense and contains, in addition to 

 the usual organelles, rod-like bodies, typically in pairs or perhaps 

 triplets, which presumably correspond to the paired organelle 

 described by the Garnham group. 



