Freeze-fracture studies on Pneumocystis carinii. II. Fine structure of the trophozoite.

Ultrastructure of the trophozoite of Pneumocystis carinii was studied by the freeze-fracture technique. Nuclei and cytoplasmic organelles such as the endoplasmic reticulum, mitochondria, cytoplasmic vacuoles and small round bodies were observed. The mean number of nuclear pores was 8 per micron 2, which is small compared with that reported for other human pathogenic protozoa. In general, the density of nuclear pores is considered to be related to the metabolic activity of the nucleus. This result, therefore, suggests that the nucleus of P. carinii may be less metabolically active than those of other protozoa thus far examined. Both the nuclear envelope and the endoplasmic reticulum showed a similar distribution of intramembraneous particles (IMPs): the P face was heterogeneous and the E face was homogeneous. However, the outer membrane of mitochondria was somewhat heterogeneous in IMP distribution on both P and E faces. The cytoplasmic vacuoles always showed a lower IMP density than that of the plasma membrane. This indicates that the vacuoles of P. carinii would not be phagosomes. By means of this technique, the tubular expansions could be divided morphologically into four types: tubules, lobopodia, branching and beaded structures. Furthermore, it was noted that the daughter trophozoite in the endogenous form was not different from the usual free trophozoites in the IMP distribution pattern.