Parasitology (1981), 82, 195-204 195 With 4 plates An ultrastructural study of developing stages of exo-erythrocytic Plasmodium berghei in rat hepatocytes J. F. G. M. MEIS*, J. P. VERHAVE*, P. H. K. JAPf, F. HESSj and J. H. E. T. MEUWISSEN* Departments of Medical Parasitology* and of Cytology and Histologyf, University of Nijmegen, Geert Grooteplein Zuid 24/Noord 21, Nijmegen, The Netherlands (Accepted 3 July 1980) SUMMARY The ultrastructure of immature exo-erythrocytic forms of Plasmodium berghei in rat hepatocytes was studied at stages between 25 and 51 h of development. A new method was successfully applied to localize the parasites in a small portion of the liver by temporary ligature of blood vessels to the majority of the liver, and from the spleen and the pancreas. Nuclear profiles appeared to be part of a highly lobed nuclear reticulum. Peripheral vesiculation and vacuolization of the cytoplasm was increas- ingly prominent and ushered in the formation of pseudocytomeres. Spacing between host- and parasite-derived membranes could first be observed after 43 h of development. In general, organellar development was found to follow closely that described for plasmodial oocysts. INTRODUCTION There have been very few fine-structural observations on the development of the exo-erythrocytic forms (EEF) of mammalian plasmodia in liver parenchymal cells. The extant work on rodent malaria was reviewed by Sinden (1978). The few publications on EEF of rodent plasmodia dealt exclusively with nearly mature stages of about 2 days, where formation of island structures or pseudocytomeres was under way. Garnham, Bird, Baker & Killick-Kendrick (1969) were the first to describe mature EEF of Plasmodium yoelii. At this stage merozoites were already present. Yoeli, who discovered the conditions required for cyclical transmission of rodent plasmodia, was involved in the study of EEF of P. berghei that were only a few hours younger (Desser, Weller & Yoeli, 1972). Bafort (1971) described the fine structure of P. vinckei and P. berghei EEF at about the same stage of develop- ment. Recently Terzakis, Vanderberg, Foley & Shustak (1979) added a short description of exo-erythrocytic merozoites of P. berghei taken up by Kupffer cells. Light microscopy clearly shows that the younger the stage, the smaller the EEF and hence the more difficult it is to localize them for ultrastructural studies. An increase in numbers of EEF per liver volume facilitates detection, especially of 0031-1820/82/0080-0616 $01.00 © 1981 Cambridge University Press