Acta Tropica, 50(1992)227-236 227 © 1992 Elsevier Science Publishers B.V. All rights reserved 0001-706X/92/$05.00 ACTROP 00183 A scanning electron microscopic study of the sporogonic development of Plasmodium falciparum in Anopheles stephensi J.F.G.M. Meis 1, P.G.P. Wismans 1, P.H.K. Jap 2, A.H.W. Lensen 1 and T. Ponnudurai 1 1Institute of Medical Microbiology, University Hospital Nijmegen and 2Department of Cell Biology and Histology, University of Nijmegen, Nijmegen, Netherlands (Received 5 June 1991; accepted 27 August 1991) The full development of Plasmodiumfalciparum in Anopheles stephensi mosquitoes was studied by scanning electron microscopy. Ookinete development was described from in vitro cultures. Growing oocysts beneath the basal lamina of the midgut wall mechanically stretch this lamina until it is torn and displaced by day 7. In young oocysts the wall appears smooth. In older oocysts wrinkles in the wall are visible after routine fixation. Osmium tetroxide postfixation greatly reduced the occurrence of these wrinkles. Intracapsular development of sporozoites was visualized after mechanical manipulation of the oocysts during sample preparation. In contrast to P. berghei, no ectopic development was seen in P.falciparum in the mosquito midgut. The mechanism of sporozoite escape from the oocyst appears to be similar to that described for rodent malaria. Fracturing of salivary glands provided the first view by scanning electron microscopy of sporozoites located in proximal and distal gland cells and in the draining duct. Key words: Plasmodium falciparum; Plasrnodium berghei; Anopheles stephensi; Sporogony; Sporozoites; Scanning electron microscopy Introduction The fine structure of the sporogonic cycle of the malaria parasite has been studied extensively by transmission electron microscopy (TEM) (Sinden, 1984). However very few reports on sporogony have been published using the scanning electron microscope (SEM) and of these studies those on the rodent malaria parasites are in the majority (Sinden, 1974, 1975; Strome and Beaudoin, 1974; Rastogi et al., 1988). Only one extensive TEM study on the ultrastructure of Plasmodium faleiparum in Anopheles gambiae has been published including some SEM findings (Sinden and Strong, 1978). Our study aimed to provide a complete insight of the sporogonic development of P. faleiparum in A. stephensi by SEM. We were particularly interested in describing the internal organization of oocysts; the interaction of oocysts with the midgut wall; sporozoite release from the oocyst; interaction of sporozoites with the Correspondence address: J.F.G.M. Meis, Institute of Medical Microbiology, University Hospital Nijmegen, P.O. Box 9101, 6500 HB Nijmegen, Netherlands.