TRENDS in Parasitology Vol.17 No.5 May 2001 http://parasites.trends.com 1471-4922/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1471-4922(01)01928-6 209 Research Update Research News In the past five years, there has been renew ed interest in the early development of the malaria parasite in the mosquito. Numerous exciting studies have examined in more detail the cellular and molecular interactions of the ookinete w ith the peritrophic matrix, midgut epithelium and basal lamina of the mosquito midgut, and a plethora of new responses by the mosquito to this invasion process have been described. Two extracellular, invasive stages of Plasmodium (the ookinete and sporozoite) are responsible for the successful migration of the parasite through the mosquito vector. During this migration, the parasite passes through two population bottlenecks. Tens of thousands of gametocytes can be ingested into the mosquito bloodmeal, but normally just 50 –100 ookinetes are produced; from these, typically fewer than five survive to produce oocysts on the midgut wall. Similarly, of the 50 000 sporozoites produced in these oocysts, only 15–80 might be inoculated into a host by the bite of each infectious mosquito. These reductions largely result from difficulties experienced by the parasite when invading and surviving within appropriate target tissues. Recent publications have shed new light on possible mechanisms evolved by the parasite to mediate these events. There are similarities between the migration of the ookinete and of the sporozoite in the mosquito (Fig. 1): both are subject to protease-rich environments; invasion is preceded by a multistep recognition of complex extracellular matrix/matrices and the target-cell plasma membrane; transient migration through the host epithelium is achieved without permanent formation of a parasitophorous vacuole and results in the short-term activation of the systemic immune response; while within the epithelial cell, the parasite could be killed and might attempt to suppress the immune response; having escaped from the host cell, the parasites reside in an extracellular location for many days. The ookinete Early studies Classic observations on the ‘vermicule’ 1,2 and the penetration of the mosquito midgut wall by the Plasmodium ookinete 3 were summarized by Wenyon 4 . He reported the contractile, twisting and gliding motility of the parasite, the direct penetration of the anopheline midgut cell, and the eventual transformation of the lanceolate ookinete into a spherical oocyst in the space between the midgut epithelium and the basal lamina. Subsequently, Huff suggested that the ookinete of P. cathemerium penetrated between midgut cells of Culex pipiens 5 , whereas Indacochea noted that, while inside the epithelial cell, the ookinete was surrounded by a ‘hyaline colourless capsule’ 6 . Subsequent observations on ookinete motility 7,8 further supported these early observations 1–4 . Much of the subcellular detail of invasion of the midgut epithelium was then revealed by electron microscopy (EM) 9–13 . These studies demonstrated that the intracellular ookinete, unlike the merozoite, was not surrounded by a parasitophorous vacuole membrane (PVM), and that the invaded midgut cell suffered significant osmotic and structural damage. Surprisingly, the controversy over whether the ookinete migrated by an inter- or intracellular route was not resolved, and it was proposed the route might vary between parasite–vector species combinations. The P. gallinaceum ookinete secretes one or more chitinases as pro-enzymes that can be activated by trypsins in the midgut to break down the chitin polymer of the peritrophic matrix in Aedes 14 . Ongoing studies in P. gallinaceum, P. falciparum and P. berghei could soon determine how essential and conserved these events are between parasite species 15 . Recent research Having crossed the peritrophic matrix the ookinete then invades the midgut epithelium. One of the more provocative hypotheses proposed was that a subgroup of epithelial cells (‘Ross cells’) in Aedes midgut epithelium, which are recognized by their expression of vesicular ATPase Plasmodium invasion of mosquito cells:hawk or dove? Robert E. Sinden and Peter F. Billingsley TRENDS in Parasitology Haemocoele Midgut cell Gametocytes (1000s) Midgut lumen Salivary duct lumen Ookinetes (50–100) Infective bite (15–80 sporozoites inoculated) Peritrophic matrix Microvillus- associated network Oocysts (~5) Sporozoites (~50 000 released) Basal lamina Sporozoites (~11 000 in glands) Chitin lining of the salivary duct Salivary gland cell Fig. 1. The passage of Plasmodium through the mosquito vector, indicating the barriers to be crossed (both cellular epithelia and extracellular matrices) and the parasite numbers commonly found at each stage of development.