ADHESION OF PLASMODIUM-GALLINACEUM OOKINETES TO THE AEDES-AEGYPTI MIDGUT - SITES OF PARASITE ATTACHMENT AND MORPHOLOGICAL-CHANGES IN THE OOKINETE

Plasmodium gallinaceum ookinetes adhered to Aedes aegypti midgut epith elia when purified ookinetes and isolated midguts were combined in vit ro. Ookinetes preferentially bound to the microvillated luminal surfac e of the midgut, and they seemed to interact with three types of struc tures on the midgut surface. First, they adhered to and migrated throu gh a network-like matrix, which we have termed microvilli-associated n etwork, that covers the surface of the microvilli. This network forms on the luminal midgut surface in response to blood or protein meals. S econd, the ookinetes bound directly to the microvilli on the surface o f the midgut and were occasionally found immersed in the thick microvi llar layer. Third, the ookinetes associated with accumulations of vesi cular structures found interspersed between the microvillated cells of the midgut. The origin of these vesicular structures is unknown, but they correlated with the surface of midgut cells invaded by ookinetes as observed by TEM. After binding to the midgut, ookinetes underwent e xtensive morphological changes: they frequently developed one or more annular constrictions, and their surface roughened considerably, sugge sting that midgut components remain bound to the parasite surface. Our observations suggest that, in a natural infection, the ookinete inter acts in a sequential manner with specific components of the midgut sur face. Initial binding to the midgut surface may activate the ookinete and cause morphological changes in preparation for invasion of the mid gut cells.