Sporogony, development and ultrastructure of extrinsic stages of Plasmodium mexicanum.

Klein T. A., Akin D. C., Young D. G., Telford S. R., Jr. and Butler J. F. 1988. Sporogony, development and ultrastructure of extrinsic stages of Plasmodium mexicanum. International Journal for Parasitology18: 463–476. The sporogonic stages of Plasmodium mexicanum in the midgut and salivary glands of Lutwmyia vexalor were studied by light and electron microscopy. The sporogonic cycle of P. mexicanum maintained at 27 °C was completed by 6.5 days after an infective bloodmeal. Sporozoites were relatively short (4–6 μm) when compared to most other Plasmodium sporozoites. The nucleus, with a distinct nucleolus, was relatively large in the early non-vacuolated oocyst. Oocyst differentiation was initiated by the formation of subcapsular spaces and internal vacuolization which eventually coalesced and formed cytoplasmic clefts (sporoblastoids). Sporozoite formation was initiated early in the differentiated oocyst by the formation of a pair of convex electron dense membranes. A moderately electron dense inclusion body. a nucleus, and a single spherical mitochondrian were observed migrating into the evaginating sporozoite. Rhoptries and micronemes appeared to arise de novo from the moderately dense inclusion bodies which degenerated and were not observed in mature Sporozoites. After their release from oocysts, Sporozoites appeared in the salivary glands of L. vexator. Sporozoites were constricted at the parasite-host cell junction during entry into the salivary gland and were enclosed within a parasitophorous vacuole formed by the host cell membrane after entering the salivary gland cell. Sporozoites became extracellular as they continued to migrate into the lumen of the salivary gland, in similar manner to that seen when entering the salivary gland cell. The arrangement of the subpellicular microtubules (asymmetrically, with 23 of the microtubules occupying 12 of the sporozoite circumference, and 13 in the remaining half) was different from that observed in mammalian malarias, where all but one microtubule lie in two-thirds of the sporozoite circumference, but was similar to Plasmodium agamae, a saurian malaria. Except for the arrangement of the subpellicular microtubules, the structure and arrangement of organelles was similar to that observed for other Plasmodium species.