Fine Structure and Invasive Behaviour of the Early Developmental Stages of Theileria Annulata In Vitro

Abstract

The interaction, in vitro, between bovine peripheral blood by lymphocytes and sporozoites of Theileria annulata (Ankara) was studied by light and electron microscopy. Beginning five minutes following incubations, samples were taken for Giesma-stained smears and glutaraldehyde-fixed pelletes, for light and electron microscopy, respectively. Sporozoites of T. annulata measure an average of 0.9 μm long, 0.8 μm broad and posses a limiting unit membrane, the pellicle; a round-to-ovoid, eccentrically situated, non-chromocentric nucleus, double-membraned, tubular, acristate mitochondria, varying numbers of anisocytic, densely osmiophilic and pleomorphic organelles, the rhoptries which together with the polar ring form the apical complex; and numerous, loosely scattered, electron-dense ribosomal particles. As early as 5 min of incubation, sporozoites had made contact with, and penetrated, lymphocytes. Sporozoites consistently attached to the lymphocyte plasmalemma by their basal end, possibly at specific receptor sites. Apparently only a proportion of lymphocytes (up to 40% and more commonly 10–20%) were susceptible. Two subpopulations of the susceptible lymphocytes were observed; one which appeared to have receptor sits localized on one pole of the plasmalemma and the other subpopulation in which the receptor sites were distributed evenly around the plasmalemmal surface. With individual susceptible lymphocytes, the number of interiorized sporozoites increased from 1 to 3 at 5–10 min to as many as 15 or more parasites at around 60 min of incubation. Theileria annulata sporozoites were interiorized by the invagination of the host cell palsmalemma which remained intact throughout the process but the later fragmented. Within 30 min of interiorization, each sporozoite underwent dedifferentiation by the loss of its rhoptries and transformed into a trophozoite. Around 24 h, the trophozoite, a uninucleate, motile and feeding stage of the parasite, developed into a schizont an acentric, closed mitosis.