Ultrastructural Analysis of Posterior Polar Endocytic Phagocytosis by Leishmania Amastigotes

Background: Leishmania spp. go through a complex life cycle. Developmental forms alternate between elongated, flagellated, motile, extracellular-promastigote forms; and aflagellar, ovoid amastigote-bodies. Mechanisms for survival and procurement of nutrients differ according to stage-specific requirements. Objectives: To present a mechanistic insight into phagocytosis by protists, and the developmental regulation of endocytosis, achieved by investigation of the ultrastructural interaction of two Leishmania isolates from Egypt, one cutaneous and one visceral, with their host cell cytoplasm. Materials and Methods: Two leishmanial strains were used. One isolate was from a patient’s cutaneous lesion acquired in northern Sinai (SH), and the other obtained from pools of livers and spleens of stray dogs from Agamy resort in Alexandria (D3). Isolation and maintenance of promastigote and amastigote stages was extracellularly in Offut’s medium and intracellularly in laboratory out bred male Syrian hamsters. Culture pellets and biopsies 1 mm thick from various hamster tissues were fixed in cold 2.4% gluteraldehyde in 2% Ca cacodylate buffer (pH 7.2). The specimens were processed for electron microscopy. Ultrathin sections were cut on a LBK Reichard ultra microtome and post stained with uranyl acetate in lead citrate solution. Sections were examined in a Zeiss EM 952 electron microscope. Results: In amastigotes the subpellicular microtubules, which form the cytoskeleton support for the pellicular membrane, showed pronounced free endings at the posterior pole of the parasites, still covered by the plasma membrane. The posterior area deficient of microtubules formed a cup-like invagination of variable depths according to the parasite’s endocytic activity. Evidence of phagocytosis by tissue forms of both isolates is exhibited by the similarity of homogenous bodies engulfed in the posterior invaginations and the host cell cytoplasm. Conclusion: The exhibition of endocytic phagocytosis by intracellular Leishmania supports the concept of a common origin of protists from a phagocytic cell ancestor, and is evidence that phagocytosis, as a criterion, is not secondarily lost in the genus.