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Abstract
The shape and form of protozoan parasites are inextricably linked to their pathogenicity. The evolutionary pressure associated with establishing and maintaining an infection and transmission to vector or host has shaped parasite morphology. However, there is not a ‘one size fits all’ morphological solution to these different pressures, and parasites exhibit a range of different morphologies, reflecting the diversity of their complex life cycles. In this review, we will focus on the shape and form of Leishmania spp., a group of very successful protozoan parasites that cause a range of diseases from self-healing cutaneous leishmaniasis to visceral leishmaniasis, which is fatal if left untreated.
Highlights
Leishmania have two major different cell morphologies, exemplified by the promastigote morphology in the sand fly and the amastigote morphology in the mammalian host
At the base of the flagellum is an invagination of the cell membrane forming a vase-like structure called the flagellar pocket, which is important in these parasites as it is the only site of endocytosis and exocytosis and is a critical interface between the parasite and its host environment [9]
The second site of Leishmania attachment in the sand fly is at the stomodeal valve, where a specialized cell type called the haptomonad is observed attached to the cuticle lining of the valve by hemidesmosomal structures that are found in the enlarged tip of a relatively short flagellum [54]
Summary
Leishmania have a digenetic life cycle involving both a mammalian host and an insect vector. The sand fly vector adds a further layer of complexity: there are many species capable of carrying the Leishmania parasite; there are often specific relationships whereby some sand fly species are capable of transmitting only a single or limited number of Leishmania species (table 1) [15] Despite all these levels of complexities and differences, the morphology of the different Leishmania species shows remarkable conservation of form as they progress through their life cycle. Cells with an amastigote or promastigote morphology look dramatically different, but they retain the same basic cell layout with the kinetoplast anterior to the nucleus and a flagellum extending from the basal body (figure 1a) [5,6]. The promastigote morphology is defined by an elongated ovoid cell body with a long motile flagellum extending out of the flagellar pocket that provides propulsive force likely responsible for facilitating the traverse through the sand fly digestive tract [18]
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