amastigote

An amastigote is a small, nonmotile intracellular form of certain kinetoplastid protozoa, most notably Leishmania species and Trypanosoma cruzi. The term reflects the absence of an external flagellum. Amastigotes are typically 3–5 μm in length, ovoid or spherical, and possess a nucleus and a kinetoplast.

In the Leishmania life cycle, the amastigote is the intracellular stage that resides within macrophages and other phagocytic cells of the mammalian host. After transmission by a sand fly, the flagellated promastigote is phagocytosed and converts to an amastigote, which replicates by binary fission within the phagolysosome and can disseminate to other cells. The disease manifestations depend on species and include cutaneous, mucocutaneous, and visceral leishmaniasis.

For Trypanosoma cruzi, amastigotes form inside a wide range of host cells, including cardiac and smooth muscle cells. They replicate in the cytoplasm, then differentiate back into infectious trypomastigotes that spread to other cells. This intracellular cycle is central to pathology in Chagas disease.

Diagnosis and morphology: Amastigotes can be identified in tissue smears and histological sections stained with Giemsa or hematoxylin-eosin, showing intracellular organisms within host cell cytoplasm, with a visible nucleus and kinetoplast. They are a diagnostic hallmark in visceral and cutaneous leishmaniasis.

Etymology: The name derives from Greek a-, without, and mastig-, whip, referring to the lack of a flagellum.