Two stages, trophozoite and cyst (Figure 10.1A and B), undergoing encystation and excystation complete the simple life cycle of Giardia. The motile and dividing trophozoite has a primitive organization of a cell that lacks several intracellular organelles, including mitochondria. It is pear-shaped and has an approximate size of 10-20 by 5-15^m. The cell contains two nuclei, four pairs of flagella, and a rod-shaped median body. Its flattened ventral side contains an adhesive disc, which promotes attachment to epithelial cells in the intestine. The dormant cyst, an infective form, is oval, 8-12 by 7-10 •m in size and contains
four nuclei. Fibrillar structures and crescent-shaped chromidial bodies present in the cyst are residual material from the flagella and the adhesive disc of the trophozoite stage. Ingested cysts, stimulated to excystation by the acidic pH in the stomach, develop into trophozoites, which colonize the upper part of the intestinal tract adhering to the surface of the enterocytes. There is no evidence for the intracellular penetration by Giardia trophozoites in humans. Stimulated by bile salts and elevated pH in the intestine, parasites later undergo encystation and are excreted with faeces.
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