An isolation technique involving filtration and discontinuous density gradient centrifugation was utilized for obtaining Giardia lamblia cysts from human feces. Highly concentrated preparations of cysts were examined by electron microscopy. The cyst of G. lamblia is surrounded by a moderately dense fibrous wall 0.3 µm thick. A thin layer of cytoplasm separates the cyst wall from electron lucid intracellular spaces located around the periphery of the cyst. Four nuclei are usually present. Basal bodies and axial filaments are located between the nuclei. Two sheets of microtubules are associated with the axial filaments. No mitochondria, endoplasmic reticulum, or Golgi apparatus is present. Parallel rows of microtubules with perpendicular ribbon-like structures are randomly distributed in the cytoplasm. Substructural units with a periodicity of 150 Å are oriented perpendicularly to the longitudinal axis of the ribbons. The ribbon-like structures and associated microtubules represent disassembled portions of the sucking disk of the trophozoite form and apparently are reorganized into sucking disks upon division of the cyst organism.
Present address: Roslagstull Hospital, Stockholm, Sweden.