The ultrastructure of Entamoeba sp. (Laredo isolate). Observations on thin sections and freeze-fracture preparations

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Abstract
The ultrastructure of Entamoeba sp. (Laredo isolate), seen in thin section and in freeze-fracture preparation, indicates that numerous structural features are common to both this E. histolytica-like amoeba and regular strains. Heavy meromyosin (HMM)-binding, actin-like microfilaments and intranuclear microtubules and microfilaments are demonstrated for the 1st time in Entamoeba. Cytoplasmic microtubules were not detected using 2 different fixation procedures. Replicas of freeze-fractured membrane surfaces revealed particle size and distribution to be heterogeneous on both fracture faces P and E and suggested a similar structural complexity to that previously described for the HK9 strain of E. histolytica. Particle density was determined in plasma and phagosome membranes, and an enrichment of 4.5-fold and 1.6-fold was estimated for the P and E faces, respectively, of phagosome membranes. Nuclear-pore distribution was heterogenous and pore density was highly variable in log-phase cells, probably reflecting different cell-cycle stages. Possible differences in nuclear-pore density and distribution relative to published accounts on regular strains of E. histolytica are considered.