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[Entamoeba histolytica]

Entamoeba histolytica/Entamoeba dispar trophozoites have a single nucleus, which have a centrally placed karyosome and uniformly distributed peripheral chromatin.  This typical appearance of the nucleus is not always observed as some trophozoites can have nuclei with an eccentric karyosome and unevenly distributed peripheral chromatin.  The cytoplasm has a granular or "ground-glass" appearance.  E. histolytica/E. dispar trophozoites usually measure 15 to 20 µm (range 10 to 60 µm), tending to be more elongated in diarrheal stool.

Erythrophagocytosis (ingestion of red blood cells by the parasite) is the only morphologic characteristic that can be used to differentiate E. histolytica from the nonpathogenic E. dispar.  However, erthrophagocytosis is not typically observed on stained smears of E. histolytica.

A	B

A, B: Trophozoites of E. histolytica with ingested erythrocytes stained with trichrome.  The ingested erythrocytes appear as dark inclusions.  The parasites above show nuclei that have the typical small, centrally located karyosome, and thin, uniform peripheral chromatin.

C	D

C, D: Trophozoites of E. histolytica with ingested erythrocytes stained with trichrome.

E	F

E: Trophozoite of E. histolytica with ingested erythrocytes stained with trichrome.
F: Trophozoite of E. histolytica trophozoite with ingested erythrocytes, under differential interference contrast (DIC) microscopy.