Entamoeba species
Entamoeba coli, cyst
Entamoeba coli is one of several non-pathogenic amebae that can be found in human stool specimens. It has both cyst and trophozoite forms. The cysts are relatively large, measuring 10 to 35 micrometers in diameter, and have 8 nuclei in their characteristic mature form. Cysts having anywhere from 1 to 16 nuclei may also be seen. The nuclei have the standard "Entamoeba-type" chromatin pattern, with a small karyosome (usually eccentric in E. coli) and a peripheral rim of condensed chromatin (so-called "dot and rim" pattern). The peripheral chromatin is often coarse and granular. The cytoplasm may contain glycogen and chromatoid bodies (usually with splintered ends).
Entamoeba histolytica, trophozoites, tissue
Entamoeba histolytica is the only proven pathogenic Entamoeba species. While it is often a commensal organism causing asymptomatic colonization, it can occasionally cause invasive disease resulting in dysentery, colitis, ameboma, ulceration, and disseminated disease (e.g. amebic liver 'abscess'). When invasive disease occurs, only the trophozoite stage is seen. In this case, trophozoites can be seen invading into the bowel wall (formalin fixed, paraffin-embedded tissue section, unknown stain). The arrows denote ingested red blood cells.
You can view whole slide images of Entamoeba histolytica in the colon HERE and a case in the liver HERE (H&E) and HERE (PAS).
Entamoeba histolytica/Entamoeba dispar, cysts
Cysts of Entamoeba histolytica/Entamoeba dispar typically measure 12 to 15 micrometers in diameter and have 1 to 4 small round nuclei. The nuclei have a 'ring and dot' chromatin pattern, with a small central karyosome and rim of condensed chromatin. The cytoplasm may contain chromatoid bodies which characteristically have rounded blunt ends.
Entamoeba histolytica/Entamoeba dispar, trophozoites
The trophozoites of Entamoeba histolytica/Entamoeba dispar measure 10 to 60 micrometers in greatest dimension (usual 15 to 20 micrometers) and have granular cytoplasm and a small round nucleus. The chromatin pattern is described as 'dot and ring' with a small central (or eccentric) karyosome and peripheral rim of condensed chromatin. The trophozoites of E. histolytica, the definitive pathogen, are morphologically identical to the trophozoites of E. dispar, as well as those of Entamoeba moskovskii and Entamoeba bangladeshi, and therefore cannot be definitively identified by morphology alone. The only exception to this is when ingested red blood cells are seen within the trophozoite cytoplasm, since this allows for a presumptive identification of Entamoeba histolytica.
Entamoeba polecki, eggs
Entamoeba polecki is a non-pathogenic ameba associated with pigs. The cysts measure 9 to 25 µm and have a large nucleus comprising up to 1/3 of the diameter of the cyst. Cysts are usually uninucleate, but may rarely be binucleate. They have the typical "Entamoeba-type" nucleus and chromatin pattern ('dot and ring'), with a small to large karyosome (central or eccentric location) and light-to-heavy peripheral chromatin. One of the most notable features of the cyst is the presence of numerous variably-sized chromatoid bodies, which may be small to large and have round or splintered ends (see examples below).
The trophozoites measure 10 to 25 µm in greatest dimension are often rounded. They possess a single nucleus (often distorted and irregularly-shaped), with a small to minute centrally-located karyosome. In contrast to the cysts, the peripheral chromatin is usually uniform and delicate. The cytoplasm is commonly vacuolated with a hyaline border, and blunt pseudopodia may be seen.
