Endolimax nana

Small, cosmopolitan, non-pathogenic amoeba that colonizes the colon.

Trophozoite

Size: 5 to 12 μm; usual range, 8-10 μm.  Slightly smaller than Iodamoeba buetschlii.

Motility: visible only in fresh, unpreserved, specimens. The trophozoite extends numerous pseudopods simultaneously, giving the appearance of a bunch of transparent bubbles.

Nucleus: not visible in fresh specimens, but only in SAF-fixed specimens or specimens stained with trichrome or hematoxylin. Thin nuclear membrane without peripheral chromatin. In this species, the karyosome is highly polymorphic, both in size and in position relative to the nucleus, and can be:

-     Round, large, and located centrally (appearing like the eye of a bird) or eccentrically, similar to that of Iodamoeba buetschlii but without achromatic granules (chromosomes);

-     Oval;

-     Irregular or Y-shaped;

-     Adjacent to the nuclear membrane, in a single mass or fragmented.

Cytoplasm: granular, often containing small, well-defined vacuoles, usually “cleaner” than that of Iodamoeba buetschlii.

If the karyosome is large, the nucleus of Endolimax nana and Iodamoeba buetschlii trophozoites are very similar. Therefore, correct species identification is possible only after the examination of numerous trophozoites, keeping in mind the distinctive morphological features of the nucleus.

Cyst

Size: 5 to 10 μm long, 3-7 μm wide.

Shape: elliptical or oval, often round when transversally observed.  In fresh, unfixed specimens, the cyst is apparently empty;  it is hyaline and nuclei are never seen.

Nuclei: 2 to 4. In immature binucleate cysts, the nuclei are paired and located at one side, while in mature tetranucleate cysts the nuclei are paired and located at each end.

In fresh, unfixed specimens the cystic cytoplasm is hyaline and the nuclei are not visible, while in unstained, formalin- or SAF-fixed samples the karyosome of the nucleus appears as a small refractile mass surrounded by a clear halo. Peripheral chromatin is absent and the nuclear membrane is not visible unless the specimen is permanently stained (with trichrome or hematoxylin).  Use of Lugol's iodine solution or Bailenger's stain helps visualize the nuclei. However, oil immersion examination (objective 100 x) is preferable to avoid confusing these small cysts with those of Entamoeba hartmanni; the rather uniform color of the cytoplasm of Endolimax nana cysts helps distinguish them from Giardia intestinalis cysts.  Chromatoid bodies are absent. In recently formed immature cysts, the presence of diffuse glycogen can be observed but this disappears rapidly when the cyst matures (Dobell 1919).  With transmission electron microscopy (TEM), one can see ill-defined, homogeneous areas where glycogen had been present.

Images