Question
Cyst of entamoeba histolytica
Answer
The cyst of Entamoeba histolytica is the environmentally resistant, infective form, excreted in the stool of an infected individual, responsible for faecal-oral transmission of amoebiasis.
Morphology: round, ranging from 10–20 μm in diameter, with a mature (infective) cyst containing four nuclei (compared to the immature cyst, which has one or two nuclei); each nucleus shows a characteristic small, centrally-placed karyosome and fine, evenly-distributed peripheral chromatin — a distinguishing feature from the non-pathogenic Entamoeba coli, whose cyst can contain up to 8 nuclei with a more eccentrically-placed, larger karyosome.
Chromatoid bodies: characteristic rod-shaped structures with rounded/blunt ends (composed of RNA and ribosomes), seen within the cytoplasm of younger (immature) cysts, tending to disappear as the cyst matures — an important distinguishing feature from E. coli cysts, whose chromatoid bodies (when present) tend to have splintered/pointed ends rather than smooth rounded ends.
Formation and role in transmission: trophozoites in the colon encyst (particularly as they pass into the more distal, drier colonic environment) by rounding up and secreting a resistant wall; the resulting cysts are excreted in formed stool (unlike the fragile trophozoite, typically found in loose/dysenteric stool, which dies rapidly outside the host) and are immediately infective on ingestion by a new host via contaminated food/water.
Laboratory diagnosis: cysts are identified on stool microscopy (direct wet mount or concentration techniques); iodine wet mount enhances visualization of the nuclei; permanent stained smears (trichrome/iron-haematoxylin) allow detailed morphological confirmation, particularly important for distinguishing pathogenic E. histolytica from the morphologically identical but non-pathogenic E. dispar (which requires antigen/molecular methods, not microscopy alone, for definitive differentiation).