Entamoeba histolytica is a microscopic, naked, parasitic amoeba.
In its life cycle, it passes through three distinct morphological stages or forms:
1. Trophozoite
2. Precystic
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3. Cystic.
1. Trophozoite Stage:
It is also known as the trophic or magna form. It is the active, motile, growing and feeding form which is pathogenic to man. It is a colourless, transparent and irregular mass of living substance, about 20-30 microns in diameter. It resembles the common amoeba in almost all the structural details.
Its surface is covered by an exceptionally thin, transparent, elastic and semi-permeable membrane, the plasmalemma. The body cytoplasm is distinctly differentiated into an outer, clear, hyaline and non-granular ectoplasm, and the inner, more fluid and granular endoplasm.
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The endoplasm contains a single nucleus, and several food vacuoles. The nucleus is usually invisible in the living parasite. In a fixed and stained specimen, it appears as a rounded and vesicular structure. It is bound by a thin and delicate nuclear membrane, whose inner surface is encrusted with a fine peripheral layer of chromatin granules.
There is a distinct but small central endosome or karyosome, often surrounded by a clear area or ‘halo’. The nucleoplasm is marked by spoke-like striations running between the endosome and the nuclear membrane.
Other inclusions of the endoplasm are food vacuoles enclosing red blood corpuscles or erythrocytes, white blood corpuscles or leucocytes, and fragments of epithelial cells and bacteria, etc. Contractile vacuoles are absent since the parasite dwells in an isotonic environment and needs no osmoregulation.
E. histolytica is monopodial, (Gr., monos, single; podos, foot), it gives out a single, large and broad, ectoplasmic pseudopodium in the direction of movement.
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In the pseudopodium, the outer clear ectoplasm remains sharply differentiated from the inner granular endoplasm. Pseudopodial movement is slow and sometimes called limax type movement, as it resembles the slow crawling of a garden slug (Limax sp.).
Food-vascuoles vary in number and size. They contain red blood corpuscles (a characteristic feature), white blood cells or bacteria in various stages of digestion.
2. Precystic or Minuta Form:
It is pre-cystic form which is smaller, spherical, non-feeding, non-motile and nonpathogenic. It measures to about 7-10 microns in diameter and resembles to the trophozoite form in its structure except that it is smaller in size having no pseudopodium and contractile vacuole. It lives only in the lumen of intestine and rarely found in the tissues. It undergoes encystations and helps in the transmission of the parasite from one host to another.
3. Cystic Stage:
During encystations, the minuta form becomes rounded and is surrounded by a thin highly refractile, resistant, flexible, colourless and transparent cyst-wall. The mature cyst of E. histolytica is-a spherical body, 12 to 15 microns in diameter. Its cytoplasm is clear and hyaline containing reserve food in the form of one or two glycogen masses which gradually disappear.
There is also present 1 to more characteristic, refractile, bar-like chromatoid bodies or chromatoid bars with rounded ends. These stain deeply with haematoxylene, like chromatin, hence this name. According to Pitelka (1963), the chromatoid bodies are composed of ribonucleoprotein.
These also disappear as the cysts mature. According to Neal (1966), their disappearance is due to the dispersion of their ribonucleoprotein throughout. The nucleus retains the characters of the trophozoite. To start with, the cyst is uninucleate, but the nucleus divides to form a binucleate and finally a tetranucleate or quadrinucleate cyst.
The tetranucleate cysts pass out with the faeces of the patient and form the infective stage. They appear as minute, shining, greenish, refractile spheres. At low temperature, they can survive for 5-6 weeks and at room temperature for about 1 week. The cysts die if dried or desiccated.