AMEBE INTESTINALI

INTESTINAL AMOEBAE

In parasitology handbooks and atlases, the description of the morphology of amoebic trophozoites, above all of the nucleus of the trophozoite, is usually rather incomplete, as it refers only to vegetative forms between two mitoses (interphase), i.e. during a phase of vegetative rest. In permanent stained smears (e.g. treated with trichrome or hematoxylin), the morphology of the trophozoite and the size and appearance of its nucleus can actually be very different from what is commonly described. This is because a single specimen can contain vegetative forms at different stages of cellular division, frozen in time by the fixative. For example, it is possible to observe trophozoites with two nuclei (because in amoebae the cytoplasmic and nuclear divisions are not synchronized) or with nuclei that are larger than normal, atypical, apparently degraded or badly fixed.  In reality, all these appearances correspond to one of the division stages of the vegetative forms.

Trophozoite division (mitosis)

The various stages of mitosis of the trophozoite can be summarized as follows:

-     Prophase: the chromosomes condense and become visible at high magnification under a light microscope.

-     Prometaphase (or late prophase): the nuclear membrane ruptures.

-     Metaphase: the centromeres of the chromosomes are lined up along the equatorial plane (metaphase plate).

-     Anaphase:  the chromatids detach at the level of the centromere and move in opposite directions to the poles (centrioles).

-     Telophase: the chromosomes uncoil, the mitotic spindle disappears and the nuclear envelope reforms around each set of separated chromosomes, resulting in the formation of two new nuclei.

-     Cytokinesis:  the cytoplasm divides and two trophozoites are formed.

Between two successive mitoses, there is a period of the cell cycle called interphase in which the DNA and cellular organelles are duplicated and the mass and size of the cell increase.  During this phase, the chromosomes are not visible inside the nucleus.

During mitosis of Entamoeba histolytica and other amoebae, it must be kept in mind that:

-     RNA is synthesized or accumulated on the peripheral chromatin of the nuclear membrane, so that the visible granules of peripheral chromatin can be considered nucleoli.

-     The karyosome (also referred to as kinetic center, endosome or nucleolus) is now properly called the microtubule organizing center (MTOC), but in this atlas the original term karyosome is used.

-     Achromatic granules (which surround the karyosome) are composed of chromosomes, explaining their inconstant presence;

-     Radial fibers that originate from the karyosome and reach the nuclear membrane correspond to the microtubules of the mitotic spindle; their presence is inconstant too.

Images

Intestinal amoebae affecting man

Genus Entamoeba

The nucleus is composed of:

-     A nuclear membrane lined with a layer of chromatin more or less uniformly distributed;

-     A karyosome that is either small and compact or large and diffuse, centrally or eccentrically located.

This genus includes the following species:

-     Entamoeba histolytica

-     Entamoeba dispar

-     Entamoeba moshkovskii

-     Entamoeba hartmanni

-     Entamoeba coli

-     Entamoeba polecki

Genus Endolimax

The nucleus is composed of:

-     A thin nuclear membrane, without visible chromatin;

-     A large karyosome located centrally (having the appearance of the eye of a bird) or eccentrically, irregular in shape or placed against one side of the nuclear membrane.

This genus includes the following species:

-     Endolimax nana

Genus Pseudolimax

The nucleus is composed of:

-     A thin nuclear membrane, without visible chromatin;

-     A large karyosome located centrally or eccentrically, often surrounded by achromatic granules (chromosomes).

This genus includes the following species:

-     Pseudolimax (Iodamoeba) buetschlii

 

The amoeba’s microscopic diagnostic stages

TROPHOZOITE 

Microscopic identification of the trophozoite is based on the following features:

-     Size: in some cases, size is difficult to measure due to the variable, irregular shape;

-     Type of motility: unidirectional, anarchic, sur place (non-progressive, in place).  Motility is visible only in fresh, not fixed, specimens;

-     Appearance and content of the cytoplasm: finely granular, “clean” or “dirty”;

-     Cytoplasmic inclusions: red blood cells, bacteria, yeast, fungi (e.g. Sphaerita), starch, parasites (e.g. Giardia), etc.;

-     Nucleus:

     Generally only one nucleus is present;

     Karyosome: size and position;

     Peripheral chromatin: absence or presence, and distribution (may be evenly distributed or aggregated in coarse, irregular masses of different sizes)

The nucleus can be parasitized by a fungus (Nucleophaga sp.).

CYST

Microscopic identification of the cyst is based on the following features:

-     Thickness, clarity of the wall and refractile nature of the cyst;

-     Shape and size;

-     Cystic contents: chromatoid bodies (shape and number), vacuoles or   glycogen masses (size and number), inclusion masse (present or absent);

        CHROMATOID BODIES

        These are colorless, refractile elements made of a crystalline array of ribosomes, more frequently found in young cysts. They can be tubby or elongated with rounded ends, straight or slightly curved (sausage-shaped), or needle-like, of various lengths and unorganized, bunched together, or forming a grid. 

-     Nucleus/i:

     Number, which varies according to the species and state of maturation of the cyst;

     Position, compared to the vacuole in immature cysts;

     Karyosome size and position;

     Peripheral chromatin: absence or presence, and distribution (may be evenly distributed or aggregated in coarse, irregular masses of different sizes)