Blastocystis - Morphology

Morphology

Blastocystis has various morphological forms.

Four commonly described forms are the vacuolar (otherwise known as central body), granular, amoeboid, and cyst forms. The appearance of the organism is largely dependent upon environmental conditions as it is extremely sensitive to oxygen. Whether all of these forms exist in the host intestine is unclear.

Vacuolar form

The vacuolar form is the typical cell form of Blastocystis seen in culture and is often used for the identification of the organism. These vacuolar forms vary greatly in size, with diameters ranging between 2 µm and 200 µm. The vacuolar form is otherwise known as central body form because it has a large central vacuole surrounded by a thin band of peripheral cytoplasm which contains other organelles. Flocculent material has been described as being scattered unevenly throughout the vacuole. The function of the vacuole is still unclear, however, it has been suggested that, like for many eukaryotic cells, it is for storage purposes. Other functions, such as cell division during reproduction and the deposition of apoptotic bodies, have been proposed, although more tests need to be done to validate these roles.

Granular form

The granular form is somewhat morphologically similar to the vacuolar forms except that distinct granules are observed in the central vacuole and / or cytoplasm. Within the central vacuole, these granules appear in different forms too. Three types were suggested – metabolic, lipid, and reproductive granules. Metabolic granules play a role in chemical processes that are necessary for the maintenance of life in the organism. It was also put forward that reproductive granules were involved in the development of progeny cells. These hypotheses were made based on microscopy alone, which may be deemed misleading, hence more need to be done before making a definite conclusion. It has also been suggested that the granules may be an indication that the cell is dying.

Amoeboid form

The other form that exists is the amoeboid form. The amoeboid form of Blastocystis is non-motile and strongly adhesive. A research study has reported that amoeboid forms are produced only in cultures taken from symptomatic individuals, with asymptomatic individuals producing exclusively vacuolar forms. The study suggested this method could be used for diagnosing symptomatic infection. Additionally, it suggested the symptoms could be due to the accumulation of the strongly adhesive amoeboid forms on the host's intestinal wall. A detailed ultra-structural study of amoeboid forms was published in 2007.

Cyst form

The Blastocystis cyst form is a more recent discovery and has helped in the advancement of understanding the way the infection is transmitted. As compared to the other forms, it is generally smaller in size and has a thick multilayered cyst wall. It lacks a central vacuole and few nuclei, multiple vacuoles and food storage deposits were observed. The cyst form is the most resistant form of this parasite and is able to survive in harsh conditions because of its thick multilayered cyst wall. Experiments have been carried out to show its ability to withstand acidic gastric juices. Besides, the cysts did not lyse when placed in distilled water and could survive well at room temperature for up to 19 days, indicating its strong resistance.