Trophozoites

Entamoeba histolytica is a parasitic intestinal amoebozoan, belonging to the genus Entamoeba. It mostly infects humans and some primates leading to amoebiasis. These anaerobic parasites occur in three morphological stages or forms –

• Trophozoites
• Precyst
• Cyst

Table of Content :

• Trophozoite – Definition
• Trophozoite of Entamoeba histolytica – Overview
• Trophozoites of other protozoans
• Frequently Asked Questions

Trophozoite – Definition

Trophozoite is the activated, growing or vegetative stage in the life cycle of the specific parasite. In this stage, they are usually growing and absorbing nutrients from their hosts.

Trophozoite of Entamoeba histolytica – Overview

They are typically irregular in shape because of slow gliding, pseudopodial movement. The average size is 20μm, ranging between 12-60μm. Trophozoites of E.histolytica are found in the submucosal and mucosal layers of the large intestine.

The cytoplasm is divided into the ectoplasm and endoplasm. The outer ectoplasm is transparent, clear and refractile having a jerky movement ejected under high pressure while the inner endoplasm is finely granular depicting a ground-glass appearance. This endoplasm consists of the food vacuoles (containing red blood cells or bacteria), erythrocytes, nucleus, sometimes tissue debris and leukocytes. The size of the nucleus varies between 4-6μm and is spherical shaped.

They are the only form found in the tissue. They are small in carriers and convalescents whereas large and actively motile in the freshly passed dysenteric stool. These trophozoites from acute dysenteric stools usually have phagocytosed erythrocytes – diagnostic features which are absent in any other commensal intestinal amoeba.

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In their life cycle, trophozoite is the pathogenic stage. Every 8 hours, they reproduce or divide through binary fission. Heating, drying and chemical sterilization kills them. They thrive and multiply in the crypts of mucous of the large intestine. It feeds on mucosal secretions and starches. It interacts with the normal gut bacteria and hydrolyzes the mucosal cells leading to amoebic colitis. It is this phase of the parasite that produces the characteristic lesions of amoebiasis.

The nuclear structure in stained preparation depict the following:

• Nuclear membrane is lined with a single-layer of uniformly distributed fine chromatic granules
• Karyosome is a tiny dot-like structure, centrally located, encircled by a clear halo
• The void between the nuclear membrane and the karyosome is traversed by a thin thread of linin network possessing a spoke-like radial organization

Trophozoites are non-invasive which is usually attributed to the proclivity of the carbohydrates once established in the human cecum area – where the carbohydrate content is low. The regions where the carbohydrate content is abundant, the entity stays in the lumen. The trophozoites secrete proteolytic enzymes which act on the mucosal epithelium promoting invasion.

Trophozoites of other protozoans 

Trophozoite stage is also found in other protozoans such as the Giardia group, Balantidium coli and Plasmodium. The trophozoites of Giardia colonizes and proliferates in the small intestine. In the life cycle of apicomplexans, they undergo asexual reproduction (schizogony) to form schizonts. The schizonts contain merozoites. 

The trophozoite and cyst stages of Balantidium coli causes a disease called balantidiasis.

Trophozoites are one of the three forms. Learn about related concepts at NEET BYJU’S.

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