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The acidocalcisome

Since the first morphological observations of T. cruzi, there have been descriptions of spherical structures distributed throughout the cell body. Only in 1994 was it shown that this organelle, which contains electron dense deposits (Fig. 18.6), is

Acidocalcisome morphology seen in cells processed using routine methods for electron microscopy. Bar, 200 nm

Figure 18.6 Acidocalcisome morphology seen in cells processed using routine methods for electron microscopy. Bar, 200 nm.

Source: After De Souza, W. Growth and transformation of Trypanosoma cruzi. In: Briggs AP, Coburn, JA, editors, Handbook of cell proliferation. Nova Science Publishers; 2009.

capable of transporting protons and calcium, thus prompting its designation as the “acidocalcisome.”19

The organelles can also be visualized in whole cells dried on a grid using an electron microscope, especially if the microscope is equipped with an energy filter, as shown in Fig. 18.7. Nowadays, it is known that the acidocalcisome contains calcium, phosphorous, sodium, potassium, and zinc. In some trypanosomatids, iron has also been found.20

The acidocalcisomes have many functions as: (1) the storage of calcium, magnesium, sodium, potassium, zinc, iron, and phosphorous compounds, especially inorganic pyrophosphate and polyphosphate, as determined by biochemical analysis and X-ray microanalysis; (2) pH homeostasis; and (3) osmoregulation, a function that involves interaction of the acidocalcisome with the contractile vacuole.19 This involvement is associated with the rapid hydrolysis or synthesis of acidocalcisome poly-P during hypo- or hyperosmotic stress, respectively, in T. cruzi.21

One key function of the acidocalcisomes is the accumulation of phosphate, pyrophosphate, and polyphosphate. The latter molecule is formed via the polymerization of a few to thousands of phosphate residues. These molecules are involved in the process of osmoregulation and are vital for T. cruzi survival during its life cycle, during which it comes in contact with diverse environmental conditions. Polyphosphate hydrolysis has been shown to occur during hypoosmotic stress in T. cruzi, leading to an increase in the acidocalcisome osmotic pressure, which facilitates the movement of water.22,23

Acidocalcisome morphology seen in whole cells examined using electron spectroscopic imaging. Bar, 2 pm

Figure 18.7 Acidocalcisome morphology seen in whole cells examined using electron spectroscopic imaging. Bar, 2 pm.

Source: After De Souza, W. Growth and transformation of Trypanosoma cruzi. In: Briggs AP, Coburn, JA, editors, Handbook of cell proliferation. Nova Science Publishers; 2009.

 
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