**1. Introduction**

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The Rajidae Family are weakly electric fish (Fessard, 1958). *Psammobatis extenta* (Rajidae Family) is a South American electric fish. Electric organs of the electric fish have constituted the choice system for studying the biochemistry, morphology, physiology and cell biology of the nervous cholinergic system (Barrantes *et al*., 1983; Changeux, 1981; 2010; Fox *et al*., 1990; Prado Figueroa *et al*., 1995; Wittaker, 1977). Electric organs derive embryologically from myoblasts and are constituted by cells called electrocytes.

The electric organs (EO) of *Psammobatis extenta* produce weak electrical discharges to the surrounding environment. Electrocytes have evolved and differential independently, losing the contractile ability. In previous works we employed a microanalysis (EDS-SEM) and documented the presence of aluminium and silicon in significant concentrations. Zinc, oxygen and cupper were also localized.

Silicon (Si) is an essential nutrient for animal biology (Carlisle 1982). It has been shown that silicon is required for bone, cartilage and connective tissue formation (Bissé *et al.* 2005). Silicon may function as a biological cross-linking agent and may contribute to the architecture and resilience of connective tissue (Schwarz 1973). Aluminium (Al) and Si accumulations have been detected in electric organs by a combination of scanning electron microscopy and X-ray spectrometry (EDS-SEM) (Prado Figueroa *et al*., 2008). Al and Si have been also detected in the human cerebral cortex from elderly people by using EDS/SEM (Perl and Brody 1980, Candy *et al*. 1985). These inorganic elements are related to pathological changes in the human brain (Candy *et al.* 1985, Tokutake *et al*. 1995).

Biomineralization is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Chalcedony is a microcrystalline fibrous form of silica (SiO2) and it is the product of biomineralization by silica (Erhlich *et al*., 2010). We have identified chalcedony in living fish electric organs by using a standard polarized light microscope (Prado Figueroa *et al*., 2008). In plane-polarized light, chalcedony is rounded in

The Growth of Chalcedony (Nanocrystalline Silica) in Electric Organs from Living Marine Fish 287

Understanding cellular and molecular mechanisms participating in neurodegenerative

Silicon is an essential element for animals (Carlisle, 1982). Silicon may function as a biological cross-linking agent and may contribute to the architecture and resilience of connective tissue (Schwarz, 1973). It has also been documented that silicon is present as a silanolate, i.e., an ester-like derivative of silicic acid and plays a role in the structural

We could observe aluminium and silicon in the cytoplasmic extracts of *P. extenta* electric tissue (Prado Figueroa *et al*., 2008) using a combination of scanning electron microscopy and X-ray spectrometry (EDS/SEM) (see Fig. 1). The result of this microanalysis is an energydispersive spectrum in which the peaks are localized at energy lines characteristic for each

Fig. 1. Electric organ cytoplasmic extracts from *Psammobatis extenta* on lyophilised paper and metalized with gold were microanalyzed by using EDS/SEM. This energy dispersive spectrum (a) shows high peaks localized at energy lines characteristic for oxygen, silicon and aluminium. Sodium, potassium, calcium and zinc are also observed. Weight and atomic

percents for these elements are indicated next to the spectrum (b).

organization of glycosaminoglycans and polyuronides (Schwarz, 1973).

process is thus an important field of research.

**2.2 Silicon in electric organs** 

element

shape, 12–15 micron in size, translucent, with a low refraction index. The crossed-polarizer image shows first order birefringence colour (grey–white) and radial extinction.

In this chapter, we document the visualization and identification of chalcedony crystals in electric organs, by using a Leica TCS - SP2 Laser Scanning Confocal Microscope (LSCM). Three ion lasers were used i.e.: argon with emission band in 458 nm (cyan), 476 nm (bluegreen), 488 nm (green) and 514 nm (yellow); He/Ne in 543 nm with emission band in red, and He/Ne with emission band in 633 nm (blue). The autofluorescent character of chalcedony (a mineral) allowed us to obtain images of the crystals together with a topographic study. Chalcedony consists of nanoscale intergrowths of silica polymorphs: quartz and moganite (Heaney & Post, 1992). These silica polymorphs, with their two different nanocrystal structures, are described in the present chapter. Quartz and moganite are both identified in three-dimensional (3-D) images using a LSCM and Leica software. 3-D images were generated as "surface": the blank spaces between the pixels are filled. 3-D images were also generated as "wireframe": all pixels are linked with lines, while the blank spaces remain free. Many images generated by differential interference contrast (DIC) are also shown in this chapter.
