br Results br Phytolith found in ESCC tumor tissue
4.1. Phytolith found in ESCC tumor tissue
Only in one ESCC tumor tissue sample, two phytoliths were de-tected; no phytoliths were found in the other three samples, tumor or normal tissues. Fig. 2A shows one phytolith with size and shape similar to the prickle hair on the surface of wheat bract as shown in Fig. 1B. In botanical science, these prickles are one type of the trichomes, which has three major groups: micro-hairs, macro-hairs and prickle hairs (Shakoor et al., 2014). In soil or fossil samples these prickle hairs are sometimes referred to as simply trichomes (Blinnikov et al., 2013) or ‘points’ (Lu et al., 2006). Fig. 2B shows one phytolith with a cross shape.
4.2. Higher silicon content in tumor tissue
With the elemental analysis of the mineral particles identified in the four pairs of tissue samples, the predominant elements are Si, Ca, and P, whereas Si signals are not obvious in the distant normal tissue. Fig. 3A shows sharp signals of Si in the tumor tissue while elemental signals of P and Ca were also obvious, indicating tissue calcification by calcium-phosphate deposition. The sharp Si signal and the relatively weak Al signal indicate the occurrence of free silica SiO2. Fig. 3B indicates that Si signal was not obvious in the distant normal tissue. Fig. 4 shows the identification of silica particles in the ultrathin section of the ESCC tumor tissue by the Si signals in the EDX spectrum; the Pb peaks were from lead citrate in the TEM grid staining material.
Fig. 2. Phytoliths (silica bodies) found in the resected 108321-42-2 tissue of an ESCC patient from north China. A) A phytolith with size and shape similar to the prickle in Fig. 1B. B) A cross shaped phytolith. Ecotoxicology and Environmental Safety 178 (2019) 79–85
After a lengthy review of the literature and a revisited hypothesis on biogenic silica and esophageal cancer in north China, we presented some preliminary findings on the detection of phytoliths and the higher than normal Si concentration in ESCC tumor tissue, which are con-sistent with the hypothesis. However, these findings were based on a pilot study of only three pairs of ESCC tumor and distant normal tissue samples; larger sample size and more systematic exploration along this line will be planned for the next stage of research.
The detection of siliceous prickles, with similar size and morphology to that in wheat bract, in ESCC tumor tissue in the current study war-rants renewed attention to the possible role of biogenic silica in eso-phageal cancer in north China. In the early 1980s, O'Neill et al. (O'Neill et al., 1982) suspected the silica fibers in millet bran to be the source of sharply pointed silica fragments detected in the esophagus of the ESCC patients, but this hypothesis was challenged by van Rensburg (1982) as millet was not the major staple in north China. Interestingly, two sub-sequent population studies found association between wheat (Triticum aestivum) consumption and high risk of ESCC in north China (Chen et al., 1993; Li et al., 1989), consistent with findings from South Africa and Iran (Craddock, 1993). The silicified prickles from wheat chaﬀ (instead of millet bran) might be the major source of the pointed silica fragments in the esophagus. The prickle hairs can be easily broken from wheat chaﬀ during the threshing process, evade the winnowing process because of its higher density, and hence contaminate wheat flour and lodge in the esophagus tissue. r> We have not seen studies comparing silicon content in wheat flour between regions of diﬀerent ESCC incidence in China. But such studies in Iran did not find significant diﬀerences of silicon content in wheat flour from various regions of diﬀerent ESCC rates (Besharat et al., 2007; Jabbari et al., 2008). However, the appropriate comparison might be on the wheat bracts instead of wheat seeds. In cereals, it is in the in-florescence bracts enclosing the seeds that have substantially higher silicon content than in other parts of the plant body (Shakoor et al., 2014). In future studies, silicon content and silica fibers in wheat bracts (glume, lemma, and palea) should be compared between regions of diﬀerent ESCC incidence.
While crystalline silica has been classified as a human carcinogen, the carcinogenicity of amorphous silica is still uncertain. Airborne crystalline silica exposure in occupational settings has been linked with elevated esophageal cancer incidence (Pan et al., 1999; Yu et al., 2005). The silicified prickles on wheat chaﬀ should be mostly amorphous (noncrystalline) although crystalline silica in the form of α -quartz in wheat blades has been reported (Dietrich et al., 2003). Crystalline silica is supposed to have higher durability than amorphous silica in biolo-gical tissues, which may contribute to the carcinogenicity diﬀerences of these two forms. The detection of silica fibers in the ESCC tumor tissue from north China in a previous study (O'Neill et al., 1982) and in the current study, however, indicates that some forms of biogenic silica fibers might be as durable as crystalline silica in biological tissues.