On the composition of gastroliths from broiler breeders

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When food is swallowed by chicken—which may temporarily store it in the crop— it then goes into the proventriculus or glandular stomach, which secretes mucus, hydrochloric acid and proteolytic enzymes. Next, the food passes into the ventriculus, gizzard or muscular stomach, which consists of two sets of well‐developed muscles that act as the bird's teeth. This “mechanical stomach” grinds, mixes and mashes the consumed feed and digestive juices with previously swallowed stones (softened by the acidic environment of the proventriculus and grinded by the gizzard into tiny pieces). These small stones are referred to as “gastroliths” or “gizzard stones,” and they remain in the ventriculus until they become too smooth or too small, when they are regurgitated or excreted. It is also worth noting that the gizzard has a thick lining that protects the aforementioned muscles. This tough abrasive membrane consists of a polysaccharide‐protein complex (mucoprotein) secreted by glands and hardened by hydrochloric acid: the so‐called “koilin” (Akester, 1986; Coles, 2007; Wings, 2007).
Although aforementioned gastroliths are most commonly found in ground‐living birds, they can also be found many reptiles and a few mammals (e.g., seals and whales), and fossil records confirm their presence in extinct animals such as plant‐eating dinosaurs and marine reptiles like plesiosaurs.
The composition of gizzard stones in birds remains a problem, and its resolution has been delayed as a consequence of the approach chosen in the literature, in which the composition of the gastroliths (or other liths) from different species, for alleged analogy, has been extrapolated without direct investigation. Consequently, the expected constituents of the advocated composition would have been chitosan, glycosaminoglycans, calcite, whewellite, apatite or carbonate apatite. The presence of glycosaminoglycans and chitosan has been postulated on the basis of findings of crustaceans (Luquet et al., 2012); calcite/vaterite and apatite have been suggested due to their usual involvement in biomineralisation processes (Luquet et al., 2012; Müller, 2011; Sigel, Sigel, & Sigel, 2008) and because they are components—together with bilirubinate—of gallbladder stones (San Miguel Hernández et al., 2014); and, in the case of whewellite, because it is the most frequent component of urinary tract stones (Martín‐Gil & Represa, 1982). Nonetheless, if the location of the gizzard in the upper digestive tract is considered, a closer correspondence of the gizzard stones’ composition with the polysaccharides of the outermost layers of bezoars found in herbivorous mammals (Allred‐Crouch & Young, 1985; Martín‐Gil, Blanco‐Álvárez, Barrio‐Arredondo, Ramos‐Sánchez, & Martín‐Gil, 1995) would be expected. Unfortunately, this later model has not been contemplated.
In the study presented herein, we report the vibrational and structural analyses (by ATR‐FTIR and X‐ray powder diffraction respectively) of gizzard stones from broiler breeders.
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