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Introduction
The hen's eggshell comprises six layers, formed in the acellular environment of the uterine fluid (Nys et al., 1991; 1999) which is supersaturated with calcium and bicarbonate and contains organic precursors of the shell matrix (Parsons, 1982; Hincke et al., 2000; Fernandez et al., 2003; Nys et al., 2004). The eggshell layers are deposited sequentially as the egg passes through the different regions of the oviduct (Wyburn et al., 1973; Hodges, 1974; Gautron et al., 1997; Johnson, 2000; Lavelin et al., 2000; Jonchere et al., 2010; Hincke et al., 2010). From inside to outside, the eggshell comprises: the inner and outer shell membranes (70µm thick), mammillary layer or cone layer (~100[mu]m), palisade layer (~300[mu]m), vertical crystal layer (3-8[mu]m) and cuticle (0.5-12.8µm) (Cain and Heyn, 1964; Gilbert, 1971; Simons, 1971; Board, 1982; Johnson, 2000; Rodriguez-Navarro et al., 2002; Lammie et al., 2005; Wellman-Labadie et al., 2008b; Rose and Hincke, 2009; Kusuda et al., 2011). The eggshell is composed mainly of calcite (the most stable form of calcium carbonate) but a thin layer of hydroxyapatite is present in the inner cuticle (Dennis et al., 1996). The cuticle layer of the eggshell is composed of inner calcified and outer non-calcified water insoluble layers which are deposited directly onto the vertical crystal layer of the eggshell (Parsons, 1982). Not all avian eggs have a cuticle layer, and the distribution of cuticle is often patchy (Board and Halls, 1973; Board, 1974; Becking, 1975). The structure of the cuticle is a moderately heritable (r2=0.27±0.13) trait in brown egg laying hens (Bain et al., 2009). It has been suggested the cuticle may function as a lubricant to facilitate egg rotation in the uterus (Rahman, et al., 2009; Kusuda et al., 2011) and it plays an important role in the termination of calcite crystal growth during shell formation (Dennis et al., 1996; Fraser et al., 1999).
An extensive scanning electron microscopic study of the eggshell surface has shown that few eggshells have 100% cuticle cover and that it is possible to score cuticle cover from 'intact' to 'absent' (Samiullah, 2012; Figures 1, 2, 3).