## 3 Results and Discussion
Red arrows indicate: ShMb\(\_\)O—outer shell membrane; ShMb\(\_\)—inner shell membrane; Cp—cuitable pore; PsL—palisade layer; MaL—mammillary layer; CuL—cuitable layer. Note: SEM micrographs were taken at the initial stage of storage (0 days). (**A**)—eggs from cage-free housed laying hens (H/HN/BCE); (**B**)—eggs from cage-free housed laying hens (H/D/BWE); (**C**)—eggs from enriched cage-housed laying hens (H/LS/CCE); (**D**)—eggs from enriched cage-housed laying hens (H/HN/CCE). the inhibition of the processes of ion transmission and the crystallization of eggshell formation [32]. The egg's integrity and physical appearance make the first representation to the consumer. If the product does not meet perceived expectations, consumer confidence de-creases [33]. The quality of the eggshell is essential to the processing industries because structurally, whole eggs will arrive at the consumer in the best condition. It has been reported that eggs injured during routine handling and transport to retail outlets cause major financial losses to the egg industry [34]. Therefore, according to visual inspection techniques, defective eggs that demonstrate even minor signs of cracking are sorted out already at the initial processing stage. Unfortunately, the ovoscope, as a frequently used instrument for assessing the quality of eggs, has certain limitations in detecting microcracks, and occasionally defective eggs reach retailers and eventually consumers. In particular, the abundance of long and thick cracks on the outer eggshell surface is reported to promote egg quality deterioration and endanger consumer health due to a risk of microbial cross-contamination, especially when eggs are stored at room temperature [35; 36]. For a more detailed examination of eggshell microcracks, backscattered electrons (BSEs) imaging accomplished by the SEM system was utilized, allowing phase separation between dense and thin surface regions, thereby identifying amorphic regions. BSE imaging revealed obvious microcracks on the outer surface of the eggshell, as illustrated in Figure 2A-C. The availability of thick and long microcracks was observed on the surface of the eggshell of eggs derived from enriched cage-housed hens H/LS/CCE, followed by cage-free housed hens H/D/BWE. The thickness values corresponded to an average of 1.41 and 1.48 \(\upmu\)m, respectively. In both cases, spider-like cracking was seen on the surface of the eggshell. The least number of cracks, as well as their thickness, were found on the surface of the eggshell of eggs derived from cage-free H/HN/BCE and cage H/HN/CCE hens, corresponding to 0.67 and 0.91 \(\upmu\)m, respectively. On the eggshell surface of eggs obtained from cage hens H/HN/CCE, obvious hairline cracks could be seen. Interestingly, [PERSON] et al. [37] highlighted the presence of hairline cracks on the surface of the eggshells that had promoted the most substantial weight loss and decrease in hatchability compared to star-cracked eggs. It is worth noting that from the total number of eggs investigated (\(n=50\)), the eggs derived from cage-free hens of H/HN/BCE represented no apparent signs of eggshell surface cracking. The EDS mapping analyses of the entire eggshell from four egg samples revealed almost similar profiles and composition of elements (data not represented). In general, six base elements, i.e., Na, Mg, P, S, Ca, and O, were detected in the eggshell (Figure 3). As indicated, an intensive signal corresponding to the S element was deposited predominantly on the membrane layer (on average, 0.94% per weight). The availability of S was due to the presence of sulfur-containing amino acids in the collagen-rich fraction, such as cystine (9-10%) and methionine (3-4%) [38].