Avian eggshell structure and osteopontin

Development of hydrogel-based composite scaffolds containing eggshell particles for bone regeneration applications

This study describes the development and characterization of novel composite scaffolds, made of an alginate-chitosan hydrogel matrix containing eggshell (ES) particles, for bone tissue engineering applications. Scaffolds with ES particles, either untreated or treated with phosphoric acid to create a nanotextured particle surface, were compared to scaffolds without particles. Results indicate that the nanotexturing process exposed occluded ES proteins orthologous to those in human bone extracellular matrix. Scaffolds with ES or nanotextured ES (NTES) particles had a higher porosity (81 ± 4% and 89 ± 5%, respectively) than scaffolds without particles (59 ± 5%) (p = .002 and p < .001, respectively). Scaffolds with NTES particles had a larger median pore size (113 μm [interquartile range [IQ]: 88-140 μm]) than scaffolds with ES particles (94 μm [IQ: 75-112 μm]) and scaffolds without particles (99 μm [IQ: 74-135 μm]) (p < .001 and p = .011, respectively). The compressive modulus of the scaffolds with ES or NTES particles remained low (3.69 ± 0.70 and 3.14 ± 0.62 kPa, respectively), but these scaffolds were more resistant to deformation following maximum compression than those without particles. Finally, scaffolds with ES or NTES particles allowed better retention of human mesenchymal stem cells during seeding (53 ± 12% and 57 ± 8%, respectively, vs. 17 ± 5% for scaffolds without particles; p < .001 in both cases), as well as higher cell viability up to 21 days of culture (67 ± 17% and 61 ± 11%, respectively, vs. 15 ± 7% for scaffolds without particles; p < .001 in both cases). In addition, alkaline phosphatase (ALP) activity increased up to 558 ± 164% on day 21 in the scaffolds with ES particles, and up to 567 ± 217% on day 14 in the scaffolds with NTES particles (p = .006 and p = .002, respectively, relative to day 0). Overall, this study shows that the physicochemical properties of the alginate-chitosan hydrogel scaffolds with ES or NTES particles are similar to those of cancellous bone. In addition, scaffolds with particles supported early osteogenic differentiation and therefore represent a promising new bone substitute, especially for non-load bearing applications.

Removal of Methylene Blue from Aqueous Solution by Mixture of Reused Silica Gel Desiccant and Natural Sand or Eggshell Waste

The purpose of this work was to develop, characterize and test new low-cost materials suitable for removing methylene blue dye from water and wastewater by adsorption. The solid materials consisted of silica gel powder (SG), silica gel mixed with eggshell powder (SG-ES) and a mixture of silica gel with sand from the western Iraqi desert (SG-SI). The samples were milled by using an electrical mixer and a ball mill, followed by a drying step. In addition, desert sand was acid-treated in order to remove impurities. The structure and chemical composition of the samples were investigated by X-ray diffraction (XRD), a scanning electron microscopy technique equipped with an energy-dispersive X-ray spectrometer (SEM-EDX), a low-temperature nitrogen adsorption (BET) technique, thermo-analytical (TG/TGA) measurements and Fourier-transformed infrared spectroscopy (FTIR). The previously mentioned materials were tested to remove methylene blue from an aqueous solution. The adsorption experiments were monitored by ultraviolet-visible (UV-Vis) spectrophotometry and showed that SG and SG-ES gave promising results for the methylene blue removal from water. After 40 min of treatment of the aqueous solution containing 10 mg/L of MB at room temperature, the tested SG, SG-ES and SG-SI materials were found to have 86%, 80% and 57% dye adsorption efficiency, respectively. Taking into consideration not only the adsorption activity of the studied material but their availability, cost and concepts of cleaner production and waste minimization, the developed silica gel with eggshell can be considered as a good, cost-effective alternative to commercially available activated-carbon-based adsorbents. Different kinetic and isotherm models were fitted to the experimental results. A pseudo-second-kinetics-order model revealed high correlation fitting, while the Freundlich model was found to appropriately describe the adsorption isotherm. The thermal stability during the possible regeneration process of the SG-ES adsorbent mixture and its interaction mechanism with cationic dye was discussed.

Analysis of ultrastructure and microstructure of blackbird (Turdus merula) and song thrush (Turdus philomelos) eggshell by scanning electron microscopy and X-ray computed microtomography

The unique structure of the egg allows for efficient reproduction on land. Although the functions of the egg are ensured by the concomitant cooperation of all its structures, the eggshell also plays a significant role. Apart from maintaining an aqueous environment within the egg along with controlled gas exchange, the color and pigmentation pattern of eggshell contributes to identification and protection. As a result of all these functions, the structure, shape, and pigmentation of eggshell greatly vary across the class of birds, and understanding these three variability-determining factors may aid in better interpretation of evolutionary mechanisms. In this study, we analyzed for the first time the structure, mineral composition, and characteristics of the pigmentation of blackbird (Turdus merula) and song thrush (Turdus philomelos) eggshells. The shell of blackbird eggs is much thicker compared to the shell of song thrush eggs which is due to a much thicker crystalline and palisade layers. In both species, strongly elongated mammillary knobs are observed, which create a large space between the mineralized shell and the egg membranes. The blackbird egg shell has a higher water vapor conductivity which is due to the larger diameter of the circle and the surface area of individual pores. The primary compound entering the mineral composition of the shell in both species is CaCO₃ however, the thrush egg shells contained more Mg in all layers except the crystalline layer, and S in the crystalline and palisade layers. The two species clearly differ in the size and distribution of pigment spots on the eggshell. We suppose that the differences in shell structure and pigmentation presented in this study may in the future provide a basis for explaining the reasons for the much lower reproductive efficiency of song thrush compared to blackbird.

Concomitant Morphological Modifications of the Avian Eggshell, Eggshell Membranes and the Chorioallantoic Membrane During Embryonic Development

The chicken eggshell (ES) consists of 95% calcium carbonate and 3.5% organic matter, and represents the first physical barrier to protect the developing embryo, while preventing water loss. During the second half of development, calcium ions from the inner ES are progressively solubilized to support mineralization of the embryonic skeleton. This process is mediated by the chorioallantoic membrane (CAM), which is an extraembryonic structure that adheres to the eggshell membranes (ESM) lining the inner ES. The CAM surrounds the embryo and all egg contents by day 11 of incubation (Embryonic Incubation Day 11, EID11) and is fully differentiated and functionally active by day 15 of incubation (Embryonic Incubation Day 15, EID15). In this study, we explored the simultaneous morphological modifications in the ES, ESM and the CAM at EID11 and EID15 by scanning electron microscopy. We observed that the tips of the mammillary knobs of the ES remain tightly attached to the ESM fibers, while their bases become progressively eroded and then detached from the bulk ES. Concomitantly, the CAM undergoes major structural changes that include the progressive differentiation of villous cells whose villi extend to reach the ESM and the ES. These structural data are discussed with respect to the importance of ES decalcification in providing the calcium necessary for mineralization of embryo's skeleton. In parallel, eggshell decalcification and weakening during incubation is likely to impair the ability of the ES to protect the embryo. It is assumed that the CAM could counteract this apparent weakening as an additional layer of physical, cellular and molecular barriers against environmental pressures, including pathogens, dehydration and shocks. However, such hypothesis needs to be further investigated.

Impact of Different Layer Housing Systems on Eggshell Cuticle Quality and Salmonella Adherence in Table Eggs

The bacterial load on the eggshell surface is a key factor in predicting the bacterial penetration and contamination of the egg interior. The eggshell cuticle is the first line of defense against vertical penetration by microbial food-borne pathogens such as Salmonella Enteritidis. Egg producers are increasingly introducing alternative caging systems into their production chain as animal welfare concerns become of greater relevance to today’s consumer. Stress that is introduced by hen aggression and modified nesting behavior in furnished cages can alter the physiology of egg formation and affect the cuticle deposition/quality. The goal of this study was to determine the impact of caging systems (conventional, enriched, free-run, and free-range), on eggshell cuticle parameters and the eggshell bacterial load. The cuticle plug thickness and pore length were higher in the free-range eggs as compared to conventional eggs. The eggshells from alternative caging (enriched and free-range) had a higher total cuticle as compared to conventional cages. A reduction in bacterial cell counts was observed on eggshells that were obtained from free-range eggs as compared to the enriched systems. An inverse correlation between the contact angle and Salmonella adherence was observed. These results indicate that the housing systems of layer hens can modify the cuticle quality and thereby impact bacterial adherence and food safety.

Evolution of the Avian Eggshell Biomineralization Protein Toolkit - New Insights From Multi-Omics

The avian eggshell is a remarkable biomineral, which is essential for avian reproduction; its properties permit embryonic development in the desiccating terrestrial environment, and moreover, are critically important to preserve unfertilized egg quality for human consumption. This calcium carbonate (CaCO₃) bioceramic is made of 95% calcite and 3.5% organic matrix; it protects the egg contents against microbial penetration and mechanical damage, allows gaseous exchange, and provides calcium for development of the embryonic skeleton. In vertebrates, eggshell occurs in the Sauropsida and in a lesser extent in Mammalia taxa; avian eggshell calcification is one of the fastest known CaCO₃ biomineralization processes, and results in a material with excellent mechanical properties. Thus, its study has triggered a strong interest from the researcher community. The investigation of eggshell biomineralization in birds over the past decades has led to detailed characterization of its protein and mineral constituents. Recently, our understanding of this process has been significantly improved using high-throughput technologies (i.e., proteomics, transcriptomics, genomics, and bioinformatics). Presently, more or less complete eggshell proteomes are available for nine birds, and therefore, key proteins that comprise the eggshell biomineralization toolkit are beginning to be identified. In this article, we review current knowledge on organic matrix components from calcified eggshell. We use these data to analyze the evolution of selected matrix proteins and underline their role in the biological toolkit required for eggshell calcification in avian species. Amongst the panel of eggshell-associated proteins, key functional domains are present such as calcium-binding, vesicle-binding and protein-binding. These technical advances, combined with progress in mineral ultrastructure analyses, have opened the way for new hypotheses of mineral nucleation and crystal growth in formation of the avian eggshell, including transfer of amorphous CaCO₃ in vesicles from uterine cells to the eggshell mineralization site. The enrichment of multi-omics datasets for bird species is critical to understand the evolutionary context for development of CaCO₃ biomineralization in metazoans, leading to the acquisition of the robust eggshell in birds (and formerly dinosaurs).

Avian eggshell biomineralization: an update on its structure, mineralogy and protein tool kit

BACKGROUND

The avian eggshell is a natural protective envelope that relies on the phenomenon of biomineralization for its formation. The shell is made of calcium carbonate in the form of calcite, which contains hundreds of proteins that interact with the mineral phase controlling its formation and structural organization, and thus determine the mechanical properties of the mature biomaterial. We describe its mineralogy, structure and the regulatory interactions that integrate the mineral and organic constituents during eggshell biomineralization. Main Body. We underline recent evidence for vesicular transfer of amorphous calcium carbonate (ACC), as a new pathway to ensure the active and continuous supply of the ions necessary for shell mineralization. Currently more than 900 proteins and thousands of upregulated transcripts have been identified during chicken eggshell formation. Bioinformatic predictions address their functionality during the biomineralization process. In addition, we describe matrix protein quantification to understand their role during the key spatially- and temporally- regulated events of shell mineralization. Finally, we propose an updated scheme with a global scenario encompassing the mechanisms of avian eggshell mineralization.

CONCLUSION

With this large dataset at hand, it should now be possible to determine specific motifs, domains or proteins and peptide sequences that perform a critical function during avian eggshell biomineralization. The integration of this insight with genomic data (non-synonymous single nucleotide polymorphisms) and precise phenotyping (shell biomechanical parameters) on pure selected lines will lead to consistently better-quality eggshell characteristics for improved food safety. This information will also address the question of how the evolutionary-optimized chicken eggshell matrix proteins affect and regulate calcium carbonate mineralization as a good example of biomimetic and bio-inspired material design.

Cuticle and pore plug properties in the table egg

Food safety of table eggs is vital since many pathogens can contaminate the unfertilized egg, leading to increased risk of foodborne illness for consumers. The eggshell cuticle is the first line of defense to restrict the entry of egg-associated pathogens, such as Salmonella Enteritidis. The thickness and completeness of coverage of the cuticle layer are heritable traits that are strongly associated with egg resistance to bacterial penetration. The present study characterizes the chemical composition of the eggshell cuticle and structure of pore plugs from table eggs. Eggs collected from both brown and white egg laying Lohmann flocks (early, mid, and late lay) were either unwashed or washed. Pore plugs were characterized using scanning electron microscopy (SEM), and elemental composition was determined using energy-dispersive x-ray spectroscopy (EDS). SEM observations confirmed that the plug formed by the cuticle layer within the eggshell pore remains firmly lodged throughout the commercial washing process. The eggshell thickness and cuticle pore length visualized in brown eggs was significantly higher than in white eggs in hens of all ages. EDS analysis revealed that the pore inner surface was enriched in phosphorus and chemically different from the surrounding bulk eggshell mineral. Detailed assessment of the cuticle chemical composition was performed by Fourier transform infrared spectroscopy (FTIR). Washing of eggs removed cuticle from the eggshell surface. There was a trend of lower cuticle coverage with increasing hen age for white eggs. A significant reduction in the amount of proteins and phosphates and polysaccharides was observed in the cuticle of brown unwashed eggs with hen age. In white unwashed eggs, amides and lipids decreased with hen age; by contrast, the amount of sulfate was highest at mid-lay. The results from our research will assist selective breeding programs that target cuticle integrity and pore plug stability to enhance egg resistance to pathogen penetration and improve food safety.

New zonal structure and transition of the membrane to mammillae in the eggshell of chicken Gallus domesticus

Avian eggshell is a typical bio-engineered ceramics characterized by layer structures. These layers are categorized mainly by the form of crystalline calcite. Whether there exist other layer structures, how the membrane layer is transformed to the carbonate one, what form the carbonate takes after the transition. These questions remain to be clarified. Here we examine the eggshell of chicken Gallus domesticus by optical microscope, scanning electron microscope and transmission electron microscope. We find that there exists another layer structure defined by variation of organic matrices. The transition from the membrane to the mammillary cones is implemented through the calcium reserve assemblies or the mammillary cores. The integrity of the transitional structure was weakens as the reserved calcium is displaced, and loses completely in about 10 days of incubation. As the first deposited carbonate layer after the transition, the mammillary cones comprise amorphous calcium carbonate and clusters/assemblies of calcite crystallites the size about a nanometer, plus bubble pores extending preferentially in the lateral direction. Our results provide new insights into the structure and component of the avian eggshell, and may help decipher the constitution of the bio-ceramics in the perspective of material science.

Nanostructure, osteopontin, and mechanical properties of calcitic avian eggshell

Avian (and formerly dinosaur) eggshells form a hard, protective biomineralized chamber for embryonic growth-an evolutionary strategy that has existed for hundreds of millions of years. We show in the calcitic chicken eggshell how the mineral and organic phases organize hierarchically across different length scales and how variation in nanostructure across the shell thickness modifies its hardness, elastic modulus, and dissolution properties. We also show that the nanostructure changes during egg incubation, weakening the shell for chick hatching. Nanostructure and increased hardness were reproduced in synthetic calcite crystals grown in the presence of the prominent eggshell protein osteopontin. These results demonstrate the contribution of nanostructure to avian eggshell formation, mechanical properties, and dissolution.

Osteopontin regulates dentin and alveolar bone development and mineralization

The periodontal complex is essential for tooth attachment and function and includes the mineralized tissues, cementum and alveolar bone, separated by the unmineralized periodontal ligament (PDL). To gain insights into factors regulating cementum-PDL and bone-PDL borders and protecting against ectopic calcification within the PDL, we employed a proteomic approach to analyze PDL tissue from progressive ankylosis knock-out (Ank^-/-) mice, featuring reduced PP_i, rapid cementogenesis, and excessive acellular cementum. Using this approach, we identified the matrix protein osteopontin (Spp1/OPN) as an elevated factor of interest in Ank^-/- mouse molar PDL. We studied the role of OPN in dental and periodontal development and function. During tooth development in wild-type (WT) mice, Spp1 mRNA was transiently expressed by cementoblasts and strongly by alveolar bone osteoblasts. Developmental analysis from 14 to 240days postnatal (dpn) indicated normal histological structures in Spp1^-/- comparable to WT control mice. Microcomputed tomography (micro-CT) analysis at 30 and 90dpn revealed significantly increased volumes and tissue mineral densities of Spp1^-/- mouse dentin and alveolar bone, while pulp and PDL volumes were decreased and tissue densities were increased. However, acellular cementum growth was unaltered in Spp1^-/- mice. Quantitative PCR of periodontal-derived mRNA failed to identify potential local compensators influencing cementum in Spp1^-/- vs. WT mice at 26dpn. We genetically deleted Spp1 on the Ank^-/- mouse background to determine whether increased Spp1/OPN was regulating periodontal tissues when the PDL space is challenged by hypercementosis in Ank^-/- mice. Ank^-/-; Spp1^-/- double deficient mice did not exhibit greater hypercementosis than that in Ank^-/- mice. Based on these data, we conclude that OPN has a non-redundant role regulating formation and mineralization of dentin and bone, influences tissue properties of PDL and pulp, but does not control acellular cementum apposition. These findings may inform therapies targeted at controlling soft tissue calcification.

Anatomically-specific intratubular and interstitial biominerals in the human renal medullo-papillary complex

Limited information exists on the anatomically-specific early stage events leading to clinically detectable mineral aggregates in the renal papilla. In this study, quantitative multiscale correlative maps of structural, elemental and biochemical properties of whole medullo-papillary complexes from human kidneys were developed. Correlative maps of properties specific to the uriniferous and vascular tubules using high-resolution X-ray computed tomography, scanning and transmission electron microscopy, energy dispersive X-ray spectroscopy, and immunolocalization of noncollagenous proteins (NCPs) along with their association with anatomy specific biominerals were obtained. Results illustrated that intratubular spherical aggregates primarily form at the proximal regions distant from the papillary tip while interstitial spherical and fibrillar aggregates are distally located near the papillary tip. Biominerals at the papillary tip were closely localized with 10 to 50 μm diameter vasa recta immunolocalized for CD31 inside the medullo-papillary complex. Abundant NCPs known to regulate bone mineralization were localized within nanoparticles, forming early pathologic mineralized regions of the complex. Based on the physical association between vascular and urothelial tubules, results from light and electron microscopy techniques suggested that these NCPs could be delivered from vasculature to prompt calcification of the interstitial regions or they might be synthesized from local vascular smooth muscle cells after transdifferentiation into osteoblast-like phenotypes. In addition, results provided insights into the plausible temporal events that link the anatomically specific intratubular mineral aggregates with the interstitial biomineralization processes within the functional unit of the kidney.

Efficacy of dietary phytase supplementation on laying performance and expression of osteopontin and calbindin genes in eggshell gland

This study was conducted to evaluate the effects of different levels of dietary phytase supplementation in the layer feed on egg production performance, egg shell quality and expression of osteopontin (OPN) and calbindin (CALB1) genes. Seventy-five White Leghorn layers at 23 weeks of age were randomly divided into 5 groups consisting of a control diet with 0.33% non-phytate phosphorus (NPP) and 4 low phosphorus (P) diets: 2 diets (T1 and T2) with 0.24% NPP + 250 FTU/kg laboratory produced phytase or commercial phytase and another 2 diets (T3 and T4) with 0.16% NPP + 500 FTU/kg laboratory produced phytase or commercial phytase with complete replacement of inorganic P. The results indicated that there were no significant differences (P > 0.05) in egg production performance and quality of egg during the first 2 months of trial. However, in next 2 months, a significant drop in egg production and feed intake was observed in birds fed diets with low P and 500 FTU/kg supplementation of laboratory produced phytase. Osteopontin gene was up-regulated whereas the CALB1 gene was down regulated in all phytase treatment groups irrespective of the source of phytase. The current data demonstrated that 250 FTU/kg supplementation of laboratory produced phytase with 50% less NPP supplementation and 500 FTU/kg supplementation of commercial phytase even without NPP in diet can maintain the egg production. The up-regulation of OPN and down regulation of CALB1 in egg shell gland in the entire phytase treated group birds irrespective of the source of enzymes is indicative of the changes in P bio-availability at this site.

Eggshell quality: a comparison between Fayoumi, Gimieizah and Brown Hy-Line strains for mechanical properties and ultrastructure of their eggshells

This experiment was designed to study the relationship between mechanical properties and ultrastructure of three genetic groups (Fayoumi, Gimieizah and Brown Hy-Line strains). To assess eggshell mechanical parameters a total of 120 eggs from each strain was used. This study suggested predicting the mechanical properties and ultrastructure of eggshells when egg weight was equal for the three strains (39.8 g). There was a significant difference among strains for shell thickness, eggshell breaking strength and stiffness, with the Fayoumi eggshells recording the highest values compared with other strains. The palisade layer of Gimieizah and Brown Hy-Line eggshells was thinner than that of Fayoumi eggshells. There were significant correlations between palisade length and breaking strength (0.51), shell thickness (0.89) and stiffness (0.48); however, the length of the palisade layer was significantly negatively correlated with elasticity (–0.85). The Fayoumi (local Egyptian strain) showed higher resistance for eggshell breakage than the Hy-Line commercial strain due to their longer palisade layer noting that there were no significant differences in egg weight between the two. The length of the palisade layer plays an important role in eggshell strength, so it is of interest to include these parameters in selection programs aimed to improve eggshell strength. However, a negative correlation between length of palisade layer and elasticity prevented improvement of these two parameters.

Dietary calcium deficiency in laying ducks impairs eggshell quality by suppressing the process of shell biomineralization

The objective of this study was to determine the effects of dietary calcium deficiency on the process of shell formation. Four hundred and fifty female ducks (Anas platyrhynchos) of 22 wk were randomly assigned to 3 groups. Ducks were fed one of two calcium-deficient diets (containing 1.8% or 0.38% calcium, respectively) or a calcium-adequate control diet (containing 3.6% calcium) for 67 d (depletion period), and then ducks of the 3 groups were fed a calcium-adequate diet for an additional 67 d (repletion period). As compared with the calcium-adequate control, the average shell thickness, egg shell weight, breaking strength, mammillae density and mammillary knob thickness of shell from ducks that consumed the diet with 0.38% calcium was significantly decreased (P&lt;0.05) during the depletion period, accompanied by reduced quality of shell and tibia. The mRNA expression of both secreted phosphoprotein 1 (SPP1) and carbonic anhydrase 2 (CA2) in uterus were decreased after feeding calcium-deficient diets (1.8% or 0.38% calcium). Transcripts of calbindin 1 (CALB1), an important protein responsible for calcium transport, and matrix protein gene ovocalyxin-32 (OCX-32) and ovocleidin-116 (OC-116) were reduced in the ducks fed 0.38% calcium but not the 1.8% calcium. Plasma estradiol concentration was decreased by both of the calcium-deficient diets (P&lt;0.05). The impaired shell quality and suppressed functional proteins involved in shell formation could be reversed by repletion of dietary calcium. The results of the present study suggest that dietary calcium deficiency negatively affects the eggshell quality and eggshell microarchitecture probably through suppressing the process of shell biomineralization.

Cell-specific and temporal aspects of gene expression in the chicken oviduct at different stages of the laying cycle

Egg formation and embryonic development occur as the yolk passes through the magnum, isthmus, and shell gland of the oviduct before oviposition in hens. The present study identified candidate genes associated with secretory function of the chicken oviduct after ovulation and contributing to egg formation and oviposition. Hens (n = 5 per time point) were euthanized to recover the reproductive tract when the egg was in the magnum (3 h after ovulation) and the shell gland (20 h after ovulation). Total RNA was extracted from each segment of the oviducts and subjected to Affymetrix chicken GeneChip analysis. Quantitative PCR and in situ hybridization analyses of selected genes confirmed the validity of the gene expression patterns detected using microarray analysis. In particular, ACP1, CALB1, CYP26A1, PENK, RCAN1 and SPP1 expression increased significantly in the shell gland between 3 h and 20 h postovulation, whereas only RCNA1 expression increased significantly in the magnum between 3 h and 20 h postovulation. Results of the high-throughput analysis revealed cell-specific and temporal changes in gene expression in the oviduct at 3 h and 20 h postovulation in laying hens provide novel insight into changes at the molecular and cellular levels of candidate genes related to formation of the egg and oviposition.

Protein constituents of the eggshell: eggshell-specific matrix proteins

In this article, we review the results of recent proteomic and genomic analyses of eggshell matrix proteins and draw attention to the impact of these data on current understanding of eggshell formation and function. Eggshell-specific matrix proteins from avian (ovocleidins and ovocalyxins) and non-avian (paleovaterin) shells are discussed. Two possible roles for eggshell-specific matrix proteins have been proposed; both reflect the protective function of the eggshell in avian reproduction: regulation of eggshell mineralization and antimicrobial defense. An emerging concept is the dual role (mineralization/antimicrobial protection) that certain eggshell matrix proteins can play.

Calcium transport in strongly calcifying laying birds: mechanisms and regulation

Birds that lay long clutches (series of eggs laid sequentially before a "pause day"), among them the high-producing, strongly-calcifying Gallus gallus domesticus (domestic hen) and Coturnix coturnix japonica (Japanese quail), transfer about 10% of their total body calcium daily. They appear, therefore, to be the most efficient calcium-transporters among vertebrates. Such intensive transport imposes severe demands on ionic calcium (Ca2+) homeostasis, and activates at least two extremely effective mechanisms for Ca2+ transfer from food and bone to the eggshell. This review focuses on the development, action and regulation of the mechanisms associated with paracellular and transcellular Ca2+ transport in the intestine and the eggshell gland (ESG); it also considers some of the proteins (calbindin, Ca2+ATPase, Na+/Ca2+ exchange, epithelial calcium channels (TRPVs), osteopontin and carbonic anhydrase (CA) associated with this phenomenon. Calbindins are discussed in some detail, as they appear to be a major component of the transcellular transport system, and as only they have been studied extensively in birds. The review aims to gather old and new knowledge, which could form a conceptual basis, albeit not a completely accepted one, for our understanding of the mechanisms associated with this phenomenon. In the intestine, the transcellular pathway appears to compensate for low Ca2+ intake, but in birds fed adequate calcium the major drive for calcium absorption remains the electrochemical potential difference (ECPD) that facilitates paracellular transport. However, the mechanisms involved in Ca2+ transport into the ESG lumen are not yet established. In the ESG, the presence of Ca2+-ATPase and calbindin--two components of the transcellular transport pathway--and the apparently uphill transport of Ca2+ support the idea that Ca2+ is transported via the transcellular pathway. However, the positive (plasma with respect to mucosa) electrical potential difference (EPD) in the ESG, among other findings, indicates that there may be major alternative or complementary paracellular passive transport pathways. The available evidence hints that the flow from the gut to the ESG, which occurs during a relatively short period (11 to 14 h out the 24- to 25.5-h egg cycle), is primarily driven by carbonic anhydrase (CA) activity in the ESG, which results in high HCO3(-) content that, in turn, "sucks out" Ca2+ from the intestinal lumen via the blood and ESG cells, and deposits it in the shell crystals. The increased CA activity appears to be dependent on energy input, whereas it seems most likely that the Ca2+ movement is secondary, that it utilizes passive paracellular routes that fluctuate in accordance with the appearance of the energy-dependent CA activity, and that the level of Ca2+ movement mimics that of the CA activity. The on-off signals for the overall phenomenon have not yet been identified. They appear to be associated with the circadian cycle of gonadal hormones, coupled with the egg cycle: it is most likely that progesterone acts as the "off" signal, and that the "on" signal is provided by the combined effect of an as-yet undefined endocrine factor associated with ovulation and with the mechanical strain that results from "egg white" formation and "plumping". This strain may initially trigger the formation of the mammillae and the seeding of shell calcium crystals in the isthmus, and thereafter initiate the formation of the shell in the ESG.