Effects of Eggshell Thickness, Calcium Content, and Number of Pores in Erosion Craters on Hatching Rate of Chinese Alligator Eggs

The Chinese alligator (Alligator sinensis), found only in a small region in southeastern Anhui Province, is listed as critically endangered (CR) by the International Union for Conservation of Nature (IUCN) due to its current declining population trend. Any abnormalities in the physical properties of an egg can decrease the hatching rate. In particular, eggshells play an essential role in embryo development, motivating us to analyze the microstructures of the eggshells of Chinese alligators. In this study, we categorized the eggshells into two groups, based on the hatching rates, and analyzed the relationship between the eggshell parameters (eggshell thickness, calcium content, and number of pores in erosion craters) and the hatching rate, as well as the relationships between the eggshell parameters. We found that the shells of the eggs with high hatching rates were thicker than those of the eggs with low hatching rates. There were also fewer erosion-crater pores on the surfaces of the eggs with high hatching rates than on the surfaces of the eggs with low hatching rates. Moreover, the shell Ca content was significantly higher in the eggs with high hatching rates than in the eggs with low hatching rates. Cluster modeling indicated that the highest hatching rate occurred when the eggshell thickness was 200-380 µm and there were 1-12 pores. These results suggest that eggs with adequate Ca contents, thicker shells, and less air permeability are more likely to hatch. Furthermore, our findings can inform future studies, which will be vital for the survival of the critically endangered Chinese alligator species.

Calcium provision to oviparous and viviparous embryos of the reproductively bimodal lizard Lacerta (Zootoca) vivipara

Embryos of oviparous squamate reptiles typically obtain calcium from both yolk and eggshell but differ from other oviparous amniotes (turtles, birds and crocodilians) because they are heavily dependent on calcium-rich yolk. Eggs of viviparous squamates lack calcareous eggshells, and embryos receive calcium solely from yolk or from both yolk and placenta. The pattern of calcium mobilization by amniote embryos has been predicted to influence the evolution of viviparity if embryos are dependent on calcium from the eggshell and calcium placentotrophy evolves subsequent to viviparity. We studied the pattern of maternal provision and embryonic utilization of calcium of an oviparous and a viviparous population of the reproductively bimodal lizard Lacerta vivipara to test the hypotheses: (1) oviparous embryos are not dependent on eggshell calcium and (2) calcium content of viviparous hatchlings does not differ from oviparous hatchlings. Our findings do not support either of these hypotheses because oviparous females oviposited eggs with heavily calcified shells and calcium-poor yolk, and embryonic mobilization of shell calcium was greater than for other oviparous squamates. The calcium content of yolk from viviparous females did not differ from oviparous yolk, but viviparous eggs lacked calcareous eggshells. Uterine secretion by viviparous females compensated for the low calcium content of yolk, and placental calcium transfer was among the highest recorded for squamates. The pattern of calcium provision in these two populations suggests that dependence on uterine calcium, either stored temporarily in an eggshell or transferred directly across a placenta, did not constrain the evolution of reproductive mode in this lineage.

Expression of calbindin-D28K by yolk sac and chorioallantoic membranes of the corn snake,Elaphe guttata

The yolk splanchnopleure and chorioallantoic membrane of oviparous reptiles transport calcium from the yolk and eggshell to the developing embryo. Among oviparous amniotes, the mechanism of calcium mobilization to embryos has been studied only in domestic fowl, in which the mechanism of calcium transport of the yolk splanchnopleure differs from the chorioallantoic membrane. Transport of calcium is facilitated by calbindin-D(28K) in endodermal cells of the yolk splanchnopleure of chickens but the chorioallantoic membrane does not express calbindin-D(28K). We used immunoblotting to assay for calbindin-D(28K) expression in yolk splanchnopleure and chorioallantoic membrane of the corn snake, Elaphe guttata, to test the hypothesis that the mechanism of calcium transport by extraembryonic membranes of snakes is similar to birds. High calbindin-D(28K) expression was detected in samples of yolk splanchnopleure and chorioallantoic membrane during late embryonic stages. We conclude that calbindin-D(28K) is expressed in these extraembryonic membranes to facilitate transport of calcium and that the mechanism of calcium transport of the chorioallantoic membrane of the corn snake differs from that of the chicken. Further, we conclude that calbindin-D(28K) expression is developmentally regulated and increases during later embryonic stages in the corn snake.

Mineral status of embryos of domestic fowl following exposure in vivo to the carbonic anhydrase inhibitor acetazolamide

Eggs of domestic fowl were given daily injections of vehicle (DMSO) or vehicle plus acetazolamide, a potent inhibitor of the enzyme carbonic anhydrase, beginning on day 12 of incubation. Embryos were removed from eggs on days 16 and 18, and carcasses and yolks were analyzed for calcium, magnesium, and phosphorus. Treatment with acetazolamide did not affect the quantity of calcium or phosphorus in carcasses and the effect, if any, on magnesium in carcasses was small. However, calcium content of yolk was reduced substantially by acetazolamide both on day 16 and day 18. The reduction in calcium content of yolk led, in turn, to a reduction in the total quantity of calcium in eggs on days 16 and 18. Embryos exposed to acetazolamide seemingly mobilized less calcium from the eggshell than did control embryos. When faced with a shortfall in the availability of calcium from the eggshell, embryos defended carcass calcium, and the shortfall was reflected in a reduction in the quantity of calcium deposited in yolk. The results of this study support the concept that the enzyme carbonic anhydrase plays a role in solubilization of the eggshell and provision of calcium to embryos.

The effects of nest environment on calcium mobilization by leatherback turtle embryos (Dermochelys coriacea) during development

We investigated the effect of sand moisture content and sand temperature on developmental success and the mobilization of calcium during development using laboratory incubated eggs (n=251) collected from leatherbacks nesting at Parque Nacional Marino Las Baulas, Costa Rica. Calcium concentrations of egg components [eggshell, yolk plus albumen (Y+A) and embryo] changed significantly through incubation for both viable and undeveloped eggs. In developed eggs, eggshell calcium content decreased 42.9% by day 60 of incubation. The Y+A calcium decreased by 20.8% until the last quarter of incubation, and then increased to 0.99% above initial Y+A calcium concentrations just prior to hatching. In undeveloped eggs, eggshell calcium content decreased by 25.7%, with the rate of decrease slowing significantly beyond day 30 of incubation. In contrast, Y+A calcium increased steadily through the 60-day incubation period. Embryos incorporated a higher proportion of calcium when incubated at a lower sand moisture content (5% H(2)O>12% H(2)O) and at lower sand temperatures (28.5 degrees C, 29.5 degrees C>31.0 degrees C). The total wet mass of freshly oviposited eggs was negatively correlated with calcium concentration per gram of eggshell (r=-0.569; P<0.001). Thus, each yolked egg, regardless of initial wet mass, had an average of 1.23 g (+/-0.43 g) of calcium per egg (Mean egg mass: 76.24+/-1.21 g). Both developmental success (24.1%) and hatching success (7.4%) of laboratory-incubated eggs were dependent to a greater extent on temperature than on moisture, with an increase in mortality as sand temperature increased. For natural nests on Playa Grande, developmental success (37.4%) and hatching success (19.8%) were similar in magnitude to the results obtained from the laboratory. The recent ENSO (El Niño Southern Oscillation) event and increased tidal activity may be responsible for the high embryonic mortality measured during the 1997-1998 nesting season.

Evaluation of a protocol for studying the chick chorioallantoic membrane in vitro

Explants of eggshell with and without the chorioallantoic membrane were taken from fertile chicken eggs on day 16 of incubation and exposed in vitro to inhibitors (acetazolamide and benzolamide) of carbonic anhydrase to determine if enzyme inhibition affected release of calcium from the shell. A separate experiment examined the effect of the metabolic poison dinitrophenol (DNP) on release of calcium from explants. Explants with the chorioallantois in situ released more calcium than those lacking the epithelium, but neither the enzyme inhibitors nor DNP affected release of calcium. The lack of effect of the enzyme inhibitors could indicate that activity of carbonic anhydrase is not as important to the release of calcium from the eggshell as has been assumed. However, the absence of an effect of DNP instead indicates that release of calcium mediated by the chorioallantois in vitro simply lacks physiological relevance. Thus, results of this investigation raise doubts that the mechanism underlying release of calcium from the eggshell can be assessed in vitro.

The effect of calcium-regulating hormones on transport of calcium across the chorioallantoic membrane of the chicken embryo

The hormonal form of vitamin D3 (1,25(OH)2D3), parathyroid hormone (PTH), or appropriate vehicle were injected into the yolk sac of eggs of domestic fowl on days 16 and 17 of incubation. The chorioallantoic membrane (CAM) and overlying inner shell membrane were removed from eggs on day 18 and mounted in a Ussing-type apparatus. Transport of calcium was assessed by monitoring movements of radiolabeled calcium. Transport of calcium from the chorionic aspect of the CAM to the allantoic aspect increased considerably with time for all treatment groups except the one receiving PTH. "Back-flux" of calcium (movement of calcium from the allantoic aspect to the chorionic) was negligible for all treatment groups at all sampling periods. PTH treatment did not affect flux of calcium from allantois to chorion but reduced flux from chorion to allantois considerably. The underlying cause of this effect has not been identified. The hormonal form of vitamin D3 did not affect flux of calcium in either direction. These data raise the possibility that control of calcium transport by the CAM may not be the primary function of the vitamin D hormone.

The effect of strain of donor eggs, culture conditions, and experimental protocol on release of eggshell calcium by the chick chorioallantoic membrane in vitro

Factors affecting release of calcium by the chick chorioallantoic membrane in vitro were assessed by culturing explants of shell with and without the chorioallantoic epithelium. Explants were removed from eggs on day 16 of incubation and cultured using techniques similar to those used to examine resorption of bone in vitro. The region of an egg from which an explant is removed, the strain of donor eggs used, and addition of bovine serum albumin to the culture medium did not affect release of calcium. In contrast, exposure of explants to a variety of different culture media (DMEM, Medium 199, BGJb, and RPMI) introduced considerable variation in calcium release. Calcium release in vitro was also examined by removing the blunt end of a fertile egg as well as the embryo and yolk. The remaining preparation was filled with culture medium, covered, and cultured for two 24-h periods, with a change of medium after the first 24 h. Preparations from which the chorioallantoic membrane had been removed released very little calcium during culture, whereas those with the epithelium in place released considerable quantities. However, release of calcium by preparations with the membrane declined during the second interval. When surface area is taken into account, such preparations mobilize less calcium than explants cultured under similar conditions.