Gas diffusion in the shell membranes of the hen's egg: lateral diffusion in situ

Non-invasive measurement of oxygen partial pressure, lateral diffusion and chorioallantoic blood flow under the avian eggshell

We measured P(O2) under the shell of avian eggs indirectly, by sealing 0.05 mL glass tubes to the shell, sealing them with mercury and using an oxygen microelectrode to measure the contained gas that equilibrates with the gas in the shell membranes. This technique requires a smaller area of contact with the shell and a shorter equilibration period than established techniques, and allows measurements at several locations simultaneously and over a long period of time without endangering the embryo. P(O2) under the shell of chicken eggs decreased to 14.3 kPa on the day before hatching (day 19). P(O2) was unstable during late development and differences up to 3.1 kPa occurred transiently on opposite sides of the equator. By waxing the shell around sampling tubes, we estimated Krogh's coefficient for lateral oxygen diffusion in the shell membranes at 1.1 mmol cm(-1) d(-1) kPa(-1), a value about a third of a previous estimate. Sampling of gas under sufficiently large regions of waxed shell allowed indirect measurements of chorioallantoic venous P(O2), without affecting embryonic respiration. Venous P(O2) was 3.8 kPa on day 19. Assuming 14.3 kPa represents arterialized blood leaving the chorioallantois, it became possible to calculate the effective chorioallantoic blood flow rate, which was 3.5 mL min(-1) on day 19.

Avian embryos in hypoxic environments

Avian embryos at high altitude do not benefit of the maternal protection against hypoxia as in mammals. Nevertheless, avian embryos are known to hatch successfully at altitudes between 4,000 and 6,500 m. This review considers some of the processes that bring about the outstanding modifications in the pressure differences between the environment and mitochondria of avian embryos in hypoxic environments. Among species, some maintain normal levels of oxygen consumption ( VO2) have a high oxygen carrying capacity, lower the air cell-arterial pressure difference ( PAO2 - PaO2 ) with a constant pH. Other species decrease VO2, increase only slightly the oxygen carrying capacity, have a higher PAO2 - PaO2 difference than sea-level embryos and lower the PCO2 and pH. High altitude embryos, and those exposed to hypoxia have an accelerated decline of erythrocyte ATP levels during development and an earlier stimulation of 2,3-BPG synthesis. A higher Bohr effect may ensure high tissue PO2 in the presence of the high-affinity hemoglobin. Independently of the strategy used, they serve together to promote suitable rates of development and successful hatching of high altitude birds in hypoxic environments.

Effect of local shell conductance on the vascularisation of the chicken chorioallantoic membrane

The vascularisation of the chorioallantoic membrane (CAM) of avian embryos is influenced by environmental oxygen partial pressure (P(O(2))) on a global level: incubation at high P(O(2)) reduces the density of pre- and post-capillary vessels of the CAM and decelerates the thinning of the blood-gas barrier, and vice versa. This study investigates the effects of local P(O(2)) on vascular development during the formative period of days ten to fifteen, by making half of the egg hypoxic and the other half hyperoxic. The densities of arterioles, venules and capillaries were reduced under the hypoxic side, compared to untreated eggs, but not significantly changed on the hyperoxic side. Harmonic mean thickness of the tissue barrier and total CAM blood volume were not affected by the treatments. Vascular development of the CAM was therefore only partly influenced by local P(O(2)).

Effect of regional changes to shell conductance on oxygen consumption and growth of chicken embryos

Diffusive and perfusive gas conductances of the shell and chorioallantois of avian eggs are not homogeneous, leading to disequilibrium between the air cell gas and chorioallantoic venous blood. This study measured the embryonic ability to maintain oxygen consumption rate and growth under experimentally enhanced inhomogeneity of diffusive conductance, achieved by waxing half of the eggshell and exposing the other half to graded hyperoxia that doubled the oxygen gradient across the shell. Thus, the overall gas conductance was effectively normal but highly inhomogeneous. No differences were observed between the experimental and the control eggs in respiration, residual yolk or embryo growth rate from day 12 until day 20. The embryo was able to utilise the increase of oxygen in the environment to compensate for the loss of about half of its respiratory surface area under the wax. The results indicate that an even distribution of pores in the eggshell and uniformity of chorioallantoic perfusion are not required for successful avian development.

Regional hypoxia elicits regional changes in chorioallantoic membrane vascular density in alligator but not chicken embryos

Hypoxic incubation increases vascularization in the chick chorioallantoic membrane (CAM). The effect of regional hypoxia on the vascular density of American alligator (Alligator mississippiensis) and chicken (Gallus gallus) CAMs was studied to determine if hypoxic proliferation of blood vessels is localized or global across the CAM. Eggs were incubated under normoxic conditions with a portion of the eggshell covered with non-toxic beeswax to induce external regional hypoxia. CAMs were examined under a microscope with a 'bulls eye' coverslip and a vascular density index (VDI) was determined. The hypoxic portions of the alligator CAMs were more vascular than the normoxic portions (VDI = 200.9 versus 157. 8, respectively). Presumably this response is maladaptive by causing increased blood flow to the poorly oxygenated portions, i.e. increased 'shunt'. Thus, we hypothesize increased vascularity due to hypoxic incubation is due to local release and subsequent rapid local breakdown or uptake of angiogenic factors. In contrast, the hypoxic and normoxic portions of the chick CAMs exhibited virtually no difference in VDI (VDI= 211.5 versus 217.9, respectively). We suggest the air cell and air space of the chicken eggs allows for circulation of gas in ovo, eliminating the possibility of regional internal hypoxia.

Effects of drilling holes into the air cell of incubated goose eggs on distribution of oxygen partial pressures under the shell

The purpose of this work was to measure changes in oxygen pressure in the air cell and under the eggshell (P(A)O2) of pre-pipping goose eggs before and after drilling holes into the air cell. Drilling a 0.6 mm (diameter of 0.9 mm) hole into the air cell caused an increase in air cell P(A)O2 of about 10 Torr. The rate of increase attenuated as hole area increased and reached about 21 Torr when the drilled area was 8.5 mm2. The P(A)O2 of intact eggs was not equally distributed under the shell. It was high in the air cell area (108 Torr) and decreased towards the pointed end (86 Torr). The increase in P(A)O2 after drilling a 4.9 mm2 hole was high in the air cell (18 Torr) and decreased with distance, becoming non-significant at the pointed end. The significant increase in P(A)O2 after drilling was limited to a distance of up to 38 mm along the shell from the edge of the air cell. This indicates that lateral diffusion in the shell membranes under the shell is limited. Drilling a hole of 3.5 to 4.9 mm2 was enough to increase air cell P(A)O2 in most of the eggs above the critical value of 100 Torr for hatching success. The increase in P(A)O2 was limited to about half the area of the shell and the average increase in P(A)O2 was 6.3 Torr (equivalent to a 0.9% increase in ambient O2). However, the blood perfusing chorioallantoic areas further away from the air cell edge may not be fully saturated with O2 and may not be sufficient to compensate fully for the low O2 availability caused by low eggshell conductance.

Artificial increase of eggshell conductance improves hatchability of early laid goose eggs

1. The purpose of this work was to test the possibility of increasing the hatchability of goose eggs with low mass specific eggshell gas conductance (Gsp), by drilling holes through the eggshell into the air cell, and thus solving both the low water loss rate and low oxygen availability problems. 2. A linear relationship was found between the area of a hole drilled and the apparent increase in eggshell gas conductance (G). Drilling more than one hole increased apparent G 3-6 times more than one hole only, of the same total area. 3. Hole-drilling did not increase egg contamination. The drilling of a 5 mm2 hole on day 17 of incubation increased hatchability both in laboratory tests and in commercial hatcheries (6.1% and 10.5% respectively). 4. Drilling holes on days 15 to 22 of incubation increased hatchability when the predicted mean water loss was lower than 14%. Drilling on day 25 did not have a significant effect, and drilling on day 11 of incubation was too early. 5. Drilling a hole into the aircell (during the second half of incubation) may increase hatchability of low conductance eggs, although oxygen pressure under the eggshell should then be checked in order to evaluate oxygen availability to the embryo.

Oxygen uptake and chorioallantoic blood flow changes during acute hypoxia and hyperoxia in the 16 day chicken embryo

Oxygen consumption rate (MO2) of hen eggs was measured on incubation day 16 (37.8 degrees C, 55% humidity) during acute exposure (90 min) to ambient hyperoxia (FI02 = 0.42) or hypoxia (FIO2 = 0.105). During the last part of these exposures, an H2 washout method was used to estimate relative changes in chorioallantoic membrane (CAM) blood flow, taking as an index the net change in the H2 washout rate constant between any experimental condition and the circulation arrested egg. Doubling normoxic FIO2 increased MO2 to an asymptotic value which was 4% above the normal (P less than 0.05; MO2 in normoxia = 890 mumols/h) even after correcting for the normoxic increase in MO2 with time during development (delta MO2/delta t = 21.5 mumols/h2; P less than 0.001). Halving FIO2 reduced MO2 calculated in the same way to 388 mumols/h. The estimate of the CAM blood flow, relative to normoxia, was 1.12 in hyperoxia (not significant, P = 0.05) and 0.68 in hypoxia (P less than 0.001). The limited changes in CAM blood flow and MO2 during hyperoxia indicate that they are both already close to their maximal values in normoxia. During acute hypoxia the 16 day embryo behaves as an oxygen-conformer; however, the small relative decrease in MO2 per unit of the flow index observed during hyperoxia suggests that the embryo can regulate its CAM blood flow to a small extent. The survival of the embryo and its recovery from hypoxia without a detectable O2 repayment suggest small if any anaerobic regulatory pathways and indicate a true metabolic depression.