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Studies on egg shell calcification Grant, Elisabeth Anne

Abstract

The transport of calcium from the blood of the laying hen (gallus domesticus) to the shell of the developing egg is a very rapid but discontinuous process. These studies were carried out in order to examine the way in which this process is controlled. In the initial experiments calcium translocation across the shell gland was studied using an in vitro system similar to that described by ehrenspeck et al (1967). The magnitude of the observed fluxes was similar to those reported by ehrenspeck et al, but, in contrast to their results, no clear correlation could be found between the flux and the physiological condition of the shell gland. The two hormones of calcium homeostasis, parathyroid hormone and calcitonin, were also without influence on the system. However the dibutyryl derivative of cyclic amp increased the calcium flux from the shell gland lumen to the blood but did not affect flux in the normal physiological direction, i.e. blood → lumen. One fundamental criticism of the in vitro system is that the fluxes measured are extremely low. Calculation of the flux which must occur in vivo showed that this was 500-1000 times larger than the fluxes obtained in vitro. In view of the deficiencies of the in vitro system all subsequent studies were carried out on the intact hen. Two of the most likely mechanisms for control of shell gland function involve either the endocrine system, which the in vitro studies implied was not involved, or the autonomic nervous system. The neural influence upon shell formation was studied using neural blocking agents as the complex anatomy of the shell gland nervous supply precluded neural section. An extensive literature survey failed to provide sufficient data on blocking drugs in chickens so that an extensive series of experiments was performed in order to define appropriate doses to produce 70-80% blockade in the anesthatized hen for periods up to two hours. Using the drug dosages so obtained the effect of blockade of various branches of the autonomic nervous system on the incorporation of intravenously injected 45ca into the egg shell, and on the disappearance of this label from the plasma, was studied in anesthatized hens. The data from both types of experiment showed that, under the experimental conditions, the rate of calcium transport across the shell gland is independent of the nervous supply.

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