Micromeasurement of total and regional CO2 productions in the one-day-old chick embryo

Ion transport across the early chick embryo: I. Electrical measurements, ionic fluxes and regional heterogeneity

The chick blastoderm at the stage of late gastrula is a flat disc formed by three cell layers and exhibiting epithelial properties. Blastoderms were cultured in miniature chambers and their electrophysiological characteristics were determined under Ussing conditions. Under open-circuit condition and identical physiological solutions on both sides, spontaneous transblastodermal potential difference (Voc) of -7.5 +/- 3.3 mV (ventral side positive) was measured. Under short-circuit condition (transblastodermal delta V = 0 mV), the blastoderm generated short-circuit current (Isc) of 21 +/- 8 microA/cm2, which was entirely dependent on extracellular sodium, sensitive to ouabain applied ventrally and independent of extracellular chloride. The net transblastodermal Na+ flux fully accounted for the measured Isc, both under control conditions and with ouabain. The total transblastodermal resistance (Rtot) was 390 +/- 125 omega cm2. Frequently, the Voc, Isc and Rtot showed spontaneous oscillations with a period of 4-5 min. Removal of endoderm and mesoderm did not significantly affect the electrical properties, indicating that the electrogenic sodium transport is generated by the ectoderm. The Voc and Isc measured in the area pellucida (-1.3 +/- 0.8 mV, 9.3 +/- 4.4 microA/cm2) and extraembryonic area opaca (-7.8 +/- 1.1 mV, 31.2 +/- 12.7 microA/cm2) were significantly different. Such a heterogeneous distribution of electrical properties can explain the presence in the blastoderm of extracellular electrical currents found by using a vibrating probe.

CO2 production of the chick embryo during the first day of post-laying development

The carbon dioxide production of the chick embryo cultured in vitro has been determined during the first 24 h of post-laying development using a non-invasive conductometric microtechnique. The mean CO2 production of the whole blastoderm (1) increased from 16 nmol/h at laying to 231 nmol/h at early neurulation, (2) became dependent on exogenous glucose and (3) was closely linked to mechanical tension generated in the blastoderm (loosening from vitelline membrane resulted in a decrease of 56%). In our experimental conditions, no significant influence of carbonic anhydrase on the CO2 production has been detected. The value of the respiratory exchange ratio varied from about 3 at pregastrular stages to 1 at neurula stage and CO2 was produced transiently in presence of antimycin A. Such results indicate that the source of CO2 is not exclusively mitochondrial and that the relative proportions of mitochondrial and non-mitochondrial CO2 productions might vary significantly throughout the early development. Our findings confirm that the metabolism of the chick embryo becomes more and more oxidative from laying onwards and suggest that the modifications of metabolism observed during the studied period of development could be associated with functional differentiation.