A quantitative analysis of ovalbumin utilization by the cultured chick embryo and its relationship to growth regulation during development

Chick embryo explants of 11–13 somites were cultured on defined medium containing a 9:1 mixture of ovalbumin- 14C and conalbumin- 12C for periods up to 48 hr. After 24 hr of culture on labeled medium some explants were transferred to unlabeled medium for the remaining 24 hr of culture. The amounts of intact ovalbumin- 14C (antibody precipitable), ovalbumin- 14C as breakdown products (trichloroacetic acid soluble) and ovalbumin- 14C converted to embryonic proteins were measured in the various regions of the explants. The relatively greater quantities of intact ovalbumin in the area opaca region of the explant during the 48-hr culture period suggested that this region was the site of intact protein uptake. During the first 6-hr of culture on labeled medium, the rate of uptake of intact ovalbumin by the area opaca exceeded the rate of accumulation of ovalbumin breakdown products, and, after transfer to unlabeled medium, the rate of decrease in intact ovalbumin exceeded the rate of decrease of ovalbumin breakdown products. These results indicated that the ovalbumin was not transported intact to the embryo, but rather was degraded by the area opaca. The larger pool of ovalbumin breakdown products in the area opaca compared to that in the embryo suggested that these pools were not in direct equilibrium. Despite the different pool sizes, both the area opaca and the embryo accumulated embryonic protein derived from ovalbumin at similar rates. A model is proposed describing the flow of nitrogenous nutrients from the medium to the embryo proper.