Nucleic Acid and Protein Changes During Embryonic Organ Development in the Chick †

The rapid acceleration of an embryo into an increasingly heterogeneous organism begins early in development with a flood of biochemical changes that accompanies the appearance and growth of organs. The nucleic acids, particularly the ribonucleic acids (RNA), are involved in these profuse changes although the manner of their involvement is only partly understood. We are attempting to define further the role of RNA in embryonic differentiation and organogenesis by several methods. These include among others the histochemical localization of RNA during development and the determination of nucleotide composition and analysis of amino acid incorporation in relation to different species of RNA in the developing embryo. This paper is a report of some baseline studies of the changing levels of nucleic acids (and protein) during the development and growth of seven organs in the chick embryo. A number of reported studies of changes in nucleic acids deal almost entirely either with particular stages of the early total embryo or longitudinally with the total embryo through its development into the newborn animal.`8 These are concerned only with the aggregate of changes in the total embryo, in effect considering the embryo a homogeneous tissue. Few reported investigations deal with changes in nucleic acids in individual organs during embryonic development.9`' Measurements of nucleic acid phosphorous in several developing organs and tissues of the chick embryo were published by Leslie and Davidson'0 (brain, heart, liver, skeletal muscle) and by Szepsenwol, Mason, and Shontze' (brain, heart, liver, skeletal muscle, digestive tract muscle, chorioallantoic membrane), but the results obtained by the two groups of authors are not comparable. Studies