DNA Polymerase Activity in Extracts of the Developing Chick Skin and Down Feather Organ. ∗

The developing down feather of the chick embryo has recently become increasingly important in studies of organogenesis(1-4). Although the factors involved in feather induction and differentiation have not been precisely defined either morphologically or temporally, several investigators have described events which characterize different developmental stages.

Yessels(3) has found random incorporation of thymidine-H³ into deoxyribonudeic acid (DNA) of the skin during the period prior to feather germ formation (stages 2 7 to 29). During this time, epidermal placodes are formed by a mobilization of the cells rather than by a differential mitotic process. Mesodermal condensations are formed only at stage 30 and are believed to arise from a brief phase of mitotic activity. A nonincor-porating phase occurs from stage 30 to 32. After this, feather germ outgrowth occurs rapidly; at stage 36 [10 days(5) |. it forms a shallow cap. At stage 37 (11 days) barb ridges begin to form on the inner epidermal surface of the distal third of the feather. As the entire organ elongates into a tapered cylinder, the ridges at the base of the feather lengthen. Growth ceases abruptly at the end of the 13th day. Subsequently, keratinization proceeds rapidly with concomitant resorption of the mesodermal pulp.

The incorporation of thymidine into DXA requires the sequential action of a number of enzymes. The initial thymidine utilization may represent merely one of the pathways for the salvage of degraded DNA components. A more direct index of DNA synthesis is the measurement of the activity of a synthetic pathway enzyme. These considerations led us to measure DNA polymerase activity (de-oxynucleoside triphosphate: DNA nucleotidyl transferase, EC 2.7.7.7) in extracts from developing chick skin.

In extracts of mammalian tissues, almost all the measurable DNA polymerase activity has been found in the ultracentrifugal super-nate (S-3) fraction of the tissue homogenate (6). If, however, the homogenization medium contains magnesium or calcium ions, at least part of the activity is associated with nuclei (7). The present paper reports that the DNA polymerase specific activity in the S-3 fraction of homogenates of chick skins removed at intervals throughout the period of down feather development undergoes a cyclic variation.