Epigenesis in developing avian scales. III. Stage-specific alterations of the developmental program caused by...

As an approach to the study of a developmental program, 5-bromodeoxyuridine (BrdU) was administered to chick embryos in ovo at various stages of avian scale formation. This brought about stage-specific alterations in morphogenesis in the anterior tarsometatarsus such as feathered scales, from Day 6 through Day 6 1/2; feathers only, from Day 6 3/4 through Day 7 1/4; scalelessness and rudimentary scales, from Day 7 7/8 through Day 8 1/8; and partial ridge scales, from Day 8 1/8 through Day 10. The effects of BrdU were completely nullified by an excess dose of thymidine which instantly suppressed BrdU incorporation into nuclear DNA. Effects of BrdU causing scalelessness were further examined. The percentage of BrdU labeled cells was immunohistochemically detected. It increased linearly in both the epidermis and dermis, reaching nearly 100% 24 hr following its injection on Day 8. However, scale forming potency, as assayed by the area of scale epidermis on Day 11, decreased with the duration of BrdU incorporation into the cells and disproportionately dropped at 15 hr when about 50% of the cells had incorporated BrdU. Scalelessness was also produced when the period of the incorporation of BrdU exceeded 15 hr. Time sequence observations demonstrated epidermal cell shape, polarity, alignment, and packing density to be remarkably disordered so that the placode and interplacode failed to develop on Day 9 1/4. Epidermal-dermal recombinations were carried out by exchanging normal tissues with those treated with BrdU in the anterior tarsometatarsus. The results clearly showed defects in the dermis at the time of reassociation, giving rise to scalelessness.