Epigenesis in developing avian scales. I. Qualitative and quantitative characterization of finite cell populations.

Throughout a period from day 8.5 to day 12.5 of incubation of a chick embryo, a finite cell population of scale epidermis was characterized from various view points such as cellular organization, position, shape, area, number of constituent cells, density, and cell proliferation activity. In this study, the preparation of whole mount specimens was found to be quite valuable. On day 8.5, cells in the prospective scale region could be morphologically distinguished in the tarsometatarsus at a certain distance proximally away from the tarsometatarsal-phalangeal joint. --On day 9.25, about 1,100 cells became highly columnar in shape and densely associated, forming a placode structure. In both distally and proximally adjacent regions of this placode, the cells were semiquadrate in shape and loosely associated, leading to the formation of the interplacode structures. Such contrasting difference in cell organization between placode and interplacode was preserved from day 9.25 to day 11. During this period, both the area and number of constituent cells increased greatly in the placode and only slightly in the interplacode. However, cell proliferation activity was completely suppressed in the placode, and quite active in the interplacode. The activity in cell proliferation proved to be inversely correlated with the density of basal cells. Throughout the present study, it has been demonstrated that the early development of scale epidermis is achieved through a coordinated activity of the two discrete cell populations: the placode and interplacode.