Investigation of feather follicle development in embryonic geese.

In the present study, the process of feather follicle formation in the Zi goose, a Chinese indigenous breed, was investigated during various stages of embryonic development by using a modified histological processing method. The results showed that the feather placodes evolved initially at embryonic day (E) 12 on the spinal feather tract, emerging as symmetrical structures. Sequentially, the buds elongated from E14 to E16 with anterior-posterior and proximal-distal asymmetries, and invaginated to form the primary feather follicles, which were identified to develop the contour feathers or remiges. The remarkable observation at this stage was the formation of the feather follicle wall, which was understood to be the result of the epidermis surrounding the base and further invaginating into the dermis. With the differentiation of the barbule plates, the various types of feathers were determined. We proved that the secondary feather follicles simply had radially symmetrical barb ridges, with much smaller diameters than the primary follicles, and that they developed only downy feathers. The primary and secondary follicles evolved independently of each other and formed ranks in a linear fashion. Moreover, quantitative measurements of the densities of both follicles confirmed that the density of the primary follicles sharply reached the maximum at E18, and then decreased gradually. Coincidentally, the secondary follicles started to increase from the age of E18, and up to E26 the density of the secondary follicles exceeded that of the primary follicles. Each of the primary feather follicles was richly encircled with muscles, which pointed to a quadrangularly arranged network in the dermis. The present work lays the foundation for further study of the cellular and molecular mechanisms of feather follicle morphogenesis in geese.