Schematic representation of a feather follicle with a growing feather.

Abstract

Adapted from [4]. Legends for the mesenchymal compartment appear in pink. It includes the dermis, the dermal papilla, and the feather pulp. Legends for the epidermal compartment appear in black. The epithelia (follicular wall and feather epithelium) are made of three layers: the basal layer (in blue), the intermediate layer (in orange), and the upper layer (in brown). The feather outer sheath surrounds and protects the feather germ. The feather germ epithelium is composed of four structures from the bottom to the top: i) the epidermal collar, ii) the collar bulge, iii) the epithelial ramogenic zone (ERZ), and iv) the barb ridges. Transiently amplifying cells (hatched area) are localized in the proliferative zone. The basal layer involuted into barb ridges is called the marginal plate. As shown in the enlargement (black frame) of one barb ridge, the basal layer/marginal plate proliferates at barb ridges extremity, thus providing numerous barbs and/or barbules cells. This allows the formation of the axial and barbule plates which give rise to barbs and barbules, respectively. The space between the follicular wall and the feather outer sheath is the follicular cavity. The feather follicle is irrigated by a prominent axial artery. Gene markers expressed in various zones of the FF and used in this study are indicated below each corresponding structure.</p