Molecular signaling in feather morphogenesis

Abstract

The development and regeneration of feathers have gained much attention recently because of progress in the following areas. First, pattern formation. The exquisite spatial arrangement provides a simple model for decoding the rules of morphogenesis. Second, stem cell biology. In every molting, a few stem cells have to rebuild the entire epithelial organ, providing much to learn on how to regenerate an organ physiologically. Third, evolution and development ('Evo-Devo'). The discovery of feathered dinosaur fossils in China prompted enthusiastic inquiries about the origin and evolution of feathers. Progress has been made in elucidating feather morphogenesis in five successive phases: macro-patterning, micro-patterning, intra-bud morphogenesis, follicle morphogenesis and regenerative cycling.

Figure 1

Overview of feather morphogenesis. (a) The life cycle of a feather. The five phases of feather morphogenesis are shown in blue. Major morphological events are in red. (b) Five phases of feather morphogenesis and major events. Each event requires a network of molecular signaling pathways. Morphogenetic signaling pathways such as FGF, BMP, Shh and Wnt are used repetitively in different events. Therefore, perturbation experiments in feather morphogenesis can result in phenotypes which are time and context dependent [34^•,49,78].

Figure 2

Highlight of the periodic patterning process. Medium blue indicates the basal state; dark blue indicates bud domains; light blue is inter-bud state; white is apteric region (inter-tract regions). The timing for restrictive or de novo mode of molecular expression are also shown.

Figure 3

Highlights of follicle morphogenesis. (a) Representative feather forms. (b) Idealized proximal follicle showing the topobiological relationship of stem cell, TA cell and differentiating ramogenic zone in radial and bilateral symmetric feathers. (c) Barb ridge orientation in an open follicle preparation. (d) Components of feather follicle (left) and a single barb ridge (right). (e) Barb ridge morphogenesis. A segment from (b) is enlarged. White, basal layer; red, marginal plate; green, suprabasal layer; blue, keratinized barbs. (f) Regenerative feather cycle. The initiation, growth and resting phases are depicted. Red, dermal papilla; blue, epithelia; orange, pulp.