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Review
. 2004;48(2-3):181-91.
doi: 10.1387/ijdb.031776my.

The biology of feather follicles

Mingke Yu  1 , Zhicao Yue, Ping Wu, Da-Yu Wu, Julie-Ann Mayer, Marcus Medina, Randall B Widelitz, Ting-Xin Jiang, Cheng-Ming Chuong
Affiliations
Review

The biology of feather follicles

Mingke Yu et al. Int J Dev Biol. 2004.

Abstract

The feather is a complex epidermal organ with hierarchical branches and represents a multi-layered topological transformation of keratinocyte sheets. Feathers are made in feather follicles. The basics of feather morphogenesis were previously described (Lucas and Stettenheim, 1972). Here we review new molecular and cellular data. After feather buds form (Jiang et al., this issue), they invaginate into the dermis to form feather follicles. Above the dermal papilla is the proliferating epidermal collar. Distal to it is the ramogenic zone where the epidermal cylinder starts to differentiate into barb ridges or rachidial ridge. These neoptile feathers tend to be downy and radially symmetrical. They are replaced by teleoptile feathers which tend to be bilateral symmetrical and more diverse in shapes. We have recently developed a "transgenic feather" protocol that allows molecular analyses: BMPs enhance the size of the rachis, Noggin increases branching, while anti- SHH causes webbed branches. Different feather types formed during evolution (Wu et al., this issue). Pigment patterns along the body axis or intra-feather add more colorful distinctions. These patterns help facilitate the analysis of melanocyte behavior. Feather follicles have to be connected with muscles and nerve fibers, so they can be integrated into the physiology of the whole organism. Feathers, similarly to hairs, have the extraordinary ability to go through molting cycles and regenerate. Some work has been done and feather follicles might serve as a model for stem cell research. Feather phenotypes can be modulated by sex hormones and can help elucidate mechanisms of sex hormone-dependent growth control. Thus, the developmental biology of feather follicles provides a multi-dimension research paradigm that links molecular activities and cellular behaviors to functional morphology at the organismal level.

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Figures

Fig. 1
Fig. 1. The basics of feathers
(A) A rooster with plumages. Notice the different feather tracts on different parts of the body. Ca, caudal tract; Ce, cervical tract; Fe, femoral tract; Hu, humerus tract; Sc, scale region; Sp, spinal tract. (B) Major types of feathers: radially symmetric downy feather, bilaterally symmetric contour feather, and bilaterally asymmetric flight feather (remiges). Schematic diagrams to show (C) the three basic levels of feather branches, and (D) the major zones of cellular activities of a sectioned developing feather follicle. (A) is from Green-Armytage, (2000). (B) is from Lucas and Stettenheim, (1972).
Fig. 2
Fig. 2. Formation of feather follicles
(A) Schematic three-dimensional view of feather follicles. (B) Longitudinal and cross section view of developing feather filaments, showing the three sequences of maturation within the feather follicles: proximal - distal, posterior - anterior (rachis side), center - periphery. The two longitudinal sections represent stages of developing branches and degenerating pulps. The cross sections show forming barb ridges. (C) Different stages of barb ridge formation showing how a flat layer of cells is transformed into branching structures. The basal layer is white and the pulp mesenchyme is at the bottom. The supra-basal layer is green. The feather filament epidermis then forms barb ridges (br). The marginal plate (mp) is red. The differentiated barbule plate (bp) is blue. The growth zone (GZ) is brown and later differentiates into the ramogenic zone (rm). Ap, axial plate. Pe, pulp epithelium. (A) and (B) are modified from Lucas and Stettenheim, (1972). (B′) is from Sengel, 1976. (C) is modified from Chuong and Edelman, 1985.
Fig. 3
Fig. 3. Lineage of feather epidermal and dermal cells
The epithelial and mesenchymal lineage and their potential interactions are summarized. (Modified from Lucas and Stettenheim, 1972).
Fig. 4
Fig. 4. Examples of molecular expression in feather follicles
(A) Longitudinal sections with SHH in situ hybridization. (B) In situ hybridizations of BMP4, BMP2 and Noggin (From Yu et al., 2002).
Fig. 5
Fig. 5. Retroviral-mediated gene transduction in regenerating feathers
(A) Strategy showing feathers plucked, infected with RCAS retroviral vectors and allowed to regenerate. (B,C) X-gal staining of sections from feather follicles infected with RCAS-LacZ (From Yu et al., 2002). (D,E) Detection of Noggin transcripts in cross sections of feather follicles infected with RCAS-Noggin (From Yu et al., 2002). (F) UV light view of a cross section of feather follicles infected with RCAS-GFP (unpublished data).
Fig. 6
Fig. 6. Altering feather branch patterns with molecular misexpression
Feathers regenerated from follicles injected with RCAS-BMP2; RCAS-BMP4; RCAS-noggin and RCAS-Shh antisense showed altered rachis, barb and bar-bule conformation. While misexpression of BMP2 or BMP4 caused barb fusions and ectopic rachis-like structures, Noggin caused further branching of barbs and rachis. Blocking of Shh by RCAS-Shh antisense caused a failure of barb separation to form a web-like epithelial sheet (modified from Yu et al., 2002).
Fig. 7
Fig. 7. Pigment patterns of feathers
(A) Representative pigment patterns within a feather. Feathers are from chicken, zebra finch and peacock. (B) From these, some basic patterns such as barbs, chevrons, circles, dots, etc. are deduced. Note pigments in the left and right vane are under different control. Please see text. (C) There are also pigment patterns at the level of the whole body.
Fig. 8
Fig. 8. Connection of feather follicles with muscles
(A) The exquisite muscle connections of mature feather follicles (from Lucas and Stettenheim, 1972). (B-D) During development, the interbud mesenchyma are at first homogenous (E10), but gradually form specifically arranged muscle and tendon fibers which link feather follicles (E18).
Fig. 9
Fig. 9. Innervations of feather follicles
(A) The nerve network around the feather follicles (Lucas and Stettenheim, 1972). (B) During development, nerves gradually grow in and form arcades surrounding the feather buds (visualized by DiI labeling). (C,D) Nerves are labeled green by antibody to neural filaments.
Fig. 10
Fig. 10. Feather molting cycle
(A) Diagram showing the first three feather cycles. (B) Table showing cycling time of different feathers. (Modified from Lucas and Stettenheim, 1972).
Fig. 11
Fig. 11. Sexual dimorphism of feathers
(A) Left, male chicken. Right, female chicken. There are several differences in the integument between the rooster and the hen. These include the erect comb, bigger wattle, long and slender saddle feathers, bigger and curved caudal feathers. (B) Comparison of caudal feathers (rectrices). Male feathers are longer, wider, and have a curvature. Images in (A) are from www.feathersite.com. The male picture is courtesy of Andy Vardy, Melbourne National 1995.
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