Towards a comprehensive model of feather regeneration

Abstract

Understanding of the regeneration of feathers, despite a 140 year tradition of study, has remained substantially incomplete. Moreover, accumulated errors and mis-statements in the literature have confounded the intrinsic difficulties in describing feather regeneration. Lack of allusion to Rudall's (Rudall [1947] Biochem Biophys Acta 1:549-562) seminal X-ray diffraction study that revealed two distinct keratins, beta- and alpha-, in a mature feather, is one of the several examples where lack of citation long inhibited progress in understanding. This article reviews and reevaluates the available literature and provides a synthetic, comprehensive, morphological model for the regeneration of a generalized, adult contour feather. Particular attention is paid to several features that have previously been largely ignored. Some of these, such as the beta-keratogenic sheath and the alpha-keratogenic, supra-umbilical, pulp caps, are missing from mature, functional feathers sensu stricto because they are lost through preening, but these structures nevertheless play a critical role in development. A new developmental role for a tissue unique to feathers, the medullary pith of the rachis and barb rami, and especially its importance in the genesis of the superior umbilical region (SUR) that forms the transition from the spathe (rachis and vanes) to the calamus, is described. It is postulated that feathers form through an intricate interplay between cyto- and histodifferentiative processes, determined by patterning signals that emanate from the dermal core, and a suite of interacting biomechanical forces. Precisely regulated patterns of loss of intercellular adhesivity appear to be the most fundamental aspect of feather morphogenesis and regeneration: rather than a hierarchically branched structure, it appears more appropriate to conceive of feathers as a sheet of mature keratinocytes that is "full of holes.

On the identification of feather structures in stem-line representatives of birds: evidence from fossils and actuopalaeontology

“…Exactly the same morphology-a small pennaceous portion and a long feather sheath-can be observed in the newly developing wing feathers of Rhea americana (Fig. 4d, e), Gallus gallus (Lucas and Stettenheim 1972), Coragyps atratus black vulture (Maderson et al 2009) and Bubo virginianus great horned owl (Prum 2010) as the distal part of the new feather breaks up the feather sheath. As in the early juvenile Similicaudipteryx (STM4-1), half of the developing feather in these birds remains stuck in the sheath, and the distal pennaceous part, its extremely slim rhachis and the broad sheath are clearly separated from each other.…”

Selective biodegradation of keratin matrix in feather rachis reveals classic bioengineering

“…Classical histological studies, recently enhanced by TEM analysis of barb differentiation, recognized three distinct cellular morphologies of b-keratogenic tissues of the feather (Alibardi 2007a,b): (i) a very flattened, proximodistally (with reference to the feather) elongated form that characterizes the outer cortex (epicortex) of the rachis and of the barb rami (epitheloid cells)-not considered any further here; (ii) a cylindrical form, elongated proximo-distally, which characterizes the barbules and down feathers-syncitial barbule cells-the main focus here, and (iii) a polyhedral form, whose axes lack obvious spatial relation to feather axes, which characterizes the medulloid pith that is unique to the rachis and barb rami-mentioned here only functionally (Alibardi 2002; reviewed in Maderson et al 2009). …”

Fault bars in bird feathers: mechanisms, and ecological and evolutionary causes and consequences

“…A major gap in our understanding is that we do not know how fault bars are generated in growing feathers by the feather follicle (Prum & Williamson, 2001;Maderson et al, 2009). Whether fault bars are created by some mechanical force, or by a change in the physiology of the follicle collar cells, or by a combination of the two is unclear.…”

“…Fault bars are oriented approximately perpendicular to the rachis (Riddle, 1908;Prum & Williamson, 2001;Maderson et al, 2009) and run parallel to the alternating light/dark feather growth bands that can be seen in some feathers and that usually record 24 h of feather growth (Brodin, 1993;Jovani & Diaz-Real, 2012). This leaves little doubt that they are generated by some trauma or perturbation affecting the collar cells of the feather follicle that generate the complex structure of growing feathers.…”