Evo-Devo of feathers and scales: building complex epithelial appendages

Abstract

IntroductionThe vertebrate body is covered by either scales, feathers or fur to provide warmth and protection. Comparing and contrasting the formation of these different integument appendages may provide insights into their common embryonic origin as well as evolutionary divergence. The reptile integument is mainly made of scales [1]. In birds, there are two major integument appendages: scales on the foot and feathers on most of the rest of the body [2••]. Scales provide protection and prevent water loss. The major innovation of the avian integument was the evolution of feathers, which provide novel functions such as insulation, display (communication), and flight.Chickens have three major types of scales, which are morphologically similar to reptile scales (Figure 1a,b [1,3]). Reticulate scales are found on the foot pad: they are radially symmetric and express α-keratin only. Scutate scales are large and rectangular and are the major type found on the anterior meta-tarsal shank and dorsal part of the toes. Scutella scales are distributed lateral to the scutate scales and are smaller in size but are also rectangular. Both scutate and scutella scales have anterior-posterior polarity, with an outer surface composed of β-keratin and an inner surface and a hinge region composed of α-keratin. Cell proliferation is distributed diffusely in scales [4•] without a localized growth zone (e.g. hair matrix or feather collar), dermal papillae, or follicular structures.Feathers are arranged in specific tracts over the body which are divided by apteric zones (regions without feathers [2••]). The base of each feather follicle contains protected tissues, permitting the epithelial-mesenchymal interactions (epidermal collar and dermal papillae) that provide a source for continuous feather elongation and molting. Epithelial and dermal sheaths lie along the exterior part of the feather, whereas pulp is found within the epithelial cylinder during development. A typical feather is composed of a rachis (primary shaft), barbs (secondary branches), and barbules (tertiary branches; Figure 1c). The variation in feather size, shape and texture is complex. With regard to size, feathers of the same bird are of different length and diameter, and often distributed in a gradient. For shape, types range from down feathers that are mainly radially symmetric (the rachis is either absent or very short) and contour feathers the symmetry of which is mainly bilateral. Flight feathers are © 2000 Elsevier Science Ltd. All rights reserved. The feather is the most complex vertebrate integument appendage ever evolved. How is a flat piece of epidermis transformed into a three level branched structure? Here we present ten complexity levels of integument appendages that correspond to developmental stages of chicken skin and feather precursors recently identified in dinosaur/primitive bird fossils. Cellular and molecular events that convert one complexity level to the next are discussed, including those converting avian foot scales to feathers. HHS Public Acce...

Intestinal TSH production is localized in crypt enterocytes and in villus ‘hotblocks’ and is coupled to IL-7 production: evidence for involvement of TSH during acute enteric virus infection

“…There now is evidence that T cells in conjunction with thyroid hormones from the thymus and the thyroid, which are pharyngeal endoderm derivatives [31][32][33], act together with intestinal T cells to control the cellularity, function and integrity of the chordate intestine, an organ formed from hindgut endoderm [34][35]. This functional interdependence may reflect an ancient relationship, characteristic of all vertebrates, between thyroid hormones and T cells regardless of their origin (thymus or intestine).…”

Adaptation to the sky: Defining the feather with integument fossils from mesozoic China and experimental evidence from molecular laboratories

“…These scientists consider that the bird and dinosaur share common ancestors, such as the basal archosaur, an ancient reptile, and the "feathers" found on dinosaurs resulted from later convergent evolution (Feduccia, 1999;Chatterjee, 1997). In either case, one has to agree that feathers evolved from the reptile integument, and that there is a gradual transformation from the simple scale to the advanced forms of feathers (Prum, 1999;Chuong et al, 2000;Maderson and Homberger, 2000;Sawyer et al, 2003a, b), or through heterochrony of appendage morphogenetic events during embryogenesis (Sawyer and Knapp, this issue).…”

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Development and evolution of the amniote integument: Current landscape and future horizon

“…By that time, feathers had gradually emerged as a hot topic due to the spectacular discoveries of feathered fossils in China (see Maderson and Homberger, 2000). From another end, progress had been made at the molecular level, trying to identify molecular basis of tissue interactions as described by Sengel ('76) (Chuong, '93;Chuong et al, 2000a, and references therein). Both Chuong and Homberger had started to work on the development and functional morphology of the avian integument, respectively, in the mid-1980s (Chuong and Edelman,'85;Homberger and Brush,'86) and had recently organized symposia and edited proceedings related to the development and evolution of feathers (Chuong, '98;Maderson and Homberger, 2000).…”

“…Evolutionary novelties that add new axes to organs, or new levels of organization based on molecular pathway modules, have been proposed (Chuong et al, 2000a;Harris et al, 2002). Analyses of the horse hoof and avian feather showed remarkable branching patterns at the level of epitheliomesenchymal interactions that may be based on similar morphogenetic principles (Homberger, 2001b;Bragulla and Hirschberg, 2003).…”