Evo Devo of the Vertebrates Integument

All living jawed vertebrates possess teeth or did so ancestrally. Integumental surface also includes the cornea. Conversely, no other anatomical feature differentiates the clades so readily as skin appendages do, multicellular glands in amphibians, hair follicle/gland complexes in mammals, feathers in birds, and the different types of scales. Tooth-like scales are characteristic of chondrichthyans, while mineralized dermal scales are characteristic of bony fishes. Corneous epidermal scales might have appeared twice, in squamates, and on feet in avian lineages, but posteriorly to feathers. In contrast to the other skin appendages, the origin of multicellular glands of amphibians has never been addressed. In the seventies, pioneering dermal-epidermal recombination between chick, mouse and lizard embryos showed that: (1) the clade type of the appendage is determined by the epidermis; (2) their morphogenesis requires two groups of dermal messages, first for primordia formation, second for appendage final architecture; (3) the early messages were conserved during amniotes evolution. Molecular biology studies that have identified the involved pathways, extending those data to teeth and dermal scales, suggest that the different vertebrate skin appendages evolved in parallel from a shared placode/dermal cells unit, present in a common toothed ancestor, c.a. 420 mya.

Histology of the Skin of Three Limbless Squamates Dwelling in Mesic and Arid Environments

The skin of limbless squamates has an increased contact with the substrate compared with limbed counterparts. Comparatively, the contact with the substrate is intensified in fossorial species, where the whole circumference of the body interacts with the soil during underground locomotion. Although fossoriality in Squamata, specifically lizards and snakes, has been studied ecologically and morphologically (e.g., osteological changes), not enough detail is yet available regarding changes in organs critical for underground lifestyle such as the skin. Here we used histological and microscopical techniques (scanning electron microscopy and transmission electron microscopy) to uncover the structural detail of the epidermis and dermis in three limbless reptiles, the amphisbaenian Diplometopon zarudnyi, and two snakes, Indotyphlops braminus (Typhlopidae) and Cerastes cerastes (Viperidae). The skin of these taxa shows pronounced morphological diversity, which is likely associated to different environmental and functional demands upon these reptiles. Anat Rec, 299:979-989, 2016. © 2016 Wiley Periodicals, Inc.

Immunocytochemical and autoradiographic studies on the process of keratinization in avian epidermis suggests absence of keratohyalin

The process of keratinization in apteric avian epidermis and in scutate scales of some avian species has been studied by autoradiography for histidine and immunohistochemistry for keratins and other epidermal proteins. Acidic or basic alpha-keratins are present in basal, spinosus, and transitional layers, but are not seen in the corneous layer. Keratinization-specific alpha-keratins (AE2-positive) are observed in the corneous layer of apteric epidermis but not in that of scutate scales, which contain mainly beta-keratin. Alpha-keratin bundles accumulate along the plasma membrane of transitional cells of apteric epidermis. In contrast to the situation in scutate scales, in the transitional layer and in the lowermost part of the corneous layer of apteric epidermis, filaggrin-like, loricrin-like, and transglutaminase immunoreactivities are present. The lack of isopeptide bond immunoreactivity suggests that undetectable isopeptide bonds are present in avian keratinocytes. Using immunogold ultrastructural immunocytochemistry a low but localized loricrin-like and, less, filaggrin-like labeling is seen over round-oval granules or vesicles among keratin bundles of upper spinosus and transitional keratinocytes of apteric epidermis. Filaggrin-and loricrin-labeling are absent in alpha-keratin bundles localized along the plasma membrane and in the corneous layer, formerly considered keratohyalin. Using ultrastructural autoradiography for tritiated histidine, occasional trace grains are seen among these alpha-keratin bundles. A different mechanism of redistribution of matrix and corneous cell envelope proteins probably operates in avian keratinocytes as compared to that of mammals. Keratin bundles are compacted around the lipid-core of apteric epidermis keratinocytes, which do not form complex chemico/mechanical-resistant corneous cell envelopes as in mammalian keratinocytes. These observations suggest that low amounts of matrix proteins are present among keratin bundles of avian keratinocytes and that keratohyalin granules are absent.

Avian skin development and the evolutionary origin of feathers

The discovery of several dinosaurs with filamentous integumentary appendages of different morphologies has stimulated models for the evolutionary origin of feathers. In order to understand these models, knowledge of the development of the avian integument must be put into an evolutionary context. Thus, we present a review of avian scale and feather development, which summarizes the morphogenetic events involved, as well as the expression of the beta (beta) keratin multigene family that characterizes the epidermal appendages of reptiles and birds. First we review information on the evolution of the ectodermal epidermis and its beta (beta) keratins. Then we examine the morphogenesis of scutate scales and feathers including studies in which the extraembryonic ectoderm of the chorion is used to examine dermal induction. We also present studies on the scaleless (sc) mutant, and, because of the recent discovery of "four-winged" dinosaurs, we review earlier studies of a chicken strain, Silkie, that expresses ptilopody (pti), "feathered feet." We conclude that the ability of the ectodermal epidermis to generate discrete cell populations capable of forming functional structural elements consisting of specific members of the beta keratin multigene family was a plesiomorphic feature of the archosaurian ancestor of crocodilians and birds. Evidence suggests that the discrete epidermal lineages that make up the embryonic feather filament of extant birds are homologous with similar embryonic lineages of the developing scutate scales of birds and the scales of alligators. We believe that the early expression of conserved signaling modules in the embryonic skin of the avian ancestor led to the early morphogenesis of the embryonic feather filament, with its periderm, sheath, and barb ridge lineages forming the first protofeather. Invagination of the epidermis of the protofeather led to formation of the follicle providing for feather renewal and diversification. The observations that scale formation in birds involves an inhibition of feather formation coupled with observations on the feathered feet of the scaleless (High-line) and Silkie strains support the view that the ancestor of modern birds may have had feathered hind limbs similar to those recently discovered in nonavian dromaeosaurids. And finally, our recent observation on the bristles of the wild turkey beard raises the possibility that similar integumentary appendages may have adorned nonavian dinosaurs, and thus all filamentous integumentary appendages may not be homologous to modern feathers.

Keratinization and ultrastructure of the epidermis of late embryonic stages in the alligator (Alligator mississippiensis)

Using specific anti-beta keratin and general anti-alpha keratin antibodies, keratins were located in the epidermis of the alligator during the final developmental stages by ultrastructural and immunocytochemical methods. The maturation of the bilayered periderm (= embryonic epidermis) coincides with the disappearance of cell organelles, including the 25-35-nm-thick coarse filaments, and the coalescing of alpha-keratin filaments into a compact mass. The plasmalemma of peridermal cells forms a 15-25-nm-thick electron-dense corneous envelope. These changes start at stage 25, about 3 weeks before hatching, and continue until hatching when the embryonic epidermis is shed. Immature beta-keratogenic cells beneath the embryonic epidermis accumulate immunolabelled beta-filaments which are packed into thin, electron-pale beta-keratogenic cells in the corneous layer. Together, electron-pale and electron-dense materials form a compact 3-4-nm filament pattern of beta-keratin. Melanosomes from epidermal melanocytes, incorporated into beta-cells, give rise to the banded skin pattern of hatchlings. Beta-keratin production is much reduced in the hinge regions, where many alpha-filaments remain packed together with lipid droplets or mucous granules into thinner, more electron-dense, alpha-cells. The keratinaceous material of the alpha-cells is mostly concentrated along the cell membrane, while the lipid/mucous material remains centrally located, as in sebokeratinocytes of the apteric areas of avian skin. Some lipid and mucus is also incorporated into typical beta-cells of the outer scale surface, so that lipids are part of the fully keratinized hard keratin layer of the alligator. Lipids within beta-cells of outer scale surfaces and alpha-cells of the hinge region are probably responsible for limiting water loss and ion movements across the skin. Neither typical mammalian keratohyalin granules nor lepidosaurian keratohyalin-like granules were detected anywhere in alligator epidermis. The combination of anti-beta and anti-alpha keratin antibodies revealed different distributions of beta- and alpha-keratins. In late embryonic stages (25-26 to hatching), beta-keratin occurs only in the upper suprabasal cells, in prekeratinized and keratinized layers, whereas alpha-keratin bundles (tonofilaments) remain only in the lowest layers. The cross-reactivity of the beta-antibody, produced against a chick scale keratin, further shows that avian and crocodilian hard (beta) keratins share common antigenic sites, reflecting a phylogenetic affinity between these taxa.

Keratinization and lipogenesis in epidermal derivatives of the zebrafinch, Taeniopygia guttata castanotis (Aves, Passeriformes, Ploecidae) during embryonic development

Little is known of the lipid content of beta-keratin-producing cells such as those of feathers, scutate scales, and beak. The sequence of epidermal layers in some apteria and in interfollicular epidermis in the zebrafinch embryo (Taeniopygia guttata castanotis) was studied. Also, the production of beta-keratin in natal down feathers and beak was ultrastructurally analyzed in embryos from 3-4 to 17-18 days postdeposition, before hatching. Two layers of periderm initially cover the embryo, but there are eventually 6-8 over the epidermis of the beak. In the beak and sheath cells of feathers, peridermal granules are numerous at 12-14 days postdeposition but they are less frequent in apteria. These granules swell and disappear during sheath or peridermal degeneration at 15-17 days postdeposition. A thin beta-keratin layer forms under the periderm among feather germs of pterylous areas but is discontinuous or disappears in apteria. In differentiating cells of barbs, barbules, and calamus cells of natal down, electron-dense beta-keratin filaments form bundles oriented along the main axis of these cells. Cells of the pulp epidermis and collar, at the base of the follicle, contain lipids and bundles of alpha-keratin filaments. Degenerating pulp cells show vacuolization and nuclear pycnosis. During beta-keratin packing, keratin bundles turn electron-pale, perhaps due to the addition of lipids to produce the final, homogenous beta-keratin matrix. In contrast to the situation in feathers, in the cells of beak beta-keratin packets are irregularly oriented. In both feather and beak epidermal cells the Golgi apparatus and smooth endoplasmic reticulum produce vesicles containing lipid-like material which is also found among forming beta-keratin. The contribution of lipids or lipoprotein to the initial aggregation of beta-keratin molecules is discussed.

CHOXC-8 and CHOXD-13 expression in embryonic chick skin and cutaneous appendage specification

We studied the expression of two distantly clustered Hox genes which could, respectively, be involved in specification of dorsal feather- and foot scale-forming skin in the chick embryo: cHoxc-8, a median paralog, and cHoxd-13, located at the 5' extremity of the HoxD cluster. The cHoxc-8 transcripts are present at embryonic day 3.5 (E3.5) in the somitic cells, which give rise to the dorsal dermis by E5, and at E6.5-8.5 in the dorsal dermal and epidermal cells during the first stages of feather morphogenesis. The cHoxd-13 transcripts are present at E4.5-9.5 in the autopodial mesenchyme and at E10.5-12.5 in the plantar dermis during the initiation of reticulate scale morphogenesis. Both the cHoxc-8 and cHoxd-13 transcripts are no longer detectable after the anlagen stage of cutaneous appendage morphogenesis. Furthermore, heterotopic dermal-epidermal recombinations of dorsal, plantar, and apteric tissues revealed that the epidermal ability or inability to form feathers is already established by the time of skin formation. Retinoic acid (RA) treatment at E11 induces after 12 hr an inhibition of cHoxd-13 expression in the plantar dermis, followed by the formation of feather filaments on the reticulate scales. When E7.5 dorsal explants are treated with RA for 6 days, they form scale-like structures where the Hox transcripts are no more detectable. Protein analysis revealed that the plantar filaments, made up of feather beta-keratins, corresponded to a homeotic transformation, whereas the scale-like structures, composed also of feather beta-keratins, were teratoid. These results strengthen the hypothesis that different homeobox genes play a significant role in specifying the regional identity of the different epidermal territories.

Ultrastructural study of the keratinization of the dorsal epithelium of the tongue of Middendorff's bean goose, Anser fabalis middendorffii (Anseres, Antidae)

BACKGROUND

Comparative studies of ultrastructural features of tongues allow deductions to be made about relationships between structure and function, as reflected by an animal's feeding habits. The present study was performed to serve as a basis for further studies of avian feeding mechanisms and of relationships between the fine structure of the lingual epithelium and the development of the expression of keratins.

METHODS

The light microscope, scanning electron microscope, and transmission electron microscope were used.

RESULTS

The dorsal surface of the tongue of Middendorff's bean goose, Anser fabalis middendorffii, has a distinctive anterior region that extends for five-sixths of its length and has a clear posterior region. The anterior region, when observed macroscopically and by scanning electron microscopy, is distinguished along its forward half by a clear median line. The back half of the anterior region has an indistinct median sulcus in some parts. There are no lingual papillae on the entire dorsal surface of the anterior and posterior regions. Giant conical papillae are located in a transverse row between the anterior and posterior regions. On both lateral sides of the anterior region for five-sixths of the length of the tongue, lingual hairs are compactly distributed, and small numbers of large cylindrical papillae are arranged at almost regular intervals between these lingual hairs. Examination of the dorsal lingual epithelium by light and transmission electron microscopy provided histological and cytological criteria for distinguishing the anterior and posterior regions, both of which were composed of stratified squamous epithelium. Basal cells were similar throughout the dorsal epithelium. The intermediate layer of cells in the anterior region contained numerous tonofibrils in electron-dense bundles composed of tonofilaments of 10 nm in diameter. The outer layer was composed of electron-dense, well-keratinized cells, with layers of electron-lucent cells on the outermost surface. The cells in the intermediate layer in the posterior region of the tongue were almost completely filled with unbundled tonofilaments. The surface layer exhibited features of parakeratinization. In all of the giant conical papillae, the large cylindrical papillae, and the lingual hairs, the epithelium was strongly keratinized.

CONCLUSIONS

The three-dimensional microanatomy and cytological features of the dorsal lingual epithelium of avians seem to be related to the functional role and shape of the tongue of each species in feeding.

Fine structure of the dorsal lingual epithelium of the little tern, Sterna albifrons Pallas (Aves, Lari)

The dorsal surface of the tongue of the little tern, Sterna albifrons, has a distinctive anterior region for five-sixths of its length and a terminal posterior region. The anterior region observed by scanning electron microscopy is distinguished along its forward half by a median line from which median papillae protrude. The hind half of the anterior region has a median sulcus without papillae. The deciduous epithelium on both sides of the median line and sulcus bears scattered epithelial protrusions. The posterior lingual region has neither median papillae nor deciduous epithelium. So-called giant conical papillae are located in a transverse row between anterior and posterior regions. Delicate microridges adorn the surfaces of all outer epithelial cells in both regions. Examination of the dorsal lingual epithelium by light and electron microscopy provides histologic and cytologic criteria for distinguishing anterior and posterior regions. Basal cells are nearly alike throughout the dorsal epithelium. Intermediate layer cells of the anterior region contain numerous tonofibrils in electron-dense bundles composed of 10 nm tonofilaments. The outer layer is composed of electron-dense, well-keratinized cells, and electron-lucent epithelial protrusions are present on the exposed surface of the outermost cells. Median papillae are composed of typical keratinized cells, which are nearly filled with keratin filaments. Intermediate layer cells in the posterior region of the tongue are nearly filled with unbundled tonofilaments. There is only a very thin outer keratinized layer in this region.

Avian scale development. XVII: The epidermis of the scaleless (sc/sc) anterior metatarsal skin is determined, but the dermis lacks permissive cues for the patterned expression of the determined state

Embryos homozygous for the gene scaleless (sc/sc) completely lack scutate scales and the beta strata which characterize terminal differentiation of the scale ridges located on the anterior metatarsal region of the foot. Although the sc/sc epidermis cannot undergo scale morphogenesis, it can respond to the inductive dermal ridges of normal scutate scales by generating beta strata. Recently, we discovered that the anterior metatarsal epidermis of normal embryos becomes committed to the formation of beta strata prior to morphogenesis of definitive scale ridges. Here, we examined the possibility that the sc/sc anterior metatarsal epidermis also becomes determined, i.e., committed to scutate scale-specific terminal differentiation. Experimental tissue recombinants were used to assess the ability of the sc/sc epidermis to generate beta strata. The results show that the germinative cells of the 15-day sc/sc epidermis are committed to generating beta strata, even though they have not undergone scutate scale morphogenesis. Thus, the mechanisms involved in establishing epidermal determination must differ form those regulating scale morphogenesis. In addition, we examined the formation of patterned, permissive cues in the anterior metatarsal and footpad dermises of sc/sc embryos. Analysis of recombinants showed that both the 15- and 20-day dermises from the sc/sc anterior metatarsal region fail to provide cues for beta stratum formation, when associated with the determined 15-day scutate scale epidermis. Likewise, the 15-day sc/sc footpad dermis cannot support beta stratum formation. However, 20-day sc/sc footpad dermis is able to support the generation of a few abnormally patterned beta strata, demonstrating that sc/sc dermis which has experienced even limited morphogenesis is able to provide permissive cues for the terminal differentiation of the scutate scale epidermis.

Avian scale development

Germinative cells of the scutate scale epidermis from 15-day embryos are committed to appendage-specific, beta stratum formation in association with a foreign dermis. Commitment precedes the time (17 days of development) at which beta strata are actually present in their site-specific locations along the outer surface of each scutate scale. This observation suggested the possibility that commitment to beta stratum formation might be occurring as the outer epidermal surface of each scutate scale first becomes established between 12 and 13 days of development. It is at this time that the scale epidermis loses its ability to participate in feather morphogenesis and cell proliferation becomes restricted to a true stratum basale. To examined the ability of the presumptive scutate scale epidermis to generate beta strata in the absence of the inductive scutate scale dermis, scutate scale epidermis from 11-, 12-, and 13-day embryos was recombined with 15-day reticulate scale dermis and grown for 7 or 9 days. The dermis of reticulate scales does not induce beta stratum formation, but it does support differentiation of a beta stratum by the determined 15-day scutate scale epidermis. Using immunohistological and biochemical analyses of beta-keratins, we find that each of these presumptive scutate scale epidermises is competent to generate appendage-specific beta strata in the absence of the scutate scale dermis. This determination is occurring prior to scale ridge morphogenesis and differentiation of the epidermis into the distinct outer and inner epidermal surfaces of the scale ridge. The restricted distribution of beta strata to the apical domes of individual reticulate-like scales demonstrates that the germinative cells of the committed epidermises are responding to patterned cues. This study also suggests that all basal cells of the presumptive scutate scale epidermis are initially endowed with the ability to generate cells that form a beta stratum.

Biochemical identification and immunological localization of two non-keratin polypeptides associated with the terminal differentiation of avian scale epidermis

The expression of two previously uncharacterized polypeptides produced in epidermal cells of chick reticulate and scutate scales during late embryonic scale histogenesis and in hatchling birds has been studied biochemically and immunologically. These polypeptides have been identified by two-dimensional pH gradient gel electrophoresis as basic in charge, with apparent molecular weights of 20 and 23 kD, and they have been characterized immunologically and by amino acid analysis as non-keratin in nature. Monoclonal antibodies which react with both polypeptides have been used for immunohistochemical and immunogold electron-microscopic localization. Immunoreactivity was observed in suprabasal cells of reticulate scale epidermis, where it codistributed with bundles of alpha-type cytokeratins in the alpha-keratin-rich layers of epidermis known as the alpha stratum and in suprabasal cells of the outer epidermal surface of scutate scales, where it codistributed with alpha- and beta-type keratin filament bundles in the beta-keratin-rich layers of epidermis known as the beta stratum.

The initial expression and patterned appearance of tenascin in scutate scales is absent from the dermis of the scaleless (sc/sc) chicken

Morphogenesis of the anterior metatarsal skin (scutate scale region), from 9.5 to 12 days of development, results in the formation of orderly patterned scale ridges. It is after the initial formation of the Definitive Scale Ridge that the characteristic outer and inner epidermal surfaces differentiate. The hard, plate-like beta stratum, with its unique beta keratins, characterizes the epidermis of the outer surface, while the epidermis of the inner surface elaborates an alpha stratum. The anterior metatarsal region of the scaleless mutant does not undergo scale morphogenesis. Therefore, scale ridges do not form nor do the outer and inner epidermal surfaces with their characteristic beta and alpha strata. We have found that the extracellular matrix molecule, tenascin, first appears in the scutate scale dermis at 12 days of development when the scale ridge is established. Tenascin is found in the dermis only under the scale ridge and is not associated with the dermal-epidermal junction. Tenascin is not found in scaleless anterior metatarsal dermis at this time. As outgrowth of the Definitive Scale Ridge takes place, tenascin distribution correlates closely with the formation of the outer epidermal surface of each scale ridge. By 16 days of development tenascin is also found in close association with the dermal-epidermal junction. Tenascin does not appear in scaleless anterior metatarsal dermis until 16 days of development and then it is randomly and sparsely distributed at the dermal-epidermal junction. Tenascin's initial appearance and pattern of distribution in the scutate scale dermis and its abnormal expression in the scaleless dermis suggest that morphogenesis plays a significant role in regulation of its expression.

Avian scale development. XIII. Epidermal germinative cells are committed to appendage-specific differentiation and respond to patterned cues in the dermis

The ability of the germinative cell population of scutate scale epidermis to continue to generate cells that undergo their appendage-specific differentiation (beta stratum formation), when associated with foreign dermis, was examined. Tissue recombination experiments were carried out which placed anterior metatarsal epidermis (scutate scale forming region) from normal 15-day chick embryos with either the anterior metatarsal dermis from 15-day scaleless (sc/sc) embryos or the dermis from the metatarsal footpad (reticulate scale forming region) of 15-day normal embryos. Neither of these dermal tissues are able to induce beta stratum formation in the simple ectodermal epithelium of the chorion, however, the footpad dermis develops an appendage-specific pattern during morphogenesis of the reticulate scales, while the sc/sc dermis does not. Morphological and immunohistological criteria were used to assess appendage-specific epidermal differentiation in these recombinants. The results show that the germinative cell population of the 15-day scutate scale epidermis is committed to generating suprabasal cells that follow their appendage-specific pathways of histogenesis and terminal differentiation. Of significance is the observation that the expression of this determined state occurred only when the epidermis differentiated in association with the footpad dermis, not when it was associated with the sc/sc dermis. The consistent positioning of the newly generated beta strata to the apical regions of individual reticulate-like appendages demonstrates that the dermal cues necessary for terminal epidermal differentiation are present in a reticulate scale pattern. The observation that beta stratum formation is completely missing in the determined scutate scale epidermis when associated with the sc/sc dermis adds to our understanding of the sc/sc defect. The present data support the conclusion of earlier studies that the anterior metatarsal dermis from 15-day sc/sc embryos lacks the ability to induce beta stratum formation in a foreign epithelium. In addition, these observations evoke the hypothesis that the sc/sc dermis either lacks the cues (generated during scutate and reticulate scale morphogenesis) necessary for terminal differentiation of the determined scutate scale epidermis or inhibits the generation of a beta stratum.

Expression of the cell adhesion molecules, L-CAM and N-CAM during avian scale development

To examine the involvement of cell adhesion molecules in the inductive epithelial-mesenchymal interactions during avian scale development, a study of the spatiotemporal distribution of L-CAM and N-CAM was undertaken. During scutate scale development, L-CAM and N-CAM are expressed together in cells of the transient embryonic layers destined to be lost at hatching. The ongoing linkage of the cells of these layers by both CAMs sets them apart, early in development, as unique cell populations. L-CAM and N-CAM were also expressed simultaneously at the basal surface of the early germinative cells where signal transduction is presumed to occur. In spite of the differences in cell shape, adhesion, density and proliferative state between populations of epidermal placode and interplacode cells, the expression of L-CAM and N-CAM appeared to be uniform and nondiscriminating for these discrete cell lineages. The same pattern of L-CAM and N-CAM expression was observed during morphogenesis of reticulate scales that develop without placode formation. While L-CAM and N-CAM are present during the early stages of scale development and most likely function in cell adhesion, the data do not support a role for these adhesion molecules in the formation of the morphogenetically critical placode and interplacode cell populations. In both scale types, L-CAM became predominantly epithelial, and N-CAM became predominantly dermal as histogenesis occurred. Initially, N-CAM was concentrated near the basal lamina where it may be involved in the reciprocal epidermal-dermal interactions required for morphogenesis. However, as development of the scales progressed, N-CAM disappeared from the tissues. L-CAM expression continued in the epidermis and was intense on all suprabasal cells undergoing differentiation into either an alpha-stratum or beta-stratum. However, L-CAM was more prevalent on the basal cells of alpha-keratinizing regions than on the basal cells of beta-keratinizing regions.

Retinoic acid induction of featherlike structures from reticulate scales

Retinoic acid-induced transformation of reticulate scales to feather-like structures (Dhouailly and Hardy, '78) provides a useful model to study biochemical differentiation in avian skin. In this study, immunofluorescent analysis of reticulate scale-feathers (RSFs) indicates that they contain beta keratin in feather barbs and, thus, are true feathers, biochemically. Epidermal cells that would otherwise produce only alpha keratin in reticulate scales are induced to reorganize and differentiate into barb ridge cells that accumulate feather beta keratins. The mechanism for these dramatic morphological and biosynthetic responses to retinoic acid is unknown.

Avian scale development: XI. Immunoelectron microscopic localization of alpha and beta keratins in the scutate scale

Epithelial-mesenchymal interactions play important roles in morphogenesis, histogenesis, and keratinization of the vertebrate integument. In the anterior metatarsal region of the chicken, morphogenesis results in the formation of distinct overlapping scutate scales. Recent studies have shown that the dermis of scutate scales is involved in the expression of the beta keratin gene products, which characterize terminal differentiation of the epidermis on the outer scale surface (Sawyer et al.: Dev. Biol. 101:8-18, '84; Shames and Sawyer: Dev. Biol. 116:15-22, '86; Shames and Sawyer: In A.A. Moscona and A. Monroy (eds), R.H. Sawyer (Vol. ed): Current Topics in Developmental Biology. Vol. 22: The Molecular and Developmental Biology of Keratins. New York: Academic Press, pp. 235-253, '87). Since alpha and beta keratins are both found in the scutate scale and are members of two different multigene families, it is important to know the precise location of these distinct keratins within the epidermis. In the present study, we have used protein A-gold immunoelectron microscopy with antisera made against avian alpha and beta keratins to specifically localize these keratins during development of the scutate scale to better understand the relationship between dermal cues and terminal differentiation. We find that the bundles of 3-nm filaments, characteristic of tissues known to produce beta keratins, react specifically with antiserum which recognizes beta keratin polypeptides and are found in the embryonic subperiderm that covers the entire scutate scale and in the stratum intermedium and stratum corneum making up the platelike beta stratum of the outer scale surface. Secondly, we find that 8-10-nm tonofilaments react specifically with antiserum that recognizes alpha keratin polypeptides and are located in the germinative basal cells and the lowermost cells of the stratum intermedium of the outer scale surface, as well as in the embryonic alpha stratum, which is lost from the outer surface of the scale at hatching. The alpha keratins are found throughout the epidermis of the inner surface of the scale and the hinge region. Thus, the present study further supports the hypothesis that the tissue interactions responsible for the formation of the beta stratum of scutate scales do not directly activate the synthesis of beta keratins in the germinative cells but influence these cells so that they or their progeny will activate specific beta keratin genes at the appropriate time and place.

Altered keratin biosynthesis follows inhibition of scale morphogenesis by hydrocortisone

Hydrocortisone, administered onto the chorioallantoic membrane (CAM) of 7- to 10-day-old chick embryos, inhibits scale development, in a dose- and stage-dependent manner. The response is also region specific in that hydrocortisone treatment, at a specific dose and time, will completely block scutellate and interstitial scale development while leaving other scale types unaffected. Using histological, biochemical, and immunofluorescence techniques, we have shown that inhibition of scutellate scale morphogenesis prevents the subsequent formation of a beta stratum and alters expression of the alpha keratins. These data support the hypotheses that each avian scale type has its own distinctive temporal, morphological, and biochemical pattern of development; and in the case of scutellate scale development, hydrocortisone treatment alters keratin biosynthesis by interfering with earlier steps in morphogenesis.

Avian scale development. X. Dermal induction of tissue-specific keratins in extraembryonic ectoderm

Epidermal-dermal tissue interactions regulate morphogenesis and tissue-specific keratinization of avian skin appendages. The morphogenesis of scutate scales differs from that of reticulate scales, and the keratin polypeptides of their epidermal surfaces are also different. Do the inductive cues which initiate morphogenesis of these scales also establish the tissue-specific keratin patterns of the epidermis, or does the control of tissue-specific keratinization occur at later stages of development? Unlike feathers, scutate and reticulate scales can be easily separated into their epidermal and dermal components late in development when the major events of morphogenesis have been completed and keratinization will begin. Using a common responding tissue (chorionic epithelium) in combination with scutate and reticulate scale dermises, we find that these embryonic dermises, which have completed morphogeneis, can direct tissue-specific stratification and keratinization. In other words, once a scale dermis has acquired its form, through normal morphogenesis, it is no longer able to initiate morphogenesis of that scale, but it can direct tissue-specific stratification and keratinization of a foreign ectodermal epithelium, which itself has not undergone scale morphogenesis.

Development of chicken epidermis cultured with embryo extract

The epidermis from 11-day-old chick embryo shank skin was cultured with 11-day-old chick embryo extract. The growth and the differentiation of the epidermis in culture were studied histologically, electron microscopically and with polyacrylamide gel electrophoresis of keratin proteins. The epidermis cultured with the chick embryo extract proliferated and stratum structures developed simultaneously with the increase in epidermal cell layers. Finally, a keratinized layer was observed after 10 days in culture. Electron microscopic observations revealed that tonofilaments were produced after 2 days in culture and increased thereafter with culture time, becoming condensed with desmosomes. Keratohyaline granules were observed in 7-day cultures. These keratinization characteristics occurring during culture showed the general characteristics of the alpha stratum observed in the skin of in ovo embryos during the early stages of development. However, the development of peridermal and subperidermal granules was poor and the stratum granulosum, which develops at the late stages between the stratum intermedium and the stratum corneum, was not observed. Polyacrylamide gel electrophoresis of S-carboxymethylated keratin proteins showed that the keratin protein band patterns of the culture differed from those of in ovo skin epidermis.