The specification of feather and scale protein synthesis in epidermal-dermal recombinations

Keratin proteins synthesized by dorsal or tarsometatarsal embryonic chick epidermis in heterotopic and heterospecific epidermal-dermal recombinants were analyzed by polyacrylamide gel electrophoresis and were compared to those produced by normal nondissociated dorsal and tarsometatarsal embryonic skin, as well as to those produced by control homotopic recombinants. Recombinant skins were grafted on the chick chorioallantoic membrane and grown for 8 or 11 days. Recombinants comprising dorsal feather-forming dermis formed feathers, irrespective of the origin of the epidermis. The electrophoretic band patterns of the keratins extracted from these feathers were of typical feather type. Conversely recombinants comprising tarsometatarsal scale-forming dermis formed scales, irrespective of the origin of the epidermis. The band patterns of the keratins extracted from the epidermis of these scales were of typical scale type. Heterospecific recombinants comprising chick dorsal feather-forming epidermis and mouse plantar dermis gave rise to six footpads arranged in a typical mouse pattern. In these recombinants, the chick epidermis produced keratins, the band pattern of which was of typical chick scale type. These results demonstrate that the dermis not only induces the formation of cutaneous appendages in confirmity with its regional origin, but also triggers off in the epidermis the biosynthesis of either of two different keratin types, in accordance with the regional type (feather, scale, or pad) of cutaneous appendages induced. The possible relationship between region-specific morphogenesis and cytodifferentiation is discussed in comparison with results obtained in other kinds of epithelial-mesenchymal interactions.     

Either chick embryo dermis or retinoid-treated mouse dermis can initiate glandular morphogenesis from mammalian epidermal tissue

Excess retinoids can cause developing mouse vibrissa follicles to be transformed into mucous glands in organ culture. The objective was to test the hypothesis that retinoids act in this system by altering morphogenetic properties of the dermis. After inititation by retinoic acid (RA) in organ culture, glands were shown to develop further in embryonic skin grafted to the chick chorioallantoic membrane (CAM). Recombinants of 12.5 day mouse epidermis with untreated or RA-treated mouse or chick dermis were then grafted to CAM for 7 days. For homospecific recombinants, 13.5 day mouse dermis originated from 11.5 day skin cultured for 2 days, with or without 5.2 microgram/ml RA. For heterospecific recombinants, 12 day dermis came from chick embryos, previously injected with 250 microgram RA. Glands were absent from the homospecific recombinants including untreated mouse dermis, but appeared in 26% of those with RA-treated dermis. Among heterospecific recombinants, 75% of those with RA-treated chick dermis and 29% of those with untreated dermis had glands. Untreated 10-12 day chick skin contained two forms of endogenous vitamin A, retinol (4.5 microgram/g protein) and dehydroretinol (3.7 microgram/g protein), while 13-14 day mouse skin contained only retinol (1.8 microgram/g protein), as shown by high performance liquid chromatography. RA injection increased retinol and dehydroretinol in chick skin, while RA was undetectable. Thus RA can act through mouse dermis to form epithelial glands and through chick dermis to increase the incidence of glands. The glands in recombinants with untreated chick dermis may result from the higher levels of endogenous retinoids in chick skin, compared with mouse skin.     

Epidermal-dermal tissue interactions between mutant foot skin and normal back skin: a comparison of the inductive capacities of scaleless low line and normal anterior foot dermis.

The inductive capacities of 9- to 16-day anterior foot dermis of scaleless low line and normal embryos were compared by recombining them with a common source of epidermis, i.e., 7-day normal back epidermis. Tissue recombinants were cultured as grafts to the chorioallantoic membrane (CAM). Both normal and scaleless low line dermis of 12 to 13 days of incubation began to lose their ability to elicit feather production in 7-day normal back epidermis. Normal foot dermis began to elicit scale production at 12 to 13 days, whereas scaleless low line anterior foot dermis maintained feather production at a low level. It is inferred that without being associated with scale placode formation, scaleless low line anterior foot dermis does not acquire specific inductive capacities related to the production of an outer scale surface in the overlying epidermis. Feather placodes do not function as surrogates of scale placodes. The difference between normal and scaleless low line anterior foot dermis in terms of specific inductive capacities related to scale production is interpreted as a secondary effect of the action of the scaleless allele in interfering with scale placode formation in the scaleless low line anterior foot epidermis.     

Scale induction in the chick amnionic ectoderm attached to tarsometatarsal skin dermis

Under the influence of tarsometatarsal dermis of 13-17-day chick embryos, 6.8-day amnionic ectoderm can form scales and express keratins specific for scales. In contrast, 10.5-day shank dermis can induce both feather filaments and scales in the amnionic ectoderm.     

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.     

Signaling dynamics of feather tract formation from the chick somatopleure

In the chick, most feathers are restricted to specific areas of the skin, the feather tracts or pterylae, while other areas, such as the apteria, remain bare. In the embryo, the expansion and closure of the somatopleure leads to the juxtaposition of the ventral pteryla, midventral apterium and amnion. The embryonic proximal somatopleural mesoderm is determined to form a feather-forming dermis at 2 days of incubation (E2), while the embryonic distal and the extra-embryonic somatopleure remain open to determination. We found a progressive, lateral expression of Noggin in the embryonic area, and downregulation of Msx1, a BMP4 target gene, with Msx1 expression being ultimately restricted to the most distal embryonic and extra-embryonic somatopleural mesoderm. Msx1 downregulation thus correlates with the formation of the pterylae, and its maintenance to that of the apterium. Suspecting that the inhibition of BMP4 signaling might be linked to the determination of a feather-forming dermis, we grafted Noggin-expressing cells in the distal somatopleure at E2. This elicited the formation of a supplementary pteryla in the midventral apterium. Endogenous Noggin, which is secreted by the intermediate mesoderm at E2, then by the proximal somatopleure at E4, could be sufficient to suppress BMP4 signaling in the proximal somatopleural mesoderm and then in part of the distal somatopleure, thus in turn allowing the formation of the dense dermis of the future pterylae. The same result was obtained with the graft of Shh-producing cells, but Noggin and Shh are both required in order to change the future amnion into a feather-bearing skin. A possible synergistic role of endogenous Shh from the embryonic endoderm remains to be confirmed.     

Induction of epidermal mucous metaplasia by culture of recombinants of undifferentiated epidermis and retinol-treated dermis in a chemically defined medium

Epidermal mucous metaplasia of cultured skin is known to be induced by excess retinol. Studies were made on whether retinol affects primarily the epidermis or the dermis during retinol-induced epidermal mucous metaplasia of 13-day-old chick embryonic skin in culture. When recombinants of 13-day-old normal epidermis and retinol-treated dermis were cultured for 7 days in chemically defined medium in the absence of retinol, hormones, and serum, they showed altered epidermal differentiation toward secretory epithelium (mucous metaplasia). Thus retinol acted primarily on dermal cells.     

Ventral vs. dorsal chick dermal progenitor specification

The dorsal and the ventral trunk integuments of the chick differ in their dermal cell lineage (originating from the somatic and somatopleural mesoderm respectively) and in the distribution of their feather fields. The dorsal macropattern has a large spinal pteryla surrounded by semi-apteria, whereas the ventral skin has a true medial apterium surrounded by the ventral pterylae. Comparison of the results of heterotopic transplantations of distal somatopleure in place of somatic mesoderm (Mauger 1972) or in place of proximal somatopleure (our data), leads to two conclusions. These are that the fate of the midventral apterium is not committed at day 2 of incubation and that the signals from the environment which specify the ventral and dorsal featherforming dermal progenitors are different. Effectively, Shh, but not Wnt -1 signalling can induce the formation of feather forming dermis from the embryonic somatopleure. Shh is not able, however, to trigger the formation of a feather forming dermis from the extra embryonic somatopleure. This brief report constitutes the first attempt, by comparing old and new preliminary results, to understand whether dermal progenitors at different sites are specified by different signalling pathways.     

Differentiation of embryonic chick feather-forming and scale-forming tissues in transfilter cultures

The dermal-epidermal tissue interaction in the chick embryo, leading to the formation of feathers and scales, provides a good experimental system to study the transfer between tissues of signals which specify cell type. At certain times in development, the dermis controls whether the epidermis forms feathers or scales, each of which are characterized by the synthesis of specific beta-keratins. In our culture system, a dermal effect on epidermal differentiation can still be observed, even when the tissues are separated by a Nuclepore filter, although development is abnormal. Epidermal morphological and histological differentiation in transfilter cultures are distinct and recognizable, more closely resembling feather or scale development, depending on the regional origin of the dermis. Differentiation is more advanced when epidermis is cultured transfilter from scale dermis than from feather dermis, as assessed by morphology and histology, as well as the expression of the tissue-specific gene products, the beta-keratins. Two-dimensional polyacrylamide gel analysis of the beta-keratins reveals that scale dermis cultured transfilter from either presumptive scale or feather epidermis induces the production of 7 of the 9 scale-specific beta-keratins that we have identified. Feather dermis, although less effective in activating the feather gene program when cultured transfilter from either presumptive feather or scale epidermis, is able to turn on the synthesis of 3 to 6 of the 18 feather-specific beta-keratins that we have identified. However, scale epidermis in transfilter recombinants with feather dermis also continues to synthesize many of the scale-specific beta-keratins. Using transmission and scanning electron microscopy, we detect no cell contact between tissues separated by a 0.2-micron pore diameter Nuclepore filter, while 0.4-micron filters readily permit cell processes to traverse the filter. We find that epidermal differentiation is the same with either pore size filter. Furthermore, we do not detect a basement membrane in transfilter cultures, implying that neither direct cell contact between dermis and epidermis, nor a basement membrane between the tissues is required for the extent of epidermal differentiation that we observe.     

EPIDERMAL METAPLASIA OF THE CORNEAL ANTERIOR EPITHELIUM IN THE CHICK EMBRYO

The corneal anterior epithelium of younger chick embryos can be changed into a keratinized epidermis, when it is cultured in vitro combined with 6 1/2-day dorsal dermis. Even if a Millipore filter is inserted between the corneal anterior epithelium and underlying dorsal dermis, the epithelium undergoes similar metaplastic changes. In older embryos, however, the epithelium gradually loses the competence for the keratinization. Cultivation of cornea (anterior epithelium, stroma and endothelium) of 6 1/2- or 10-day embryos results in maintenance of its original pattern, and the epithelium fails to differentiate into a keratinized epidermis. The dermis isolated from 8 1/2-day dorsal or 12 1/2-day tarsometatarsal skin is not so effective in inducing the epidermal metaplasia. The mesenchyme of 5 1/2-day proventriculus or 5 1/2-day gizzard fails to bring about any endodermal metaplasia of the corneal epithelium. The corneal stroma, on the other hand, has no inhibitory action on the keratinization of the epidermis obtained from 6 1/2-day dorsal skin.     

Induction of Mucous Metaplasia in Chick Embryonic Skin by Retinol-Pretreated Embryonic Chick or Quail Dermal Fibroblasts through Cell-Cell Interaction: Correlation of a Transient Increase in Retinoic Acid Receptor β mRNA in Retinol-Treated Dermal Fibroblasts with Their Competence to Induce Epidermal Mucous Metaplasia

Epidermal mucous metaplasia of 13-day-old chick embryonic tarsometatarsal skin can be induced by culture in medium containing 20 μM retinol for only 8 hr and then in a chemically defined medium without retinol for 2 days. Retinol primarily affects the dermal cells, which then transform the epithelial cells into mucus-secreting cells. In this study, we developed a system using a combination of retinol-pretreated chick or quail dermal fibroblasts and chick skin, and showed that retinol-pretreated quail embryonic dermal fibroblasts invaded the dermis of chick embryonic skin to beneath the epidermal basal cells within 1 day of culture and induced metaplasia, suggesting that epidermal mucous metaplasia of the skin was induced by the direct interaction of retinol-pretreated dermal fibroblasts with the epidermal cells or by low diffusible paracrine factor produced by the fibroblasts.
Increase in retinoic acid receptor β (RARβ) mRNA in dermal fibroblasts was observed after 8 hr-treatment with retinol which preceded morphological changes induced by retinol and this increase was correlated with the competence of the dermal fibroblasts to induce epidermal mucous metaplasia. Thus some gene product(s) controlled by RARβ in dermal fibroblasts may be an essential signal for induction of epidermal mucous metaplasia.     

Epidermal-dermal tissue interactions between mutant and normal embryonic back skin: site of mutant gene activity determining abnormal feathering is in the epidermis.

The site of the scaleless gene's activity in the development of abnormal feathers was determined by reciprocally recombining epidermis and dermis between normal and scaleless chick embryos and culturing the recombinants for seven days on the chorioallantoic membrane. When recombined with a common dermal source, feather development is enhanced by scaleless high line as compared to scaleless low line epidermis. Against a common responding tissue, 7-day normal back epidermis, significant differences were not found in feather inducing ability between normal, scaleless high line and scaleless low line dermis. It was concluded that, in relation to abnormal feathering, these tissue interactions reveal that the site of the scaleless gene's activity is the epidermis. A model of tissue interaction in the development of normal and abnormal feathers is presented. According to the model, the focus of the scaleless mutation and the genes accumulated by selection for high or low feather numbers is the epidermis, the effect being that the reactivity of the epidermis to dermal stimuli is altered. Subsequently, the epidermis controls the morphogenetic organization of the dermis. The scaleless dermis is presumed to contain normal positional information for the determination of feather structure and pattern.     

The structure of the canary's incubation patch

The gross morphology, histology and ultrastructure of the canary's incubation patch and the ventral apterium from which it arises are described. The apterium is vascularized by pectoral, external mammary, incubation, and prepubic arteries. It is innervated by cutaneous branches of spinal nerves. It has a surface area of 6 cm². Its epidermis is a stratified squamous epithelium with basal, intermediate, transitional and cornified layers. Cells in the stratum germinativum contain a normal array of organelles, but are characterized by tonofilaments, desmosomes and interdigitating surfaces. Cellular organelles disappear in the stratum transitivum and are replaced by large vacuoles and keratohyalin bands. Nonmyelinated nerve fibers are abundant in the stratum germinativum. The dermis consists of (1) an avascular layer of dense collagen subjacent to the epidermis and containing many nonmyelinated nerves, and (2) an underlying layer of areolar connective tissue containing blood vessels, lamellar corpuscles and nerves. A layer of coarse elastic fibers, reinforced by collagen and smooth muscle, separates the dermis from subcutaneous tissue. In contrast to the ventral apterium, the incubation patch is featherless and visibly hypervascular and edematous. Its epidermis is both hypertrophic and hyperplastic. Large spaces separate cells in the stratum germinativum. The visible hypervascularity is due to hyperemia and increased number and size of blood vessels in the dermis. Visible edema is due to the accumulation of fluid interstitially. Although no histological differences exist among various regions of the ventral apterium, such differences are present in the incubation patch.     

How and when the regional competence of chick epidermis is established: feathers vs. scutate and reticulate scales, a problem en route to a solution

Most of the chick body is covered with feathers, while the tarsometatarsus and the dorsal face of the digits form oblong overlapping scales (scuta) and the plantar face rounded nonoverlapping scales (reticula). Feathers and scuta are made of beta-keratins, while the epidermis of reticula and inter-appendage or apteria (nude regions) express a-keratins. These regional characteristics are determined in skin precursors and require an epidermal FGF-like signal to be expressed. Both the initiation of appendages, their outline and pattern depend on signals from the dermis, while their asymmetry and outgrowth depend on epidermal competence. For example, the plantar dermis of the central foot pad induces reticula in a plantar or feathers in an apteric epidermis, in a hexagonal pattern starting from the medial point. By manipulating Shh levels in the epidermis, the regional appendage type can be changed from scuta or reticula to feather, whereas the inhibition of Wnt7a, together with a downregulation of Shh gives rise to reticula and in extreme cases, apteria. During morphogenesis of plantar skin, the epidermal expression of En-1, acting as a repressor both of Wnt7a and Shh, is linked to the formation of reticula. Finally, in birds, the complex formation of feathers, which can be easily triggered, even in the extra-embryonic somatopleure, may result from a basic genetic program, whereas the simple formation of scales appears secondarily derived, as requiring a partial (scuta) or total (reticula) inhibition of epidermal outgrowth and beta-keratin gene expression, an inhibition lost for the scuta in the case of feathered feet breeds.     

Differences in the histogenesis and keratin expression of avian extraembryonic ectoderm and endoderm recombined with dermis

The responses of the chorionic ectoderm and allantoic endoderm (from 8-day chick embryos) to dermal induction were compared through tissue recombinants grafted onto the chorioallantoic membrane. The chorionic epithelium formed the appropriate epidermis with a fully developed stratum corneum in response to both spur and scutate scale dermises. Analysis of these recombinant epidermal tissues by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated that tissue-specific expression of the alpha (alpha) and beta (beta) keratin polypeptides occurred. In addition, indirect immunofluorescence studies with antisera to alpha or beta keratins showed that the beta stratum, which characterizes the epidermis of spurs and scutate scales, was formed, and the alpha keratins were distributed as in the normal epidermal tissues. In contrast, although the allantoic endoderm became stratified in association with either spur or scutate scale dermis, a stratum corneum with a beta stratum did not develop. SDS-PAGE analysis demonstrated that while the characteristic beta keratins of scutate scales and spur were not detected, most of the alpha keratins normally elaborated by these structures were present, suggesting that even without histogenesis of a stratum corneum the expression of alpha keratins of endoderm could be regulated in a tissue-specific manner by dermis. This study also demonstrated that there are differences in the abilities of the chorionic and allantoic epithelia to respond to the same dermal cues, which may reflect earlier restrictions in their developmental potentials.     

Acceleration of Retinol-Induced Epidermal Mucous Metaplasia by Stimulating the Dermal Adenylate Cyclase-cAMP System in Chick Embryonic Skin: Appearance of cAMP-Dependent Phosphorylated Proteins in Dermis of Retinol-Pretreated Skin after 2 h-Treatment with cAMP

Epidermal mucous metaplasia of cultured 13-day-old chick embryonic tarsometatarsal skin can be induced by culture in medium containing excess retinol (20 μM) for only 8–24 h and then in a chemically defined medium with Bt₂cAMP (0.2–2 mM) and without retinoids or serum for 2 days. In this work, stimulation of the adenylate cyclase-cAMP system in retinol-pretreated skin by forskolin, pertussis toxin, cholera toxin or AIF₄^– was found to accelerate the synthesis of epidermal sulfated glycoprotein (mucin). In skin induced toward mucous metaplasia by retinol, treatment with forskolin for 1 day increased the cAMP content 10-fold in the dermis but only 2-fold in the epidermis over the control levels. The cAMP level of Bt₂cAMP (0.2 mM)-treated skin was 18 times higher in the dermis but rather lower in the epidermis than untreated skin. These results suggest the importance of an adenylate cyclase-cAMP system in the dermis of skin in stimulating mucous metaplasia induced by retinoids. In fact, cAMP-dependent protein phosphorylation was seen only in the dermis of retinol-pretreated skin after 2 h-treatment with cAMP. As no transfer of cAMP from the dermis to the epidermis of forskolin-treated skin was detected, there may be no gap junctional communication between the epidermis and the dermis, while the basement membrane becomes discontinuous during mucous metaplasia.     

Region-specific deposition of dermal proteins between dermis and epidermis during induction of chick feather and scale rudiments

To begin to study the role of particular proteins in inductive tissue interactions, we have used density labelling techniques to determine whether any dermal proteins are found between embryonic chick dermis and epidermis at a stage when the dermis plays an important inductive role in epidermal differentiation. Epidermis will form feathers or scales depending on whether it interacts with dorsal or foot dermis, respectively, and the dermis can still influence epidermal differentiation when direct cell contact between the tissues is blocked by a membrane filter during culturing (Peterson & Grainger, 1985). In transfilter experiments, we detect a subset of dermal proteins within the filter between the tissues. Several of these dermal proteins are deposited in a region-specific manner, that is, they are only found associated with filters from either dorsal or foot dermis. We have previously shown that the expression of some of these proteins is specific to particular regions of dermis and is also associated with the inductive potential of the dermis (Peterson & Grainger, 1986). We detect only 17 dermal proteins which are transferred across the filter in these cultures and found in direct association with epidermis; of these 14 are common to both dorsal and foot dermis, and 3 are deposited in a region-specific manner. Our results lead us to hypothesize a significant function for certain dermal proteins in this inductive interaction either as part of the extracellular matrix or in direct association with epidermis.     

Epidermis Is the Site of Action of Tabby (TA) in the Mouse

The site of action of the sex-linked tabby (Ta) locus was analyzed by the technique of dermal-epidermal recombination grafting. Skin components from normal and tabby 14-day embryos were separated, recombined and grown 21 days in testes of histocompatible mice. Grafts of the combinations normal epidermis-normal dermis and normal epidermis-tabby dermis produced predominantly zig-zag hairs. Grafts of the combination tabby epidermis-normal dermis and tabby epidermis-tabby dermis produced hairs with a morphology similar to hairs found in tabby mice. We conclude from these results that the tabby locus acts within the epidermis, and has no effect on the dermis.     

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.     

Purification and characterization of beta-galactoside-binding lectin from chick embryonic skin

Lectin activity was found in tarsometatarsal skin of chick embryo. It was specific for beta-linked galactosyl residues and required a thiol-reducing agent for hemagglutination activity. The lectin was extracted from dermis and epidermis (skin) with lactose and purified to apparent homogeneity by affinity chromatography on asialofetuin-Sepharose. Examination of their biochemical properties showed that although dermis and epidermis develop from different origins, they contain the same lectin. The apparent subunit Mr of lectin was 14000 and its isoelectric point was 7.0. Under non-dissociating conditions, the lectin exists mainly as a dimer. Radioimmunoassay showed that this skin-type lectin is present in many tissues including skin, muscle, bone, eye, heart, liver and brain at various developmental stages. A wide distribution and a marked change in its content during development strongly suggest that the lectin might have a fundamental role in cellular function, embryonic development and tissue differentiation.