Cell death in the anuran tadpole tail: thyroid hormone induces keratinization and tail-specific growth inhibition of epidermal cells

The mechanism of thyroid hormone-induced and glucocorticoid-modulated death of tail epidermal cells from tadpoles of bullfrog, Rana catesbeiana, was investigated by comparing tail epidermal cells with dorsal body epidermal cells. From morphological and biochemical criteria, there were two types of epidermal cells: basal cells and skein cells. The abundance of these cells was different between the tail and the body skin. Fifty percent of body cells and more than 95% of tail cells were skein cells. Effects of 3,3',5-triiodo-L-thyronine (T3, 10(-8) M) and cortisol (5 X 10(-7) M) were investigated with cultured epidermal cells. T3 differently regulated the keratinization of the tail and body cells. The keratinization of the tail epidermal cells was not observed without T3. T3 induced the keratinization dramatically. On the other hand, body epidermal cells were constantly undergoing keratinization without the hormone: T3 merely accelerated the rate of keratinization. Cortisol generally did not show any significant effect on keratinization. T3 showed opposite effects on DNA synthesis of the tail and body cells: suppression of tail cells and stimulation of body cells. Cortisol weakened the inhibitory effect of T3 on DNA synthesis in tail cells. Immunofluorescent micrographs with anti-BrdU showed that T3 decreased the number of cells in the S phase of the cell cycle in the case of tail cells but not of body cells. Thus, thyroid hormone plays dual roles for the tadpole epidermal cells: one is an induction and a promotion of keratinization in tail and body cells, respectively, and the other is an opposite regulation for the proliferation of both epidermal cells. These roles seem to have crucial connections to a tail-specific cell death induced by thyroid hormone.     

Reversible inhibition by DMSO of hydrocortisone-induced keratinization of chick embryonic skin

Hydrocortisone, at a physiological concentration of 10^?8 M, induces keratinization of chick embryonic tarsometatarsal skin in a chemically defined medium in 4 days [1]. The presence of 1–4% DMSO with hydrocortisone reversibly prevented this keratinization. DMSO suppressed the appearance of epidermal structural protein, which was preferentially induced by hydrocortisone. It also suppressed hydrocortisone-induced epidermal transglutaminase activity; which was presumably responsible for polymerization and decrease in solubility of epidermal protein in keratinization, and it suppressed increase of epidermal protein. When DMSO was added to differentiated skin or added concomitantly with a higher concentration of hydrocortisone, epidermal transglutaminase activity was suppressed. Electron microscopic studies showed that hydrocortisone induced tonofilament bundles and keratinized cells with cellular envelopes, which are all characterestic of α-type keratinization of chick embryonic skin [2], and that DMSO inhibited hydrocortisone induced keratinization and kept the epidermis in an undifferentiated state. Moreover, DMSO inhibited epidermal DNA synthesis and increase in thickness of the epidermis during culture of hydrocortisone-treated skin, indicating that it suppressed cell proliferation as well as cell differentiation. DMSO by itself at 1 or 2 % did not affect epidermal cell differentiation, but suppressed cell proliferation when compared with untreated control.     

Induction of the alpha-type keratinization by hydrocortisone in embryonic chick skins grown in a chemically defined medium: An electron microscopic study

Previous biochemical analyses showed the differential accumulation of the epidermal structural protein, which yielded S-carboxymethylated epidermal protein A (SCMEpA), in the hydrocortisone-induced in vitro keratinization of 13-day embryonic chick tarsometatarsal skin growing in a chemically defined medium (Sugimoto et al., 1974). Fine structural features of such an in vitro keratinization process were studied by electron microscopy in the present work.After 2 days of culture with hydrocortisone (0.02 or 0.2 μM), development of the tonofilament bundles occurred to some extent, but the keratinized layer was not formed. Keratinization was observed after 4 days of culture with hydrocortisone (0.02 or 0.2 μM). Desmosomes and tonofilament bundles were prominent in the cytoplasm of the basal and intermediate cell layers of the epidermis. Keratohyalin granules and lipid droplets appeared in the upper layer. Degradation of cellular organelles such as nuclei and mitochondria then proceeded, leaving only filament bundles and electron-dense amorphous masses in the cytoplasm. Thickened cellular envelopes, which are characteristic of keratinized cells, were also observed. These features are characteristic of alpha-type keratinization which is common for other body surfaces. Beta-type keratinization, typical of normal embryonic scales, was not observed even after 6 days of culture with hydrocortisone. Keratinization of embryonic subperiderm of beta-type did not occur either. These ultrastructural observations clearly showed that hydrocortisone induced the alpha-type keratinization. It was also suggested that SCMEpA was closely related to alpha-type keratinization.     

The Effect of Hydrocortisone on the Epidermal Proliferation in an Organ Culture of Chick Embryonic Skin

Hydrocortisone is regarded as an initiator of keratinization in embryonic skin. The present investigation dealt with the effect of hydrocortisone on the proliferation of epidermal cells during early development: Cell kinetic analyses using ³H-thymidine autoradiography were applied to a skin organ culture prepared from a 13-day chick embryo.
Hydrocortisone at a concentration between 0.01 and 1.0 μg/ml was effective in initiating a morphological change leading to the epidermal keratinization in vitro and caused a marked decrease in the mitotic and labeling indices of epidermal basal cells, the decrease being maximum at 2 days of culture previous to the morphological change.
During continuous labeling with ³H-thymidine, the number of labeled basal cells reached 100% within 2 days in the control and 4 days in the culture treated with hydrocortisone. This confirmed that the growth fraction of epidermal basal cells was 1.0 even after the administration of hydrocortisone.
The duration of each cell cycle phase at 2 days of culture was determined by percent labeled mitoses and double-labeling analyses. It was concluded that hydrocortisone extended the generation time of epidermal basal cells at this time point about three fold over the control. This extension was mainly due to the elongation of the G ₁ phase.     

Retinoids alter the direction of differentiation in primary cultures of cutaneous keratinocytes

The effects of vitamin A on the morphological expression of differentiation were studied in cell cultures of cutaneous keratinocytes from the newborn rat. The cells were first cultivated in a medium containing 0.11 mM calcium until a confluent monolayer had been formed. Stratification and terminal differentiation were then triggered by raising the calcium concentration of the medium to 1.96 mM ('normal' culture). The rise in the concentration of calcium was coupled with the addition of retinol (RL) of retinoic acid (RAC) to the medium to produce an excess of vitamin A (high-retinoid culture). Delipidized serum was used to produce a deficiency of vitamin A (low-retinoid culture). The tissue organization and the ultrastructure of the keratinocytes in the stratified culture were the same as those seen in conventional cultures and skin explants. These stratified cultures expressed the morphological features of the epidermis of intact skin. The addition of RL or RAC to the medium enhanced features characteristic of the secretory epithelium, such as the formation of an extensive endoplasmic reticulum, an enlargement of the Golgi zone, and an increase in the number of vacuoles. At the same time, the addition of retinoids diminished features characteristic of the terminal differentiation of the stratified squamous epithelium, such as stratification and keratinization. Deficiency of vitamin A in the medium resulted in a culture with many differentiated layers. The differentiated cells of the low-retinoid cultures contained densely packed tonofilaments and synthesized products that reacted with the monoclonal antibody AE2 that is specific for keratin peptides which are markers of epidermal differentiation. In the cell culture system that is presented here, an excess of retinoids redirected epithelial differentiation from a stratifying and keratinizing epithelium towards a secretory epithelium. This system is a useful tool for elucidating the mechanisms responsible for the effect of vitamin A on the differentiation of epithelial cells.     

Changes in Soybean Agglutinin (SBA) and Peanut Agglutinin (PNA) Binding Pattern in the Epidermis of the Developing Chick Embryo

Lectin binding pattern in the developing chick embryonic epidermis was studied using peroxidase labeling method. The epidermis of the 13-day-old embryo is in an undifferentiated state. Little binding of soybean agglutinin (SBA), specific for N-acetyl-D-galactosamine, and peanut agglutinin (PNA), specific for β-D-galactose, was seen in such epidermal cells. As the epidermis developed toward keratinization, the cell membrane of the differentiating flattened cells was positively stained with SBA and PNA. The positive staining was also seen in the supranuclear region of the cells located between the flattened cells and the basal cells. The basal cells remained unstained in all the stages of development. Similar staining pattern with SBA and PNA was seen in the cultured skin explants during the epidermal differentiation in vitro. These observations show that the SBA- and PNA-reactive glycoconjugates accumulate during the epidermal cell differentiation, suggesting their important roles in the maintenance of the ordered structure of the epidermis.     

Complex epidermal organs of Phascolion (Sipuncula): insights into the evolution of bimodal secretory cells in annelids

The epidermal organs of an undescribed Phascolion species from the Balearic Islands were investigated using SEM, TEM, LM, CLSM and μCT methods. We found axial receptor cells confirming the previously assumed sensory function of epidermal organs. Our analyses also revealed six types of secretory cells. Some secretory cells types are capable of secreting filamentous and amorphous secretion in two different ways simultaneously (bimodal secretion). The high diversity of cell types, the complex pattern of acinar units, and the absence of a common gland pore make epidermal organs of Phascolion unique amongst sipunculans (Phascolion type). Our reconstruction of the evolution of the epidermal organs of Sipuncula revealed that Phascolion‐type epidermal organs may have derived from either Golfingia‐, Sipunculus‐ or Phascolosoma‐type epidermal organs. The oldest known sipunculans were Golfingia‐like and had epidermal organs, which might resemble the architecture of the Golfingia‐type epidermal organs in extant taxa. Thus, it can be hypothesized that bimodal secretory cells (e.g. basophilic secretory cells) were part of the sipunculan ground pattern. Moreover, bimodal secretory cells of Phascolion look strikingly similar to those found in various annelid glands and thus might even be part of the ground pattern of stem species of Sipuncula + Pleistoannelida.     

Succinic dehydrogenase activity in the epidermis ofNatrix piscator during its sloughing cycle

Succinic dehydrogenase activity, in the epidermis of Nairix piscutor in different stages of sloughing cycle, has been localized using a nitro-BT technique with appropriate controls. The staining properties of different layers in scale epidermis are similar to the corresponding layers in hinge epidermis.
In the stratum germinativum, the layers of undifferentiated epidermal cells in all stages of the sloughing cycle, and in the lacunar tissue of Stages 3,4 and 5, a positive though weak reaction for SDH activity reflects the active metabolic state of the cells in these layers. Loss of SDH activity in Stage 6 indicates an inactive metabolic state of the lacunar tissue cells, corresponding with their disintegration owing to the cessation of nutrients as a result of keratinization of cells in the underlying layers.
The Oberhautchen, mesos and alpha layers in all Stages, and the clear layer cells in Stages 5 and 6 (outer epidermal generation), the presumptive Oberhautchen, presumptive mesos layer and presumptive alpha layer in all stages of their differentiation, and the presumptive beta layer in Stages 3 and 4 (inner epidermal generation) all stain purple with nitro-BT technique even in sections incubated in the medium without the substrate-succinate. The reaction is inhibited by prior treatment with 0.1 M N-ethyl maleimide blocking protein-bound -SH groups. This suggests that the reaction is due to the presence of protein-bound -SH groups in these sites. The reduced intensity of reaction in the mature beta layer of the outer epidermal generation, and in the presumptive beta layer in Stages 5 and 6 of the inner epidermal generation, is due to simultaneous loss of their content of -SH groups with maturation and keratinization.     

Expression of Hex homeobox gene during skin development: Increase in epidermal cell proliferation by transfecting the Hex to the dermis

A number of homeobox genes have been found to be expressed in skin and its appendages, such as scale and feather, and appear to be candidates for the regulation of the development of these tissues. We report that the proline-rich divergent homeobox gene Hex is expressed during development of chick embryonic skin and its appendages (scale and feather). In situ hybridization analysis revealed that, during development of the skin, a transient expression of the Hex gene was observed. While the expression of Hex in the dermis was closely correlated with proliferation activity of epidermal basal cells, that in the epidermis was related to a suppression of epidermal differentiation. When dermal fibroblasts were transfected with Hex, stimulation of both DNA synthesis and proliferation of the epidermal cells followed by two-fold scale ridge elongation and increase in epidermal area was observed during culture of the skin, whereas epidemal keratinization was not affected. This is the first study to demonstrate that Hex is expressed during development of the skin and its appendages and that its expression in the dermal cells regulates epidermal cell proliferation through epithelial mesenchymal interaction.     

TGF-beta and retinoic acid: regulators of growth and modifiers of differentiation in human epidermal cells.

In the epidermis of skin, a fine balance exists between proliferating progenitor cells and terminally differentiating cells. We examined the effects of TGF-beta s and retinoic acid (RA) on controlling this balance in normal and malignant human epidermal keratinocytes cultured under conditions where most morphological and biochemical features of epidermis in vivo are retained. Our results revealed marked and pleiotropic effects of both TGF-beta and RA on keratinocytes. In contrast to retinoids, TGF-beta s acted on mitotically active basal cells to retard cell proliferation. Although withdrawal from the cell cycle is a necessary prerequisite for commitment to terminal differentiation, TGF-beta s inhibited normal keratinization in suprabasal cells and promoted the type of differentiation commonly associated with wound-healing and epidermal hyperproliferation. The actions of TGF-beta s and RA on normal keratinization were synergistic, whereas those on abnormal differentiation associated with hyperproliferation were antagonistic. These observations underscore the notion that environmental changes can act separately on proliferating and differentiating cells within the population. Under the conditions used here, the action of TGF-beta s on human keratinocytes was dominant over RA, and TGF-beta s did not seem to be induced as a consequence of RA treatment. This finding is consistent with the fact that RA accelerated, rather than inhibited, proliferation in raft cultures. Collectively, our data suggest that the effects of both factors on epidermal growth and differentiation are multifaceted and the extent to which their action is coupled in keratinocytes may vary under different conditions and/or in different species.     

Effects of theophylline, epidermal chalone and x-irradiation on proliferation and differentiation of human keratinocytes in vitro.

Outgrowth cultures of normal human epidermis were used to study a possible relationship between growth inhibition and differentiated function. The effects of theophylline, epidermal chalone and x-irradiation on mitoses and the characteristic production of epidermal keratohyaline granules (KG) were examined at various intervals after the treatment. Theophylline (an inhibitor of cyclic nucleotide phosphodiesterase) or epidermal chalone inhibited mitoses and enhanced KG production. X-irradiation inhibited mitoses but had no effect on KG formation. These results indicate that inhibition of proliferation per se is not sufficient to enhance keratinization of human epidermal cells.     

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 morphogenesis, can direct tissue-specific statification 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.     

Uncoupling of the calcium-induced terminal differentiation and the activation of membrane-associated transglutaminase in murine keratinocytes by type-beta transforming growth factor

Calcium is an important regulator of terminal differentiation of cultured epidermal cells. In order to investigate the relationship between the termination of proliferative activity and the process of keratinization, we studied the time course of events induced by a sudden increase of extracellular calcium (calcium-switch) in cultures of established murine skin keratinocytes (BALB/c MK-1). These cells displayed density-dependent growth arrest without undergoing terminal differentiation in the presence of serum- and mitogen-free medium with a calcium concentration less than 0.10 mM. The calcium-switch alone was sufficient to induce a dose-dependent burst of DNA synthesis, which was followed by a state in which the cells became progressively refractory to mitogenic stimulation with epidermal growth factor. Treatment of cultures with type beta transforming growth factor during the first 6- to 10 h following the calcium-switch completely eliminated the initial burst of DNA synthesis as well as the terminal differentiation in response to calcium. On the other hand, the calcium-switch also caused the induction of a four- to fivefold increase of the activity of the membrane-associated form of transglutaminase that is required for keratinization, which was not affected by the presence of type beta transforming growth factor. These observations suggest that type beta transforming growth factor regulates the calcium-induced terminal cell division independently of the induction of phenotypic markers of keratinization, such as transglutaminase.     

MORPHOGENESIS OF CAPITATE GLANDULAR HAIRS OF CANNABIS SATIVA (CANNABACEAE)

The glandular secretory system in Cannabis sativa L. (marihuana) consists of three types of capitate glandular hairs (termed bulbous, capitate-sessile, and capitate-stalked) distinguishable by their morphology, development, and physiology. These gland types occur together in greatest abundance and developmental complexity on the abaxial surface of bracts which ensheath the developing ovary. Bulbous and capitate-sessile glands are initiated on very young bract primordia and attain maturity during early stages of bract growth. Capitate-stalked glands are initiated later in bract growth and undergo development and maturation on medium, to full sized bracts. Glands are epidermal in origin and derived, with one exception, from a single epidermal initial. The capitate-stalked gland is the exception and is of special interest because it possesses a multicellular stalk secondarily derived from surrounding epidermal and subepidermal cells. Glands differentiate early in development into an upper secretory portion and a subtending auxiliary portion. The secretory portion, depending on gland type, may range from a few cells to a large, flattened multicellular disc of secretory cells. The secretory portion produces a membrane-bound resinous product which caps the secretory cells. Capitate-stalked glands are considered to be of particular evolutionary significance because they may represent a gland type secondarily derived from existing capitate-sessile glands.     

Ultrastructure of epidermal glands in neotenic reproductives of the termite Prorhinotermes simplex (Isoptera: Rhinotermitidae)

The ultrastructure of epidermal glands in neotenic reproductives of Prorhinotermes simplex is described and their development is compared among young and old neotenics of both sexes. Secretory cells forming the epidermal gland are attached to the cuticle all over the body. The glands are formed by class 1 and class 3 secretory cells and corresponding canal cells with secretory function. Class 1 cells are sandglass-like and class 3 secretory units are located among them. Class 1 cells contain predominantly tubular endoplasmic reticulum, the major part represents the smooth and the minor the rough form. Numerous electron dense granules occur in the cytoplasm, they are always disintegrated prior to be released. Class 3 secretory cells contain a large amount of vacuoles, which are always lucent in males while newly produced vacuoles are dense in females. Dense vacuoles are frequently transformed into lucent ones before being released. Canal cells are locally equipped with microvilli. The conducting canal is surrounded by an electron dense secretion of regular inner structure. The cytoplasm of the canal cell contains numerous mitochondria, rough endoplasmic reticulum and a large proportion of microtubules. The young neotenic reproductives differ from the old ones by a lower amount of secretory products. Epidermal glands probably produce substances inhibiting the occurrence of superfluous reproductives.     

The outer mantle epithelium of Haliotis tuberculata (Gastropoda Haliotidae): an ultrastructural and histochemical study using lectins

The mantle of molluscs has been the subject of many studies as it is the organ that forms the shell. Microscopic studies in particular focus on the outer mantle epithelium, but few studies address this epithelium in a histochemical way. In this study, the outer mantle epithelium in adult specimens of Haliotis tuberculata is studied, that is, in specimens involved in maintaining and repairing the shell rather than in generating it. The epithelial cells are studied by scanning (SEM) and transmission electron microscopy (TEM), and by histochemical techniques, including the use of lectins for their biochemical characterization. The epithelium is composed of pigmented epidermal cells with small microvilli and junctional complexes. It furthermore contains a few ciliated cells, as well as two types of secretory cells which differ in the ultrastructural appearance of their secretory granules and their glycoconjugate content. Histochemical study shows secretory cells containing sulphated glycoconjugates such as glycosaminoglycans or mucins rich in N‐acetylgalactosamine and N‐glycoproteins rich in fucose. Furthermore, the apical regions of the epidermal cells are positive for lectins that label fucose, mannose and N‐acetylglucosamine. The role of epithelial cells in the synthesis of structural components of the shell is discussed.     

Comparative studies on petals structure,micromorphology and ultrastructure in two species of <Emphasis Type="Italic">Stephania</Emphasis> (Menispermaceae)

Petals in Ranunculales are nectariferous organs referred to as “nectary leaves” and show diversity in shape, color and structure due to various positions and structure of nectary tissue. Menispermaceae are in the core Ranunculales and have green and short nectary leaves, but the knowledge on structure and functions of the nectary leaves is limited. We use scanning electron microscopy, light microscopy and transmission electron microscopy to investigate nectary leaves structure, micro-morphology and ultrastructure in two species of Stephania in Menispermaceae. Our results show that secretory tissues present in the upper part of abaxial side of the nectary leaves and the specialized secretory epidermal cells are distinguished from other cells. In Stephania cepharantha, clusters of secreting epidermal cells are raised slightly above other cells to form some “bulges” (8–25 cells arranged in a cluster) and connected to distinct huge sieve tube elements. In contrast, in Stephania japonica, secretory epidermal cells are lower than non-secreting cells and result in many “well-like” structures (comprising 6–20 cells per “well”), and have no sieve tube element connection. Secretory epidermal cells have dense cytoplasm, large nucleus and abundant organelles. Nectar secretions are exuded via micro-channels or pores of cuticle on outer walls. The type of secretory tissue in Stephania is the variant of nectarioles.     

Avian scale development. VII. Normal keratinization follows abnormal morphogenesis of reticulate scales from the “scaleless” mutant

The scutate scales are entirely missing in chick embryos homozygous for the gene, “scaleless.” Reticulate scales of this mutant are present; however, they have undergone abnormal morphogenesis into irregular mounds and crevices. The pattern of keratinization seen along the anterior metatarsus of normal embryos differs dramatically from that seen along the anterior metatarsus of scaleless embryos. In contrast, we find that the unique pattern of keratinization seen in the epidermal cells of normal reticulate scales is retained in mutant reticulate scales, even though these scales are morphologically abnormal. We believe that differences in the initial tissue interactions (which establish the inductive ability of the dermis) of these two types of scales are responsible for the differences seen in their responses to the scaleless gene. The pleiotropic nature of the scaleless gene is discussed.     

Abnormal keratinization in the pupoid fetus (pf/pf) mutant mouse epidermis

During its development the epidermis of the pf/pf mutant mouse is invaded by cells from the underlying dermis. These invading cells establish a network of cells including fibroblasts, endothelial cells, and nerve fibers, throughout the epidermis. Subsequent to these events the keratohyalin protein, filaggrin, is drastically reduced and keratinization fails to occur. Heterotypic tissue recombinations indicate that the pf gene is not expressed in the skin. After simply grafting whole mutant dorsal skin, filaggrin synthesis is initiated and an orderly process of epidermal differentiation is achieved. These results suggest that the pf gene acts systemically and that the failure of epidermal differentiation in the mutant occurs secondary to abnormal epidermal organization.     

Monoclonal antibody analysis of keratin expression in epidermal diseases: a 48- and 56-kdalton keratin as molecular markers for hyperproliferative keratinocytes

The polypeptide composition of epidermal keratin varies in disease. To better understand the biological meaning of these variations, we have analyzed keratins from a number of human epidermal diseases by the immunoblot technique using AE1 and AE3 monoclonal antikeratin antibodies. The results reveal a continuous spectrum of keratin expression ranging from one closely resembling the normal in vivo pattern to one almost identical to cultured epidermal keratinocytes. Specifically, a 50-kilodalton (kd) (AE1-positive) and a 58-kd (AE3-positive) keratin are present in all diseases, supporting the concept that they represent "permanent" markers for keratinocytes. A 56.5-kd (AE1) and a 65-67-kd (AE3) keratin, previously shown to be markers for keratinization, are expressed only by lesions retaining a keratinized morphology. A 48-kd (AE1) and a 56-kd (AE3) keratin are present in all hyperproliferative (para- or nonkeratinized) disorders, but not in normal abdominal epidermis or in ichthyosis vulgaris which is a nonhyperproliferative disease. These two keratins have previously been found in various nonepidermal keratinocytes undergoing hyperproliferation, suggesting that these keratins are not epidermis-specific and may represent markers for hyperproliferative keratinocytes in general. In various epidermal diseases, there is a reciprocal expression of the (keratin) markers for hyperproliferation and keratinization, supporting the mutual exclusiveness of the two cellular events. Moreover, our results indicate that, as far as keratin expression is concerned, cultured human epidermal cells resemble and thus may be regarded as a model for epidermal hyperplasia. Finally, the apparent lack of any major, disease-specific keratin changes in the epidermal disorders studied so far implies that keratin abnormalities probably represent the consequence, rather than the cause, of these diseases.