Keratin Gene Expression Research Articles

Expression of keratin 14 and 19 mRNA and protein in normal oral epithelia, hairy leukoplakia, tongue biting and white sponge nevus

This study was undertaken to analyze keratin gene expression at both the mRNA and protein level in oral hairy leukoplakia (OHL). Comparisons were made with normal lingual epithelium from a similar site, tongue biting, normal buccal mucosa and another condition which disturbs oral epithelial differentiation, white sponge nevus. Combined immunocytochemical and in situ hybridization studies for keratins 14 and 19 were carried out on 2 specimens of OHL from HIV-positive males and one sample each of the other cases. Keratin 14 protein expression was uniform throughout all the epithelia. In normal epithelia and in lesions other than OHL, keratin 14 mRNA was most strongly expressed in basal cells with weaker but still significant amounts in the spinous cell layer. In both cases of OHL there was weaker basal cell expression of keratin 14 mRNA and frequent absence in koilocytoid cells. Keratin 19 protein expression was heterogeneous in the basal layer of all specimens with suprabasal staining of occasional groups of cells. Its mRNA was uniformly distributed in all cases. The findings indicate the keratin mRNA expression does not always parallel that of protein and that, in the case of keratin 14, expression may be influenced by the presence of EBV.

Retinoid-dependent transcriptional suppression of cytokeratin gene expression in human epidermal squamous cell carcinoma cells

Abstract We have previously demonstrated that cytokeratin levels are coordinately regulated in normal cultured human keratinocytes. In the present study we examine the mechanism of this regulation using human squamous cell carcinoma (SCC) cells. Treatment of SCC-13 cells with 20 or 200 n M trans-retinoic acid results in nearly complete suppression of cytokeratin K5 and K6 expression. This change is accompanied by a simultaneous reduction (> 20-fold) in the level of the mRNAs encoding K5 and K6. Transcriptional analysis indicates that the transcription rate of the K5 and K6 genes drops by approximately four to fivefold in retinoid treated nuclei. Retinol (2000 n M) also promotes this change. In contrast, cytokeratin K19 does not increase in the presence of retinoic acid, thus the normal coordinate regulation of keratin gene expression by retinoids appears to be uncoupled in SCC-13 cells. However, this does not represent a general defect in positive regulation of gene expression by retinoids, since in a transient transfection assay trans-retinoic acid positively regulates a reporter plasmid containing the retinoid response element from the retinoic acid receptor-β gene. The synthetic retinoids Ro 13–6298 (ethyl ester) and its metabolic derivative Ro 13–7410 (free acid) are both active in modulating the differentiation of normal keratinocytes. In contrast, only Ro 13–7410 is active in SCC-13 cells. As Ro 13–6298 binds poorly to the retinoic acid receptors, this suggests that SCC-13 cells, unlike normal keratinocytes, lack the ability to convert Ro 13–6298 to the active Ro 13–7410. These results further suggest that an inability to metabolize retinoid ethyl esters may be a property of transformed epidermal keratinocytes.

Regulation of Keratin Gene Expression: The Role of the Nuclear Receptors for Retinoic Acid, Thyroid Hormone, and Vitamin D3

Keratinization, the orderly process of differentiation of epidermal keratinocytes from stratum basale to stratum corneum, is influenced by hormones and vitamins. We have used expression of epidermal keratins as a paradigm of keratinization processes and analyzed the effects of retinoic acid, thyroid hormone, and vitamin D3 on keratin gene expression. DNA constructs in which keratin gene promoters drive expression of reporter genes were co-transfected with vectors expressing nuclear receptors for the above molecules into various cell types. The keratin promoters studied included K3, K5, K10, K14, and K16. The recipient cell types were HeLa and primary cultures of rabbit corneal and esophageal epithelial cells and of human epidermal keratinocytes. We found that retinoic acid, via its nuclear receptor, suppresses expression of all the above-listed keratin genes. Thyroid hormone and its receptor similarly suppressed those genes. The site of interaction between these two receptors and the promoter sequences of K10 and K14 genes has been identified. Surprisingly, vitamin D3 and its receptor had no direct effect on keratin promoters. Our results suggest that a retinoic acid has a twofold effect on keratin gene expression: by regulating keratinocyte differentiation it determines which keratins are expressed, basal cell specific or differentiation specific; by direct interaction between its receptor and keratin genes, retinoic acid determines the total amount of keratin protein within the cell. Vitamin D3, on the other hand, also regulates keratinocyte differentiation, but does not directly interact with the keratin genes.

Spatial, temporal, and hormonal regulation of epidermal keratin expression during development of the frog, Xenopus laevis

To study the mechanism of hormone-induced keratin expression in the epidermis during Xenopus metamorphosis, a monospecific antibody was raised against a unique carboxy-terminal peptide of the 63-kDa keratin. Immunohistological analysis demonstrated that the onset of 63-kDa keratin expression showed distinct regional and temporal differences. The expression started at stage 54 in the hindlimb epidermis, at stage 57 in the head, and over 1 month later at stage 63 in the tail. The amount of 63-kDa keratin was further regulated during epidermal stratification and differentiation. The 63-kDa keratin was expressed first in basal epidermal cells before stratification began. The outer layer of the larval epidermis (periderm) did not express the 63-kDa keratin. As the cells moved out of basal layer, they stained more intensely with the anti-keratin antibody indicating that 63-kDa keratin synthesis is up-regulated during differentiation. Similar results were obtained with cultures of purified epidermal cells grown in high calcium conditions. Since we have shown that thyroid hormone (T 3) induces 63-kDa keratin gene expression and hydrocortisone (HC) modulates T 3 action we examined the effects of T 3 and HC at the single cell level with the anti-keratin antibody. Immunostaining demonstrated that T 3 alone and T 3 plus HC increased the number of 63-kDa keratin-positive cells as well as the amount of 63-kDa keratin per cell. Unexpectedly these hormones had the same effects on head and tail epidermal cells even though the latter cells degenerate during metamorphosis. The major difference between tail and head cells was that the percentage 63-kDa keratin-producing cells was much greater in the head than in the tail.

Hormonal regulation of adult type keratin gene expression in larval epidermal cells of the frog Xenopus laevis

Abstract Triiodothyronin (T 3) is known to induce amphibian metamorphosis but other hormones such as glucocorticoids accelerate T 3 action. The increase in plasma concentration of both T 3 and glucocorticoids during metamorphic climax is correlated with the transformation of the epidermis from larval type (uncornified) to adult type (cornified). Previously we have shown that T 3 induced adult-type 63 Kd keratin gene expression and cornification of the larval epidermis. In this study, we have examined the effects of T 3 and hydrocortisone (HC) on the conversion of larval to adult epidermal cells in vitro. When larval epidermal cells were treated with both T 3 and HC, they had a synergistic effect on adult-type keratin synthesis (both 63 Kd and 49 Kd keratins) and epidermal cornification. The synergistic effect between T 3 and HC required a pretreatment with T 3 for 3 days. During this time, addition of HC to cultures containing T 3 did not change the amount of 63 Kd keratin mRNA. Thus, HC did not reduce the lag time for epidermal cells to respond to T 3. After 4 days of hormone treatment, T 3 increased the amount of 63 Kd keratin mRNA 9-fold while T 3 and HC induced it 18-fold. When cultures were pretreated with T 3 for 3 days, a 1 day treatment with HC was sufficient to obtain the synergistic effect. Thus the induction of 63 Kd keratin gene expression by T 3 required a much longer lag (3 days) than the lag required for the synergistic action of T 3 and HC (<1 day). These results demonstrate for the first time that the synergistic effects of T 3 and gluco-corticoid on metamorphosis occur at the mRNA level in a purified population of cells.

Regulation of Keratin Gene Expression in Hair Follicle Differentiationa

In hair growth, as the follicle bulb cells rapidly differentiate into either cortical or cuticle hair keratinocytes, about 50-100 keratin genes are transcriptionally activated. However, this complexity can be reduced to several, highly conserved gene families. In studying the regulation of keratin gene expression in the hair follicle we have isolated genes from most of these families and have examined their expression patterns by in situ hybridization. In the cortical keratinocytes striking patterns of keratin gene expression exist, suggesting that different transcriptional hierarchies operate in the various cell types. Comparisons of the keratin gene promoter regions indicates conserved sequence motifs that could be involved in determining these cell specificities. Similarly, we have isolated related sheep and human cuticle keratin genes and find conserved DNA motifs and expression patterns in cuticle cell differentiation. Additionally, the expression of sheep wool follicle IF and high-sulfur keratin genes in transgenic mice suggests that the regulatory DNA elements and proteins of hair keratin genes are functionally conserved between mammals.

Chromosomal Localization of Mouse Hair Keratin Genesa

Many genetic defects are known to cause abnormal development of the coat in mice. Hair keratin genes would seem to be particularly promising candidates among the potential targets of these mutations in mice and of inherited hair-related abnormalities in humans as well. We used specific probes from cloned and sequenced mouse hair keratin cDNAs (MHKA-2, MHKB-1, and MHKB-2) to assess linkage of hair keratin genes and mouse mutations. We analyzed DNA from the progeny of interspecies backcrossed mice for segregation of hair mutations, hair ("hard") keratin alleles, and epidermal ("soft") keratin alleles (Krt-1 and Krt-2 loci). The results suggest that most, if not all, hair keratin genes (types Ia and IIa) are part of the Krt-1 locus on chromosome 11 and Krt-2 locus on chromosome 15, respectively. Linkage of the hair keratin genes and the mutations Re, Den, and Bsk on chromosome 11, and Ca, Sha, and Ve on chromosome 15 suggests that these mutations may possibly involve altered hair keratin expression or structure. In addition, the nondispersion of homologous keratin genes in the mammalian genome suggests that a domain organization of the genes has influenced evolution of the keratin gene family and that the organization may play a significant role in tissue-specific and developmental regulation of keratin gene expression as well.

Transcription factor AP-2 is tissue-specific in Xenopus and is closely related or identical to Keratin Transcription Factor 1 (KTF-1)

This paper identifies a new, developmental role for transcription factor AP-2 in the activation of amphibian embryonic epidermal keratin gene expression. Keratin transcription factor KTF-1 is shown by several criteria to be identical or closely related to AP-2. KTF-1/AP-2 is shown to be tissue-specific from its first transcription in Xenopus embryos, and restricted to a small number of adult tissues, including skin. Epidermis-specific keratin transcription closely follows specification of the embryonic ectoderm in Xenopus, and is subject to regulation by growth factors and embryonic induction. We further show that in mouse basal keratinocytes, a KTF-1/AP-2-like factor is present and binds to a DNA sequence previously shown to be important in the regulation of the keratin K14 gene, which is actively expressed in these cells. Thus, the study of AP-2 and its role in the regulation of keratin gene transcription should enhance our understanding of both amphibian embryonic development and mammalian skin differentiation.

Keratin expression in rat intestinal crypt and villus cells. Analysis with a panel of monoclonal antibodies

Seven monoclonal antibodies were prepared against cytoskeletal components of rat intestinal brush borders. In the following paper (Chandler, J. S., Calnek, D., and Quaroni, A., J. Biol. Chem. 266, 11932-11938), three of them were shown to be specific for, respectively, keratin 8 (RK4), keratin 19 (RK7), and a newly identified type I keratin (keratin 21) (RK5). With these antibodies we have investigated the changes in keratin gene expression accompanying intestinal cell differentiation. Keratin 21 was detected exclusively in differentiated villus cells and in goblet, enteroendocrine, and Paneth cells in the crypts; in the proliferative crypt cells keratin 19 was predominant. Analysis of keratins expressed by cultured rat crypt cells (IEC cells) confirmed the absence of keratin 21 in undifferentiated intestinal cells. Changes in keratin's expression similar to those observed with cell differentiation in the adult intestinal mucosa were also demonstrated during early fetal intestinal development: the stratified epithelium present at 15-16 days of gestation contained predominantly keratin 19 with only a small amount of keratin 8; keratin 21 was first detected at 18-19 days of gestation, concomitant with the appearance of a well formed brush border and an apical cytoplasmic terminal web. These results suggest that keratin tonofilaments may play a role in the morphological and structural alterations accompanying intestinal cell differentiation in vivo.

Expression of Embryo-Specific Epidermal XK81 Keratin Gene in Dissociated Cells of Xenopus laevis Animal Caps Co-cultured with Mesoderm-Inducing Vegetal Cells: (Xenopus embryonic cells/dissociated animal cap/keratin gene expression/α-actin mRNA/mesodermal induction).

Dissociated cells of whole embryos or their animal halves were cultured until the time when sibling embryos reached the neurula stage, and expression of genes for embryo-specific epidermal XK81 keratin, muscle-specific α-actin and histone H4 was studied by Northern blot analysis. In the experiments with dissociated midblastula cells, the expression of keratin gene, but not two other genes, was found to be inhibited in animal cap cells when these cells were cultured with vegetal cells. However, in the experiments with midgastrula cells, such inhibition was not observed even when animal cap cells were co-cultured with vegetal cells. Therefore, keratin gene expression in animal cap cells is sensitive at the midblastula stage, but resistant at the midgastrula stage, to the inhibitory activity by vegetal cells. To determine when such transition occurs, we prepared animal cap cells from embryos at different stages and co-cultured them with vegetal cells from midblastulae. Results obtained showed that the transition takes place at the late blastula stage (stage 9). These results suggest that the transcriptional system for the embryo-specific epidermal XK81 keratin gene is stabilized in animal cap cells prior to gastrulation, so that it is expressed autonomously, without being inhibited by contact with vegetal cells.

Altered keratin expression in ichthyosis hystrix Curth-Macklin

The pathogenesis of a rare form of the ichthyotic diseases, ichthyosis hystrix Curth-Macklin, was investigated by immunohistochemistry and electron microscopy. Monoclonal antibodies (Mabs) against keratins expressed in normal basal cells (PKK2 and KA1), Mabs against keratins only present in normal fetal skin (PKK1), and Mabs against keratins 1, 2, 10, and 11 (KA5 and K8.60) were used. The Mabs reacting with normal basel cells showed an increased reaction with many cell layers. The Mab PKK1 distinctly reacted with the basal cell layer, suggesting an expression of fetal keratins. Electron microscopic study of both normal-looking and involved skin revealed the keratinization disorder characterized by tonofilament shells, perinuclear vacuoles, and binuclear keratinocytes. The results suggest that there is no prematurity of keratinization, but rather a pathological expression of specific keratin genes leading to expression of fetal keratins in this form of ichthyosis hystrix.

Differential keratin gene expression during the differentiation of the cement gland of Xenopus laevis

The expression of both epidermal and nonepidermal keratins has been detected in the cement gland of Xenopus laevis by antibody staining. Northern blot and in situ hybridizations with gene-specific probes indicated the expression of the nonepidermal keratin, XK endo B, and the embryonic epidermal keratin, XK70, in the cement gland. Furthermore, since explanted animal pole cells can be induced to differentiate into cement gland cells in vitro by incubation in NH 4Cl, we have demonstrated the in vitro induction of XK endo B, maintenance of XK70, and repression of another embryonic epidermal keratin, XK81. This is the first report of keratin gene expression in the cement gland.

Transcriptional regulation of a Xenopus embryonic epidermal keratin gene

XK81A1 is a type I epidermal keratin gene expressed in early developmental stages of Xenopus (Jonas et al. 1985). Fusion of the keratin promoter (-5900 to +26) to a human beta globin gene led to fully epidermis-specific accumulation of human globin mRNA and protein when this DNA was injected into fertilized eggs. Further localization of regulatory sequences was accomplished by injecting marked, 5'-deleted keratin gene DNA into fertilized eggs and evaluating tissue specificity of expression. All 5' flanking DNA upstream from -487 could be removed without interfering with keratin gene expression or regulation. These results suggest that the primary mode of regulation of epidermis-specific keratin gene expression is at the level of transcription, and that sequence elements in the 5' flanking region of the keratin gene, between -487 and +26, are responsible for this regulation.

Thyroid hormone induces constitutive keratin gene expression during Xenopus laevis development.

We have used in vitro explant cultures of Xenopus laevis skin to investigate the role that the thyroid hormone triiodothyronine (T3) plays in activating the 63-kilodalton (kDa) keratin genes. The activation of these genes in vivo requires two distinct steps, one independent of T3 and one dependent on T3. In this report we have shown that the same two steps are required to fully activate the 63-kDa keratin genes in skin explant cultures, and we have characterized the T3-mediated step in greater detail. Unlike the induction of transcription by T3 or steroid hormones in adult tissues, there was a long latent period of approximately 2 days between the addition of T3 to skin cultures and an increase in concentration of keratin mRNA. While the T3 induction of 63-kDa keratin gene transcription cannot occur until age 48, a short transient exposure of stage 40 skin cultures to T3 resulted in high-level expression of these genes 5 days later, when normal siblings had reached stage 48. This result indicates that T3 induces a stable change in epidermal cells which can be expressed much later, after extensive cell proliferation has occurred in the absence of T3. Once the 63-kDa keratin genes were induced, they were stably expressed, and by the end of metamorphosis T3 had no further effect on their expression. The results suggest that T3 induces constitutive expression of the 63-kDa keratin genes during metamorphosis.

Thyroid Hormone Induces Constitutive Keratin Gene Expression during Xenopus laevis Development

We have used in vitro explant cultures of Xenopus laevis skin to investigate the role that the thyroid hormone triiodothyronine (T3) plays in activating the 63-kilodalton (kDa) keratin genes. The activation of these genes in vivo requires two distinct steps, one independent of T3 and one dependent on T3. In this report we have shown that the same two steps are required to fully activate the 63-kDa keratin genes in skin explant cultures, and we have characterized the T3-mediated step in greater detail. Unlike the induction of transcription by T3 or steroid hormones in adult tissues, there was a long latent period of approximately 2 days between the addition of T3 to skin cultures and an increase in concentration of keratin mRNA. While the T3 induction of 63-kDa keratin gene transcription cannot occur until age 48, a short transient exposure of stage 40 skin cultures to T3 resulted in high-level expression of these genes 5 days later, when normal siblings had reached stage 48. This result indicates that T3 induces a stable change in epidermal cells which can be expressed much later, after extensive cell proliferation has occurred in the absence of T3. Once the 63-kDa keratin genes were induced, they were stably expressed, and by the end of metamorphosis T3 had no further effect on their expression. The results suggest that T3 induces constitutive expression of the 63-kDa keratin genes during metamorphosis.

Molecular Biology of Keratinocyte Differentiation

Epidermal keratinocytes (skin cells) are highly specialized epithelial cells designed to perform a very specific function, separation of the organism from its environment. To accomplish this the cells synthesize precursors and assemble them into two distinct structures, the cornified envelope and keratin intermediate filaments. The intermediate filaments are assembled from keratin monomers and the cornified envelope is assembled from a protein called involucrin and several other proteins. Expression of involucrin and the keratins genes are regulated as a function of the stage of keratinocyte differentiation and by various external agents such as calcium and vitamin A. To study the function of these structures and the regulation of precursor production we have cloned cDNA and genomic clones encoding involucrin and four of the keratin polypeptides. Retinoids profoundly alter the differentiation pattern of human epidermal keratinocytes, but the underlying biochemical basis of this change is not known. In this report we describe retinoid-promoted changes in keratin gene expression that may, in part, be responsible for the alteration in cellular phenotype in the presence of the vitamin. We also describe the novel structure of the human 40 kD keratin, a member of the keratin family that is retinoid responsive and is likely to be important during epidermal development. Finally, we describe the structure of the envelope precursor protein, involucrin, as determined from its DNA sequence and speculate on its role in cornified envelope formation.

Molecular biology of keratinocyte differentiation.

Epidermal keratinocytes (skin cells) are highly specialized epithelial cells designed to perform a very specific function, separation of the organism from its environment. To accomplish this the cells synthesize precursors and assemble them into two distinct structures, the cornified envelope and keratin intermediate filaments. The intermediate filaments are assembled from keratin monomers and the cornified envelope is assembled from a protein called involucrin and several other proteins. Expression of involucrin and the keratins genes are regulated as a function of the stage of keratinocyte differentiation and by various external agents such as calcium and vitamin A. To study the function of these structures and the regulation of precursor production we have cloned cDNA and genomic clones encoding involucrin and four of the keratin polypeptides. Retinoids profoundly alter the differentiation pattern of human epidermal keratinocytes, but the underlying biochemical basis of this change is not known. In this report we describe retinoid-promoted changes in keratin gene expression that may, in part, be responsible for the alteration in cellular phenotype in the presence of the vitamin. We also describe the novel structure of the human 40 kD keratin, a member of the keratin family that is retinoid responsive and is likely to be important during epidermal development. Finally, we describe the structure of the envelope precursor protein, involucrin, as determined from its DNA sequence and speculate on its role in cornified envelope formation.

Tissue-specific and differentiation-specific expression of a human K14 keratin gene in transgenic mice.

A construct containing approximately 2500 base pairs (bp) of 5' upstream and approximately 700 bp of 3' downstream sequence was used to drive the expression of an intronless human K14 gene in vitro and in vivo. To track the expression of the gene, a small sequence encoding the antigenic portion of neuropeptide substance P was inserted in frame 5' to the TGA translation stop codon of the gene. Surprisingly, this gene was expressed promiscuously in a wide variety of cultured cells transiently transfected with the construct. In contrast, when introduced into the germ line of transgenic mice, the construct was expressed in a fashion analogous to the endogenous K14 gene--namely, in the basal layer of stratified squamous epithelia. Our results suggest that some regulatory mechanism is overridden as a consequence of transient transfection but that sequences that can control proper K14 expression are present in the construct. The appropriate tissue-specific and differentiation-specific expression of K14.P in transgenic mice is an important first step in characterizing a promoter that could be employed to drive the foreign expression of drug-related genes in the epidermis of skin grafts.

Keratin gene expression during the resolution of psoriatic plaques: effect of dithranol, PUVA, etretinate and hydroxyurea regimens

Quantitative changes in the levels of keratin polypeptides extracted from keratotome shavings from psoriatic epidermis were measured by using one-dimensional SDS-PAGE, followed by scanning densitometry. Values obtained were compared with results for non-lesional epidermis and from epidermis from normal individuals. Patients on four different treatment regimens were investigated by repeated sampling over 3-4 months starting before therapy commenced. The levels of four keratins changed significantly: keratins I (70 kd) and 2 (66 kd) tended to rise to normal levels, while keratins 16 (50 kd) and 18 (44 kd) fell to normal levels. There were differential effects as well as differences in the rates of normalization depending upon the treatment regimen. The most rapid normalization of the levels of all four keratins was observed with topical dithranol (anthralin) treatment (five patients) with plaque resolution and keratin level normalization after 7-9 weeks. Oral hydroxyurea (three patients) had similar effects, but over a longer time scale (20 weeks). In contrast, oral etretinate (four patients) caused a normalization of all except keratin 2 (66 kd) over a period of 20-28 weeks, and keratin I (70 kd) levels tended to 'overshoot' the normal level. PUVA (five patients) caused rapid normalization (in 9-12 weeks) of keratins 2 (66 kd) and 18 (44 kd), but had much weaker effects on keratins I (70 kd) and 10 (57 kd). These results suggest that resolution of lesions as judged by clinical criteria can occur without normalization of the keratin electrophoretic profile. Possibly the most reliable marker of clinical resolution was the reduction in keratin 16 (50 kd), since treatment effects on the differentiation of keratins I (70 kd) and 2 (66 kd) were different.

Immunocytochemical localization and biochemical analysis of α and β keratins in the avian lingual epithelium

The alpha and beta keratins are found as 10-nm and 3-nm cytoplasmic filaments, respectively. While the alpha keratins are produced in essentially all vertebrate epithelia (Franke et al.: Exp. Cell Res., 116:429-445, 1978; Sun et al.: Proc. Natl. Acad. Sci. USA, 76:2813-2817, 1979), the beta keratins have been demonstrated only in specific epithelial tissues of birds and reptiles (Sawyer et al.: In: Biology of the Integument: Vertebrates. J. Bereiter-Hahn, A.G. Matoltsy, and K.S. Richards, eds. Springer-Verlag, Berlin, Vol. 2, pp. 194-238, 1986; Landmann: In: Biology of the Integument: Vertebrates. J. Bereiter-Hahn, A.G. Matoltsy, and K.S. Richards, eds. Springer-Verlag, Berlin, Vol. 2, pp. 150-187, 1986). Recently, Homberger and Brush (Zoomorphology, 106:103-114, 1986) have demonstrated that within the lingual epithelium of parrots, beta keratins are expressed exclusively in the anterior ventral region. While it is well established that epidermal-dermal interactions are important for the regional expression of the beta keratin genes in the avian scutate scales and feathers, little is known about the expression of beta keratins in other epithelial structures such as the tongue. We have used biochemical and immunocytochemical techniques to analyze the alpha and beta keratins of the lingual epithelium of the chick as an initial step in the characterization of this model system for developmental studies. We have found that alpha keratins are present throughout the lingual epithelium. The anterior ventral epithelium contains alpha keratin polypeptides characteristic of skin-type differentiation, while the epithelium of the dorsal and posterior ventral regions contains alpha keratin polypeptides characteristic of esophageal-type differentiation (O'Guin et al.: In: Current Topics in Developmental Biology: The Molecular and Developmental Biology of Keratins. A.A. Moscona and A. Monroy, eds. R.H. Sawyer, vol. ed. Academic Press, New York, Vol. 22, pp. 282-306, 1987). Beta keratins are produced only in the differentiated epithelial cells of the anterior ventral region of the tongue. Immunoelectron microscopy demonstrates that the alpha and beta keratins of the stratum intermedium and corneum of the anterior ventral region are found together in the large filament bundles characteristic of this region. The preexistence of the alpha keratins in the cells destined to produce beta keratins as well as the colocalization of these keratins in the filament bundles of these cells suggests that a functional relationship may exist between the alpha and beta keratins.