Abnormal expression and processing of keratins in pupoid fetus (pf/pf) and repeated epilation (Er/Er) mutant mice

The cell cycle regulator protein 14-3-3σ is essential for hair follicle integrity and epidermal homeostasis

The 14-3-3σ (Stratifin; Sfn) is a cell cycle regulator intimately involved in the program of epithelial keratinization. 14-3-3σ is unique in that it is expressed primarily in epithelial cells and is frequently silenced in epithelial cancers. Despite its well-documented role as a cell cycle regulator and as a tumor suppressor, the function of 14-3-3σ in the intricate balance of proliferation and differentiation in epithelial development is poorly understood. A mutation in 14-3-3σ was found to be responsible for the repeated epilation (Er) phenotype. It has previously been shown that Sfn(+/Er) mice are characterized by repeated hair loss and regrowth, whereas Sfn(Er/Er) mice die at birth displaying severe oral fusions and limb abnormalities as a result of defects in keratinizing epithelia. Here we show that mice heterozygous for the 14-3-3σ mutation have severe defects in hair shaft differentiation, resulting in destruction of the hair shaft during morphogenesis. Furthermore, we report that the interfollicular epidermis and sebaceous glands are hyperproliferative, coincident with expanded nuclear Yap1 (Yes-associated protein 1)--a critical modulator of epidermal stem cell proliferation. We also report that hair follicle stem cells in the bulge cycle abnormally, raising important questions as to the role of 14-3-3σ in the bulge.

Interferon regulatory factor 6 is necessary, but not sufficient, for keratinocyte differentiation

Regulation of epidermal proliferation and differentiation is critical for maintenance of cutaneous homeostasis. Interferon Regulatory Factor 6 (Irf6)-deficient mice die perinatally and exhibit ectopic proliferation and defective epidermal differentiation. We sought to determine whether these disruptions of epidermal function were cell autonomous, and used embryonic Irf6(-/-) keratinocytes to understand the specific role of Irf6 in keratinocyte proliferation and differentiation. In the absence of Irf6, keratinocytes exhibited a heterogeneous phenotype with the presence of large cells. Irf6(-/-) keratinocytes displayed increased colony-forming efficiency compared with wild-type cells, suggesting that Irf6 represses long-term proliferation. Irf6 was present at low levels in wild-type keratinocytes in culture, and upregulated after induction of differentiation in vitro, along with upregulation of markers of early differentiation. However, Irf6(-/-) keratinocytes did not express markers of terminal differentiation. Overexpression of Irf6 in wild-type keratinocytes was insufficient to induce expression of markers of differentiation under growing conditions. Together, these results indicated that Irf6 is necessary, but not sufficient, for keratinocyte differentiation. Finally, using a transgenic mouse expressing Lac-Z under the regulation of an enhancer element 9.7  kb upstream of the Irf6 start site, we demonstrated that this element contributes to the regulation of Irf6 in the epidermis and keratinocytes in culture.

Abnormal skin, limb and craniofacial morphogenesis in mice deficient for interferon regulatory factor 6 (Irf6)

Transcription factor paralogs may share a common role in staged or overlapping expression in specific tissues, as in the Hox family. In other cases, family members have distinct roles in a range of embryologic, differentiation or response pathways (as in the Tbx and Pax families). For the interferon regulatory factor (IRF) family of transcription factors, mice deficient in Irf1, Irf2, Irf3, Irf4, Irf5, Irf7, Irf8 or Irf9 have defects in the immune response but show no embryologic abnormalities. Mice deficient for Irf6 have not been reported, but in humans, mutations in IRF6 cause two mendelian orofacial clefting syndromes, and genetic variation in IRF6 confers risk for isolated cleft lip and palate. Here we report that mice deficient for Irf6 have abnormal skin, limb and craniofacial development. Histological and gene expression analyses indicate that the primary defect is in keratinocyte differentiation and proliferation. This study describes a new role for an IRF family member in epidermal development.

Abnormal skin, limb and craniofacial morphogenesis in mice deficient for interferon regulatory factor 6 (Irf6)

Transcription factor paralogs may share a common role in staged or overlapping expression in specific tissues, as in the Hox family. In other cases, family members have distinct roles in a range of embryologic, differentiation or response pathways (as in the Tbx and Pax families). For the interferon regulatory factor (IRF) family of transcription factors, mice deficient in Irf1, Irf2, Irf3, Irf4, Irf5, Irf7, Irf8 or Irf9 have defects in the immune response but show no embryologic abnormalities. Mice deficient for Irf6 have not been reported, but in humans, mutations in IRF6 cause two mendelian orofacial clefting syndromes, and genetic variation in IRF6 confers risk for isolated cleft lip and palate. Here we report that mice deficient for Irf6 have abnormal skin, limb and craniofacial development. Histological and gene expression analyses indicate that the primary defect is in keratinocyte differentiation and proliferation. This study describes a new role for an IRF family member in epidermal development.

Identification of 14-3-3sigma mutation causing cutaneous abnormality in repeated-epilation mutant mouse

Repeated-epilation (Er) mutation in the mouse is inherited as an autosomal and semidominant mutation. Major defects in heterozygous adults and homozygous fetuses were associated with skin and were caused by abnormal ectodermal differentiation. Heterozygous mice are characterized by repeated hair loss and regrowth, and homozygous fetuses die at birth with severe abnormality in skin, limb, tail, and face. To identify the gene causing Er mutation, we have performed gene-expression profiles of skins and mouse embryonic fibroblasts from WT and mutant Er mice by using Affymetrix (Santa Clara, CA) chip analysis. By analyzing the candidate genes generated from gene-expression profiling, we identified a Sfn mutation in Er mice. A single nucleotide insertion in the Sfn (Stratifin, also called 14-3-3sigma) coding region results in a truncated protein lacking 40 amino acid residues at the C terminus. The mutation is linked with phenotypes of Er-heterozygous and -homozygous mice. Ectopic overexpression of WT 14-3-3sigma in Er/Er keratinocytes rescues defects in keratinocyte differentiation. Our study demonstrates that 14-3-3sigma is a crucial regulator for skin proliferation and differentiation.

A mutation in stratifin is responsible for the repeated epilation (Er) phenotype in mice

Stratifin (Sfn, also called 14-3-3sigma) is highly expressed in differentiating epidermis and mediates cell cycle arrest. Sfn is repressed in cancer, but its function during development is uncharacterized. We identified an insertion mutation in the gene Sfn in repeated epilation (Er) mutant mice by positional cloning. Er/+ mice expressed a truncated Sfn protein, which probably contributes to the defects in Er/Er and Er/+ epidermis and to cancer development in Er/+ mice.

IKK alpha on center stage

Inhibitor of kappaB kinase alpha (IKK alpha) was originally identified as a component of a multiprotein kinase complex that regulates the activity of the transcription factor nuclear factor-kappaB (NF-kappaB) through phosphorylation of its inhibitor proteins, the IkappaBs. DiDonato discusses new roles that have been discovered for IKK alpha, focusing especially on its role in epidermal differentiation and on a new function of IKK alpha in B cell maturation. In epidermal differentiation, IKK alpha regulates the production of a secreted differentiation factor through a pathway that is independent of its role in activation of NF-kappaB. In B cell maturation, conventional NF-kappaB signal-induced activation of IKK alpha results in phosphorylation of p100 precursor proteins and increased proteolytic processing and constitutive NF-kappaB activation.

IKKalpha controls formation of the epidermis independently of NF-kappaB

The IKKalpha and IKKbeta catalytic subunits of IkappaB kinase (IKK) share 51% amino-acid identity and similar biochemical activities: they both phosphorylate IkappaB proteins at serines that trigger their degradation. IKKalpha and IKKbeta differ, however, in their physiological functions. IKKbeta and the IKKgamma/NEMO regulatory subunit are required for activating NF-kappaB by pro-inflammatory stimuli and preventing apoptosis induced by tumour necrosis factor-alpha (refs 5,6,7,8,9,10,11). IKKalpha is dispensable for these functions, but is essential for developing the epidermis and its derivatives. The mammalian epidermis is composed of the basal, spinous, granular and cornified layers. Only basal keratinocytes can proliferate and give rise to differentiated derivatives, which on full maturation undergo enucleation to generate the cornified layer. Curiously, keratinocyte-specific inhibition of NF-kappaB, as in Ikkalpha-/- mice, results in epidermal thickening but does not block terminal differentiation. It has been proposed that the epidermal defect in Ikkalpha-/- mice may be due to the failed activation of NF-kappaB. Here we show that the unique function of IKKalpha in control of keratinocyte differentiation is not exerted through its IkappaB kinase activity or through NF-kappaB. Instead, IKKalpha controls production of a soluble factor that induces keratinocyte differentiation.

A 5'-upstream region of a bovine keratin 6 gene confers tissue-specific expression and hyperproliferation-related induction in transgenic mice

Keratins, the constituents of epithelial intermediate filaments, are precisely regulated in a tissue- and development-specific manner, although little is known about the molecular mechanisms underlying this regulation. The expression pattern of keratin 6 is particularly complex, since besides being constitutively expressed in hair follicles and in suprabasal cells of a variety of internal stratified epithelia, it is induced in epidermis in both natural and artificially caused hyperproliferative situations. Therefore, the regulatory sequences controlling keratin 6 gene activity are particularly suitable for target gene expression in a tissue-specific manner. More interestingly, they can be skin-induced in transgenic animals or in gene therapy protocols, particularly those addressing epidermal hyperproliferative disorders. To delimit the regions containing these regulatory elements, different parts of the bovine keratin 6 gene linked to a beta-galactosidase reporter gene have been assayed in transgenic mice. A 9-kbp fragment from the 5' upstream region was able to provide both suprabasal tissue-specific and inducible reporter expression.

Induction of epidermal hyperplasia, hyperkeratosis, and papillomas in transgenic mice by a targeted v-Ha-ras oncogene

The regulatory elements of the human keratin K1 gene have been used to target expression of the v-Ha-ras oncogene exclusively in the epidermis of transgenic mice. We developed 12 transgenic mouse lines that express the HK1.ras transgene, producing epidermal hyperplasia in neonates and hyperkeratosis in juveniles. Eventually this skin phenotype diminished but with time adult animals developed papillomas that could persist or regress. The rate and frequency of tumorigenesis appeared to be limited, which suggests that v-Ha-ras requires a second or even third event to elicit and maintain a benign phenotype in transgenic mice. Since in certain transgenic lines papillomas appeared at wound sites, it appears that the promotion stimulus from wounding may be a second event. We envision that such transgenic mice that express v-Ha-ras in the epidermis will become a powerful model for assessing how environmental and molecular factors affect the process of multistage skin carcinogenesis in vivo, as well as a model for evaluating novel therapeutic protocols.

Effects of Topical Retinoids on Cytoskeletal Proteins: Implications for Retinoid Effects on Epidermal Differentiation

In vivo effects of retinoids on epidermal differentiation were investigated by analyzing cytoskeletal proteins in rhino mice treated topically with all-trans-retinoic acid (RA) and other retinoids (13-cis-retinoic acid, etretinate, TTNPB). Non-disulfide-linked cytoskeletal proteins, including keratins from the epidermal "living layers," were first selectively extracted using 9.5 M urea; subsequently, keratins of the stratum corneum were isolated using 9.5 M urea plus a reducing agent. Gel electrophoresis and immunoblot analysis showed that urea extracts of epidermis from vehicle-treated skin were composed predominantly of four major keratins (analogous to human epidermal keratins K1, K5, K10, and K14), and the keratin filament-associated protein filaggrin. In contrast, extracts of epidermis from retinoid-treated skin contained additional keratins (K6, K16, and K17) and almost no detectable filaggrin. Furthermore, similar analysis of stratum corneum keratins demonstrated that extracts from RA-treated skin did not contain the partially proteolyzed keratins typically observed in stratum corneum extracts of control animals. Hyperplasia-inducing agents (salicylic acid, croton oil) caused an increase in keratins K6, K16, and K17, but they did not effect filaggrin or alter proteolysis of stratum corneum keratins. The result that RA induced expression of keratins K6, K16, and K17, as commonly expressed in hyperproliferative epidermis, is consistent with the notion that retinoids increase epidermal cell proliferation in the basal and/or lower spinous layers. The findings that topical RA decreased filaggrin expression and reduced proteolysis of stratum corneum keratins, despite increased size and number of granular cells and the presence of an anucleate stratum corneum, suggest that topical RA may also modulate a later stage of epidermal differentiation involved in stratum corneum formation.

v-Ha-ras-induced mouse skin papillomas exhibit aberrant expression of keratin K13 as do their 7,12-dimethylbenz[a]anthracene/12-O-tetradecanoylphorbol-13-acetate -induced analogues

Introduction of the v-Ha-ras gene into primary epidermal keratinocytes, followed by grafting of these cells to animals, leads to the formation of benign epidermal tumors that resemble papillomas induced chemically by a two-stage carcinogenesis protocol. In this study, we investigated v-Ha-ras-induced papillomas for aberrant expression of type I keratin K13, previously described in 7,12-dimethylbenz[a]anthracene/12-O-tetradecanoylphorbol-13- acetate (DMBA/TPA)-induced mouse epidermal tumors. Papillomas produced from three independent infection series were removed 3 wk after grafting concomitant with control grafts originating from mock-, neo-, and v-fos-infected primary keratinocytes. Combined analysis of the grafts by western blotting of extracted keratins and immunofluorescence studies of frozen sections with a K13-monospecific antibody revealed K13 expression in all v-Ha-ras-induced papillomas and absence of this keratin in all control grafts. K13-positive cells in papillomas were restricted to the suprabasal cell layers of the lesions and, at this stage of papilloma development, occurred as foci of varying extensions. Analysis of genomic DNA from v-Ha-ras-induced papillomas for the methylation state of a CpG dinucleotide in the distant promoter region of the K13 gene revealed the occurrence of unmethylated DNA copies that were generated at the expense of methylated DNA copies ubiquitously present in normal epidermis. The ratio of unmethylated to methylated DNA copies correlated with the extent of suprabasal K13 protein expression. Thus, all features of aberrant K13 expression previously described in DMBA/TPA-induced papillomas were shared by v-Ha-ras-induced papillomas.

Increased sensitivity to two-stage skin carcinogenesis of mice heterozygous for the repeated epilation mutation (Er)

Mice heterozygous for repeated epilation mutation (Er) have cutaneous abnormalities that result in repeated loss of hair. Skin papillomas and carcinomas occur spontaneously in such Er/+ mice. BALB/c mice are generally resistant to induced skin cancers. We investigated whether Er/+ heterozygous mice of BALB/c genetic background exhibit increased susceptibility to spontaneous and induced skin tumors. Although none of the Er/+ CXB(N5) mice spontaneously developed skin tumors, they exhibited increased sensitivity to the development of skin papillomas induced by an initiation-promotion regimen. Er/+ mice developed papillomas after 20 micrograms DMBA initiation in the absence of TPA promotion, but the same dose of DMBA was subtumorigenic in +/+ (sibling) mice. Although 15 weeks of TPA promotion resulted in similar tumor susceptibilities, tumor latencies and tumor frequencies in the 2 groups of initiated mice, the papillomas were qualitatively different. Er/+ mice developed more papillomas of the delayed promoter-independent type, which occur after termination of promotion. In contrast, +/+ mice developed more promoter-dependent papillomas, which regress after termination of promotion. Therefore Er/+ mice had a significantly higher number of papillomas than +/+ mice at the termination of the experiment. These results suggest that Er-mutation-induced skin defects not only lead to the repeated loss of hair, but also influence the mode of development of skin papillomas from carcinogen-initiated cells.

Modulation of phosphorylation and dephosphorylation of keratin and other polypeptides by estradiol-17 beta in rat vaginal epithelium

Phosphorylation of keratin polypeptides was studied by incubating vaginal tissues (removed from estradiol primed and unprimed 30-day-old rats) with 32Pi. Analysis by SDS-PAGE and autoradiography showed that on treatment with estradiol phosphorylation of 63 and 58 kDa keratin polypeptides increased 3- and 2-fold respectively. Phosphorylation was maximal after 30 min of estradiol priming and decreased thereafter. Phosphorylation of some non-keratin polypeptides (37, 34, 32 and 25 kDa) also showed time dependent variation. The results showed that estradiol can modulate phosphorylation-dephosphorylation of keratins and other polypeptides in rat vaginal epithelial cells.

Tissue-specific expression of murine keratin K13 in internal stratified squamous epithelia and its aberrant expression during two-stage mouse skin carcinogenesis is associated with the methylation state of a distinct CpG site in the remote 5'-flanking region of the gene

Under normal conditions, the expression of the murine type-I keratin K13 is restricted to the suprabasal, differentiating cell layers of internal stratified squamous epithelia that line the oral cavity and the upper digestive tract. It is, however, also expressed aberrantly but constitutively in only the differentiating parts of 7,12-dimethylbenz[alpha]anthracene/12.0-tetradecanoyl-phorbol-13-acetate (DMBA/TPA) induced malignant epidermal tumors of the back skin of mice, whereas its likewise suprabasal expression in papillomas is highly variable [27]. In an approach to unravel regulatory DNA sequence elements involved in the tissue-specific and aberrant K13 expression, the 5'-flanking region of the gene was analyzed with regard to potential methylation sites and DNase hypersensitive regions. We report on the identification of a CpG dinucleotide (designated M1; located about 2.3 kb upstream of the transcriptional start site), whose methylation state correlates with the differential gene activity in various epithelia and tumors. We show that in K13-nonexpressing integumental epidermis the M1 site is methylated in both suprabasal and basal cells. In contrast, internal stratified squamous epithelia (i.e. tongue, esophagus, forestomach) exhibit an unmethylated M1 site not only in their suprabasal. K13-expressing cells, but also in basal cells--in which, however, the keratin is not yet synthesized. The identical situation is encountered in DMBA TPA-induced moderately differentiating epidermal squamous cell carcinomas with compartmentalized K13 expression. In papillomas we observed a striking correlation between the extent of both suprabasally expressed K13 protein and demonstrable DNA copies carrying an unmethylated M1 site. Moreover we found that the sequence region around the M1 site was DNAseI hypersensitive in K13-expressing malignant tumors, but DNaseI insensitive in K13-nonexpressing epithelia and cells. DNAseI hypersensitivity in K13-expressing tissues was, however, independent of an active transcription of the gene in differentiating cells or transcriptional inertia in basal cells. These results strongly suggest that the sequence element around the demethylated M1 site is involved in a multi-level control mechanism mediating the selective expression of the K13 gene in internal squamous epithelia and in DMBA/TPA-induced epidermal tumors.

Establishment and characterization of four new squamous cell carcinoma cell lines derived from oral tumors

Four cell lines were established from squamous cell carcinomas (SCC) of the oral cavity. Cell lines AW 13516 and AW 8507 were derived from poorly differentiated SCC and epidermoid carcinoma of the tongue respectively. Cell line AW 10498 was derived from moderately differentiated SCC of the lower alveolus, and AW 9803 grew from a well-differentiated SCC of a retromolar trigone. The cultures showed typical epithelial cell morphology, numerous mitotic figures, occasional multinucleated giant cells, individual cell diskeratosis and nuclear and nucleolar abnormalities. The cell lines AW 13516 and AW 8507 were fast growers with a doubling time of 35.5 h and 31.9 h, respectively, which was independent of the initial seeding density. Cell lines AW 10498 (doubling time 52.2 h) and AW 9803 (doubling time 66 h) showed slower growth and had shorter doubling times at higher seeding densities. The presence of cytokeratins was detected in all the four cell lines by using polyclonal antikeratin antisera in indirect immunofluorescence and in Western blotting. None of the cell lines expressed major histocompatibility complex (MHC) class II antigens. MHC class I antigens were expressed by three cell lines but not by AW 9803. Flow cytometric analysis of DNA content and chromosomal studies suggested the presence of polyploidy and aneuploidy in all the four cell lines. Ultrastructural studies revealed typical epithelial cell features, such as the presence of desmosomes, tonofilaments and keratin bundles.

Extensive changes in cytokeratin expression patterns in pathologically affected human gingiva

The stratified squamous epithelium of the oral gingiva and the hard palate is characterized by a tissue architecture and a cytoskeletal composition similar to, although not identical with, that of the epidermis and fundamentally different from that of the adjacent non-masticatory oral mucosa. Using immunocytochemistry with antibodies specific for individual cytokeratins, in situ hybridization and Northern blots of RNA with riboprobes specific for individual cytokeratin mRNAs, and gel electrophoresis of cytoskeletal proteins of microdissected biopsy tissue samples, we show changes in the pattern of expression of cytokeratins and their corresponding mRNAs in pathologically altered oral gingiva. Besides a frequently, although not consistently, observed increase in the number of cells producing cytokeratins 4 and 13 (which are normally found as abundant components in the sulcular epithelium and the alveolar mucosa but not in the oral gingiva) and a reduction in the number of cells producing cytokeratins 1, 10 and 11, the most extensive change was noted for cytokeratin 19, a frequent cytokeratin in diverse one-layered and complex epithelia. While in normal oral gingiva cytokeratin 19 is restricted to certain, sparsely scattered cells of --or near--the basal cell layer, probably neuroendocrine (Merkel) cells, in altered tissue of inflamed samples it can appear in larger regions of the basal cell layer(s) and, in apparently more advanced stages, also in a variable number of suprabasal cells. Specifically, our in situ hybridization experiments show that this altered suprabasal cytokeratin 19 expression is more extended at the mRNA than at the protein level, indicating that cytokeratin 19 mRNA synthesis may be a relatively early event during the alteration. These changes in cytokeratin expression under an external pathological influence are discussed in relation to other factors known to contribute to the expression of certain cytokeratins and with respect to changes occurring during dysplasia and malignant transformation of oral epithelia.

Ultrastructural mapping of a sperm plasma membrane autoantigen before and after the acrosome reaction

The rabbit sperm plasma membrane autoantigen, RSA, is a zona binding protein that binds the spermatozoon to the zona pellucida both before and after the acrosome reaction. In the present study rabbit spermatozoa undergoing the acrosome reaction in vitro are described and monospecific polyclonal mouse anti-RSA and protein A-gold label is used with the label-fracture technique (Pinto de Silva and Kan, J Cell Biol, 99:1156-1161, 1984) to map the location of RSA at the ultrastructural level before and after the acrosome reaction. RSA is most concentrated in the plasma membrane over the postacrosomal-equatorial region border. The label appears to cluster over the anterior aspects of the postacrosomal region's tooth-like projections. Following the acrosome reaction, RSA is still present in the postacrosomal region and often appears clustered in the medial aspects of the equatorial region.

Aberrant expression during two-stage mouse skin carcinogenesis of a type I 47-kDa keratin, K13, normally associated with terminal differentiation of internal stratified epithelia

Specific keratin cDNA probes and monospecific antikeratin antisera were used to analyze mouse epidermis and epidermal tumors for the expression of a type I 47-kDa keratin, K13, normally associated with terminal differentiation of internal stratified epithelia. We demonstrated that this keratin was virtually absent from the entire body epidermis at various stages of development. Also, it was not detected in various forms of acute and chronic epidermal hyperproliferation or in epidermal cells cultured under conditions that favored either cell proliferation or in vitro differentiation. In contrast, K13 was consistently expressed in squamous cell carcinomas of the skin induced by 7,12-dimethylbenz[a]anthracene and 12-O-tetradecanoylphorbol-13-acetate (TPA), whereas papillomas obtained by the same two-stage protocol were distinctly heterogeneous with regard to the expression of this keratin. These findings were true for two different strains of mice (NMRI and Sencar). Papillomas collected from Sencar mice after 12 wk or from NMRI mice after 15 wk of promotion with TPA were either negative for K13 or elicited variable amounts of this keratin. In all cases of positive expression of K13 in tumors, as in normal stratified internal epithelia, both the keratin protein and its mRNA invariably occurred in the differentiating cell compartments. In contrast to what we found in internal stratified epithelia, however, K13 was expressed without its commonly encountered type II 57-kDa partner, K4. Papillomas negative for the K13 protein were also devoid of K13 transcripts. This indicates that the aberrant K13 expression in tumors is regulated at the level of transcription. Our results suggest that K13 may provide a marker for malignant conversion in the mouse two-stage skin carcinogenesis model and may be especially suited for studies of gene expression regulation.

Centripetal transport of cytoplasm, actin, and the cell surface in lamellipodia of fibroblasts

Wound healing in Swiss 3T3 cultures was investigated with video-enhanced contrast (VEC) microscopy. The formation of protrusions at the leading edge of cells along wound was investigated in detail during the spreading stage, which usually lasted from 1 to 4 hr postwounding. Lamellipodia exhibited a continuous rearward, or centripetal, transport of a variety of cellular constituents at rates of approximately 0.26 microns/sec from the leading edge. The lamellipodia were also the sites of lateral migration as well as extension and retraction of actin microspikes. Actin fibers oriented transversely to the direction of movement were also observed to transport centripetally at similar rates. These fibers may in part give rise to large actin fibers forming at the interface between the base of the lamellipodia and the lamellae. Beads 0.5 microns in diameter attached to the dorsal surfaces of lamellipodia also transported centripetally at rates of approximately 0.21 microns/sec. Thus there is an apparent correlation between transport of a variety of structures within lamellipodia and with surface movements of lamellipodia.