The pattern of cellular organization of human epidermis

Epidermal cell turnover across tight junctions based on Kelvin's tetrakaidecahedron cell shape

In multicellular organisms, cells adopt various shapes, from flattened sheets of endothelium to dendritic neurons, that allow the cells to function effectively. Here, we elucidated the unique shape of cells in the cornified stratified epithelia of the mammalian epidermis that allows them to achieve homeostasis of the tight junction (TJ) barrier. Using intimate in vivo 3D imaging, we found that the basic shape of TJ-bearing cells is a flattened Kelvin's tetrakaidecahedron (f-TKD), an optimal shape for filling space. In vivo live imaging further elucidated the dynamic replacement of TJs on the edges of f-TKD cells that enables the TJ-bearing cells to translocate across the TJ barrier. We propose a spatiotemporal orchestration model of f-TKD cell turnover, where in the classic context of 'form follows function', cell shape provides a fundamental basis for the barrier homeostasis and physical strength of cornified stratified epithelia.

DOI:http://dx.doi.org/10.7554/eLife.19593.001

The skin surface – known as the epidermis – is made up of sheets of cells that are stacked up in layers. One of the roles of the skin is to provide a protective barrier that limits what leaks into or out of the body. A particular layer of the epidermis – referred to as the stratum granulosum – is primarily responsible for forming this barrier. The cells in this layer are sealed together in a zipper-like fashion by structures known as tight junctions.

New skin cells are continuously produced in the lowest cell layers of the epidermis, and move upwards to integrate into the stratum granulosum layer to replace old cells (which also move upwards to leave the layer). How stratum granulosum cells are replaced without disrupting the tight junction barrier was not well understood.

Yokouchi et al. used a technique called confocal microscopy to examine the stratum granulosum cells in the ears of mice, and found that the shape of these cells forms the basis of the barrier that they form. These cells resemble a flattened version of a shape called Kelvin’s tetrakaidecahedron: a 14-sided solid with six rectangular and eight hexagonal sides. This structure was proposed by Lord Kelvin in 1887 to be the best shape for filling space. Tight junctions are present on the edges of the flattened Kelvin’s tetrakaidecahedron.

Further experiments revealed that the tight junctions move from cell to cell in a spatiotemporally-coordinated manner in order to maintain a continuous barrier throughout the stratum granulosum as cells are replaced. A newly formed stratum granulosum cell appears beneath the cell that it will replace. The shape of these cells enables a new barrier of three-way tight junction contacts to form between them and the neighboring cells in the stratum granulosum. After this barrier has formed, the upper cell leaves the stratum granulosum.

Future research could address how cells adopt the flattened Kelvin’s tetrakaidecahedron shape, and discover why tight junctions only form in one layer of the epidermis.

DOI:http://dx.doi.org/10.7554/eLife.19593.002

ICRP Publication 131: Stem Cell Biology with Respect to Carcinogenesis Aspects of Radiological Protection

This report provides a review of stem cells/progenitor cells and their responses to ionising radiation in relation to issues relevant to stochastic effects of radiation that form a major part of the International Commission on Radiological Protection's system of radiological protection. Current information on stem cell characteristics, maintenance and renewal, evolution with age, location in stem cell 'niches', and radiosensitivity to acute and protracted exposures is presented in a series of substantial reviews as annexes concerning haematopoietic tissue, mammary gland, thyroid, digestive tract, lung, skin, and bone. This foundation of knowledge of stem cells is used in the main text of the report to provide a biological insight into issues such as the linear-no-threshold (LNT) model, cancer risk among tissues, dose-rate effects, and changes in the risk of radiation carcinogenesis by age at exposure and attained age. Knowledge of the biology and associated radiation biology of stem cells and progenitor cells is more developed in tissues that renew fairly rapidly, such as haematopoietic tissue, intestinal mucosa, and epidermis, although all the tissues considered here possess stem cell populations. Important features of stem cell maintenance, renewal, and response are the microenvironmental signals operating in the niche residence, for which a well-defined spatial location has been identified in some tissues. The identity of the target cell for carcinogenesis continues to point to the more primitive stem cell population that is mostly quiescent, and hence able to accumulate the protracted sequence of mutations necessary to result in malignancy. In addition, there is some potential for daughter progenitor cells to be target cells in particular cases, such as in haematopoietic tissue and in skin. Several biological processes could contribute to protecting stem cells from mutation accumulation: (a) accurate DNA repair; (b) rapidly induced death of injured stem cells; (c) retention of the DNA parental template strand during divisions in some tissue systems, so that mutations are passed to the daughter differentiating cells and not retained in the parental cell; and (d) stem cell competition, whereby undamaged stem cells outcompete damaged stem cells for residence in the niche. DNA repair mainly occurs within a few days of irradiation, while stem cell competition requires weeks or many months depending on the tissue type. The aforementioned processes may contribute to the differences in carcinogenic radiation risk values between tissues, and may help to explain why a rapidly replicating tissue such as small intestine is less prone to such risk. The processes also provide a mechanistic insight relevant to the LNT model, and the relative and absolute risk models. The radiobiological knowledge also provides a scientific insight into discussions of the dose and dose-rate effectiveness factor currently used in radiological protection guidelines. In addition, the biological information contributes potential reasons for the age-dependent sensitivity to radiation carcinogenesis, including the effects of in-utero exposure.

Label retaining cells and cutaneous stem cells

This is a comprehensive review on label retaining cells (LRC) in epidermal development and homeostasis. The precise in vivo identification and location of epidermal stem cells is a crucial issue in cutaneous biology. We discuss here the following problems: (1) Identification and location of LRC in the interfollicular epithelium and hair follicle; (2) The proliferative potential of LRC and their role in cutaneous homeostasis (3); LRC phenomenon and the Immortal Strand Hypothesis, which suggests an alternative mechanism for retention of genetic information; (4) Significance of LRC studies for development of stem cell concept. Now, it seems evident that LRC are a frequent feature of stem cell niches and revealing highly dormant LRC may be used for identification of stem cell niches in different tissues. LRC were used for screening specific markers of epidermal stem cells. Within a given tissue stem cells have different proliferative characteristics. There are more frequently cycling stem cells which function primarily in homeostasis, while LRC form a reserve of dormant, may be ultimate, stem cells, which are set aside for regeneration of injury or unforeseen need. The authors suggest that LRC dormancy described in Mammalia has much in common with developmental quiescence found in some other animals. For example in C. elegans reproductive system, vulval precursor cells have developmentally programmed cell-cycle arrest in the first larval stage, and then undergo an extended period of quiescence before resuming proliferation. Another example of developmental quiescence is the diapause, a widespread phenomenon exhibited by animals ranging from nematodes to mammals, often occurring at genetically predetermined life history stage.

Epidermal homeostasis: do committed progenitors work while stem cells sleep?

Tracking the fate of cells in murine epidermis in vivo has revealed that a committed progenitor cell population can maintain normal adult tissue in the long term without support from a long-lived, self-renewing population of stem cells. Here, we argue that these results challenge the dogma that stem-cell proliferation is required for the cellular homeostasis of the epidermis and other adult tissues, with important implications for tissue physiology and disease.

Sic transit gloria: farewell to the epidermal transit amplifying cell?

For the past 30 years, the prevailing model of epidermal homeostasis has been that epidermal stem cells give rise to transit amplifying cells, which undergo a limited number of cell divisions before initiating terminal differentiation. Recent studies challenge the existence of a transit amplifying cell compartment and suggest a new paradigm for epidermal homeostasis.

Interfollicular Epidermal Stem Cells: Identification, Challenges, Potential

Homeostatic epidermal tissue renewal is the result of the combined activity of rare but potent stem cells, and a large pool of short-lived progenitor cells termed transit amplifying cells. Although the existence of epidermal stem cells has been known for some decades, their specific role in the processes of wound repair, skin diseases, and carcinogenesis remains unelucidated. Nevertheless, significant advances have been made in the identification and functional characterization of both murine and human epidermal stem cells, which place investigators in an exciting position to gain further insights into the fundamental processes of tissue renewal and repair in the epidermis.

Nutrition 1: a vital consideration in the management of skin wounds

Nutrition is a provocative subject in tissue repair, particularly in relation to the skin which consists of many different tissues fulfilling various essential functions. This article, the first in a two-part series, will discuss the structure of the human skin and its variations, with indications as to the individual requirements of the cells in fulfilling their normal role in skin physiology and in repair systems. Nutrient gradients seem to play a major role in normal skin physiology and repair systems following injury. The second part will discuss in more detail the importance of major nutrient factors and their inter-relationships in tissue repair. It is clear that much of our available knowledge is derived from experimental studies in animals. Their relevance in human wound healing may be equivocal as the human skin is unique in the animal kingdom.

Intrinsic Patterns of Behavior of Epithelial Stem Cells

The early concepts concerning hematopoietic and epithelial stem cells that were derived from kinetic studies have been greatly enhanced by new information about a range of other properties of somatic and embryonic stem cells. Firstly, the stem and amplifying pattern characteristically established by epithelial lineages has been found to represent an intrinsic pattern that is generated by somatic epithelial stem cells without the need for additional environmental information. Secondly, it is now apparent that somatic epithelial stem cells are plastic and can be directed into a range of new pathways of differentiation by heterotypic interactions. The mechanisms of this plasticity need to be reconciled with the normally stable commitment of these cells to production only of progeny entering a tightly restricted range of phenotypic pathways. The present review discusses the intrinsic properties of epithelial stem cells and how they may be acted upon by connective tissues to generate a wide range of phenotypically different epithelial structures.

Lipid vesicles and other colloids as drug carriers on the skin

Colloids from an aqueous suspension can cross the skin barrier only through hydrophilic pathways. Various colloids have a different ability to do this by penetrating narrow pores of fixed size in the skin, or the relevant nano-pores in barriers modelling the skin. Such ability is governed by colloid adaptability, which must be high enough to allow penetrant deformation to the size of a pore in such barrier: for a 100 nm colloid trespassing the skin this means at least 5-fold deformation/elongation. (Lipid) Bilayer vesicles are normally more adaptable than the comparably large (lipid coated) fluid droplets. One of the reasons for this, and an essential condition for achieving a high bilayer adaptability and pore penetration, is a high bilayer membrane elasticity. The other reason is the relaxation of changing colloid's volume-to-surface constraint during pore penetration; it stands to reason that such relaxation requires a concurrent, but only transient and local, bilayer permeabilisation. Both these phenomena are reflected in bilayer composition sensitivity, which implies non-linear pressure dependency of the apparent barrier penetrability, for example. Amphipats that acceptably weaken a membrane (surfactants, (co)solvents, such as certain alcohols, etc.) consequently facilitate controlled, local bilayer destabilisation and increase lipid bilayer flexibility. When used in the right quantity, such additives thus lower the energetic expense for elastic bilayer deformation, associated with pore penetration. Another prerequisite for aggregate transport through the skin is the colloid-induced opening of the originally very narrow ( approximately 0.4 nm) gaps between cells in the barrier to pores with diameter above 30 nm. Colloids incapable of enforcing such widening-and simultaneously of self-adapting to the size of 20-30 nm without destruction-are confined to the skin surface. All relatively compact colloids seem to fall in this latter category. This includes mixed lipid micelles, solid (nano)particles, nano-droplets, biphasic vesicles, etc. Such colloids, therefore, merely enter the skin through the rare wide gaps between groups of skin cells near the organ surface. Transdermal drug delivery systems based on corresponding drug formulations, therefore, rely on simple drug diffusion through the skin; the colloid then, at best, can modulate drug transport through the barrier. In contrast, the adaptability-and stability-optimised mixed lipid vesicles (Transfersomes, a trademark of IDEA AG) can trespass much narrower pathways between most cells in the skin; such highly adaptable colloids thus mediate drug transport through the skin. Sufficiently stable ultra-adaptable carriers, therefore, can ensure targeted drug delivery deep below the application site. This has already been shown in numerous preclinical tests and several phase I and phase II clinical studies. Drug delivery by means of highly adaptable drug carriers, moreover, allows highly efficient and well-tolerated drug targeting into the skin proper. Sustained drug release through the skin into systemic blood circulation is another field of ultradeformable drug carrier application.

NHE1 regulates the stratum corneum permeability barrier homeostasis. Microenvironment acidification assessed with fluorescence lifetime imaging

The outermost epidermal layer, the stratum corneum (SC), exhibits an acidic surface pH, whereas the pH at its base approaches neutrality. NHE1 is the only Na(+)/H(+) antiporter isoform in keratinocytes and epidermis, and has been shown to regulate intracellular pH. We now demonstrate a novel function for NHE1, as we find that it also controls acidification of extracellular "microdomains" in the SC that are essential for activation of pH-sensitive enzymes and the formation of the epidermal permeability barrier. NHE1 expression in epidermis is most pronounced in granular cell layers, and although the surface pH of NHE1 knockout mice is only slightly more alkaline than normal using conventional pH measurements, a more sensitive method, fluorescence lifetime imaging, demonstrates that the acidic intercellular domains at the surface and of the lower SC disappear in NHE1 -/- animals. Fluorescence lifetime imaging studies also reveal that SC acidification does not occur through a uniform gradient, but through the progressive accumulation of acidic microdomains. These findings not only visualize the spatial distribution of the SC pH gradient, but also demonstrate a role for NHE1 in the generation of acidic extracellular domains of the lower SC, thus providing the acidification of deep SC interstices necessary for lipid processing and barrier homeostasis.

Estimation of size of clonal unit for keratinocytes in normal human skin

OBJECTIVE

It has been suggested that keratinocyte (KC) stem cells reside at the epicenter of a clonal population of cells. To estimate the territory or surface area covered by a single stem-cell-derived KC population in human skin, clonal skin maps were created from 3 healthy adult women and from normal skin of a psoriatic patient.

DESIGN

Two hundred fifty-eight punch biopsy samples of various sizes (ranging from 2 to 8 mm in diameter) were analyzed for clonality employing X chromosome inactivation patterns at the human androgen receptor gene (HUMARA) locus. DNA was isolated and clonality established by significant decrease of either maternal or paternal X chromosome band patterns following restriction enzyme digestion, polymerase chain reaction amplification, and gel electrophoresis.

RESULTS

Fifty-three (41%) of 128 two-mm biopsies were clonal, whereas only 6 (14%) of 43 three-mm, 5 (14%) of 36 four-mm, and 3 (8%) of 35 five-mm biopsies revealed a clonal population of KCs. By contrast, in 5 different biopsies from a psoriatic patient, including 4- or 5-mm sizes, all but 1 were clonal; even an 8-mm biopsy contained a clonal population of KCs. Mantel-Haenszel chi(2) analysis revealed a P value of.001, reflecting a strong trend in probability for presence of a single clone of KCs as related to size of the biopsy sample. By sequentially analyzing 30 contiguous 2-mm biopsy samples within a given strip of skin, 10 clonal domain changes, as reflected in maternal versus paternal switches, were observed.

CONCLUSIONS

These results provide direct evidence of a clonal population of KCs in normal and psoriatic lesion-free skin, and indicate that a clonal epidermal unit of KCs frequently can be detected in small biopsies (2 mm), but that in normal skin sampling, overlapping clones are apparently present in larger (ie, 4-5-mm) biopsies, producing nonclonal patterns. The clonal domain of progeny in normal skin has a rather limited territorial boundary (2 mm in diameter). However, in lesion-free skin from a psoriatic patient, there may be clonal expansion of KCs due to perturbation in epidermopoiesis and/or stem cell distribution.

Murine epidermal label-retaining cells isolated by flow cytometry do not express the stem cell markers CD34, Sca-1, or Flk-1

Keratinocyte stem cells are present in the murine epidermis, based on both in vitro and in vivo evidence, and better characterization of these cells remains an active goal. Because keratinocyte stem cells are believed to cycle slowly, a good method for identification is based on their ability to retain nucleoside analog, such as bromodeoxyuridine. Adult stem cells have been identified in other tissues, including hematopoietic, neural, and skeletal muscle, and stem cell surface markers have been characterized. We wanted to determine if cell-surface markers present on both hematopoietic and skeletal muscle stem cells (CD34, Sca-1, and Flk-1) were also present on keratinocyte stem cells, and could be used to identify them. The cell-surface expression of cells that retained bromodeoxyuridine label for at least 21 d was compared with that of nonlabel-retaining cells. Double-labeling for flow cytometric analysis was employed, and label-retaining cells were found to lack expression of the tested markers. Beta1 integrin levels were also evaluated, and although high expression was found on label-retaining cells, it was not specific for these cells.

Visualization of the lipid barrier and measurement of lipid pathlength in human stratum corneum

Detailed models of solute transport through the stratum corneum (SC) require an interpretation of apparent bulk diffusion coefficients in terms of microscopic transport properties. Modern microscopy techniques provide a tool for evaluating one key property-lipid pathway tortuosity-in more detail than previously possible. Microscopic lipid pathway measurements on alkali expanded human SC stained with the lipid-soluble dyes methylene blue, Nile red, and oil red O are described. Brightfield, differential interference contrast, fluorescence, and laser scanning confocal optics were employed to obtain 2-dimensional (2-D) and 3-dimensional (3-D) images. The 2-D techniques clearly outlined the corneocytes. Confocal microscopy using Nile red yielded a well-delineated 3-D structure of expanded SC. Quantitative assessment of the 2-D images from a small number of expanded SC samples led to an average value of 3.7 for the ratio of the shortest lipid-continuous pathway to the width of the membrane. This was corrected for the effect of alkaline expansion to arrive at an average value of 12.7 for the same ratio prior to swelling.

Retroviral transduction of murine epidermal stem cells demonstrates clonal units of epidermal structure

It has been suggested that the number and position of epidermal stem cells are related to the units of columnar structure in the upper epidermal strata and that the cells of each unit are derived from a single stem cell. Studies of cell lineage in developing tissues have been facilitated by the use of retroviral transduction to provide inherited expression of a histochemically demonstrable foreign gene product. To provide direct evidence about the clonal nature of epidermal units, murine epidermal keratinocytes were transduced with a replication-deficient retroviral vector carrying the beta-galactosidase gene. Subepidermal injection of virus in vivo led to infrequent transduction with only transient presence of beta-gal-staining keratinocytes within the epidermis. Transduction of keratinocytes in vitro and transplantation back to in vivo sites permitted demonstration of the transduced gene in clusters of cells within the reformed epidermis throughout a 12-wk period. The epidermis redeveloped an ordered columnar structure with restriction of transduced cells to individual columnar units. This clonal appearance is compatible with derivation of each epidermal unit from a single stem cell but is not compatible with a random pattern of cell proliferation. Transduced epidermal sheets that were recombined with oral mucosal connective tissue also redeveloped normal columnar structure with restriction of beta-gal staining to individual columnar units. These data suggest that the establishment of an epidermal stem cell pattern related to units of structure is an intrinsic property of the epithelium and is not dependent on regionally-specific connective tissue influences.

Epithelial stem cells

New molecular markers for epidermal stem cells have enabled their isolation both in vitro and from the epidermis lying between hair follicles. Micro-dissection experiments have localised a second population of stem cells within hair follicles. Epidermal stem cells have a patterned distribution in vivo. The patterning can be reconstituted in vitro, showing that it is generated by interactions between keratinocytes and that the differentiation of epidermal stem cells is regulated by signals from other keratinocytes. Recent evidence from transgenic mice suggests that stem cell behaviour in the gut may be regulated by similar cell-cell interactions in vivo. Candidate genes for mediating these interactions are the homologues of Drosophila cell fate patterning genes such as Notch and Wingless and the Cadherin family of cell-cell adhesion molecules. The roles of stem cells and of mutations of the Patched gene in epithelial carcinogenesis are discussed.

Rate of stratum corneum formation in the perinatal rat

During late gestation, the fetal rat exhibits marked hyperplasia of the interfollicular epidermis and accelerated cornification in preparation for birth. In this study, we utilized simple morphometric techniques to provide quantitative estimates of the rate of stratum corneum (SC) formation during the perinatal period. Cryostat sections of dorsal epidermis from rat pups between -48 and +72 h of age were expanded under alkaline conditions and the number of corneocyte interfaces counted from photomicrographs. This method yielded the following regression lines: prenatal, y = 0.19x + 13.07, r = 0.93; postnatal, y = 0.13x + 13.00, r = 0.93, where y = number of SC layers and x = age in hours. Lack of desquamation was assured by the postnatal persistence of the granular periderm. Adhesive stripping of the epidermis followed by phase-contrast microscopy revealed intact monolayer sheets of SC. Quantitation using a computerized image analysis system gave an average corneocyte surface area in the newborn rat of 1908 +/- 36 mu 2 (mean +/- SEM). Treatment of neonatal rats with epidermal growth factor (500 ng/g BW) increased the number of SC layers over the first 24 h of life (p < 0.05) and resulted in marked hyperkeratosis by one week of age. These results allow the following conclusions: 1) in the prenatal rat, SC forms at a rate of one layer every 5 h; 2) postnatally, SC formation slows to one layer every 8 h; 3) to support normal corneum synthesis at birth, dorsal keratinocytes must enter transition at a rate of approximately 3 cells/second/cm2 of body surface; and 4) treatment with exogenous EGF augments the rate of terminal differentiation of perinatal rat epidermis.

Structural relationship between epidermal lipid lamellae, lamellar bodies and desmosomes in human epidermis: an ultrastructural study

The water permeability of the stratum corneum (SC) appears to be regulated primarily by the lamellar arrangement of lipid bilayers between the corneocytes. A significant body of evidence already exists, suggesting that the specific structural organization of these intercellular lipid lamellae is responsible for the very low water permeability of the intact skin and that these lipid-rich structures may also influence the process of desquamation in the SC. In this electron microscopic study the structure of the intercellular domains at different levels within the SC of normal skin from 18 healthy subjects has been evaluated with a special fixation protocol utilizing acrolein vapour as primary fixation, followed by a modified ruthenium tetroxide (RuO4)-post-fixation technique. This procedure permitted an insight into the process of post-secretory extracellular processing of the lamellar body (LB)-derived lipids into lamellar lipid bilayers. This transformation takes place in unique saccular invaginations of the intercellular domains, which indent the underlying stratum granulosum (SG) cells. In this specialized environment LB lipids are first processed into broad sheets before they become part of the typical lamellar lipid structure of the SC. Furthermore, in the process of lipid maturation distinct differences between inner and outer parts of the SC emerge, in particular an increase in both the number of the lamellae per intercellular space, and their order of arrangement. Moreover, distinct structural relationships between desmosomes (at the SG/SC interface and lower SC) and desmosomal remnants (at the stratum disjunctum) on the one hand, and lipid layers on the other, have been demonstrated, pointing to an important functional interaction of these components in normal human skin.

Identification of late differentiation antigens of human cornified epithelia, expressed in re-organized desmosomes and bound to cross-linked envelope

Little is known about the process leading to desquamation in cornified epithelia. We describe late differentiation antigens (Ag) specific for human cornified squamous epithelia, defined by two murine monoclonal antibodies (MoAb), G36-19 and B17-21, produced after immunization with plantar stratum corneum (SC). Histologically, in epidermis both Ag are cytoplasmic in the lower stratum granulosum (SG), become pericellular in the upper SG, and progressively disappear in the lower SC. In contrast, they persist up to the desquamating corneocytes in the palmoplantar epidermis and hard palate epithelium, as well as in the three cornified epithelial components of the inner root sheath (IRS) of the hair follicle (HF). Cytologically, both Ag are expressed as surface spots only on rough corneocytes. They are largely preserved on cross-linked envelopes (CLE) of the fragile type. Ultrastructurally, both Ag appear in keratinosome-like cytoplasmic vesicles in the upper stratum spinosum (SS) and the SG keratinocytes, then are found in both the regular and reorganizing desmosomes of the SG keratinocytes, and lastly in the corneocyte-specific reorganized desmosomes we propose to name corneodesmosomes. On CLE, the Ag are located on fibrils gathered over the external side of the envelope. Immunochemically, the G36-19--defined epitope is sequential and shared by five non-cytokeratin protein antigens of molecular weight 33.5, 36.5, 40, 49, and 52 kD, the higher molecular weight polypeptides being possibly precursors of the 33.5-kD protein. In contrast, the B17-21 epitope, unaccessible by immunoblotting, is probably conformational. In long-term cultured keratinocytes, the Ag are only expressed when epidermal sheets are morphologically differentiated. The expression is enhanced in the absence of fetal calf serum (FCS) and of epidermal growth factor (EGF). G36-19 and B17-21 Ag participate in a corneodesmosome-CLE superstructure that is probably involved in corneocyte cohesiveness and partly responsible for the mechanical resistance of the SC. These Ag are relevant markers for studying desmosomal maturation during epidermal differentiation and desquamation.

Architectural organization of human oral epithelium as visualized by keratin staining pattern in tobacco-associated leukoplakias

The keratin staining pattern in clinical normal buccal mucosa in smokers and non-smokers and in tobacco-associated leukoplakias histologically characterized by the chevron type of keratinization were studied. No differences in keratin staining pattern were found in normal buccal mucosa in smokers and non-smokers. In the tobacco-associated leukoplakias we found distinct differences in staining pattern between areas overlying connective tissue papillae and epithelial ridge areas indicating a lateral organization of oral epithelium related to the ridge system. It seems possible to explain this pattern in terms of current hypotheses of the proliferative conditions in squamous epithelia as previously studied in detail in the epidermis and tongue filiform papillae. We conclude, that our results confirm the presence of subpopulations of suprabasal and basal cells and support the presence of proliferative units in oral epithelium. Furthermore, smoking does not seem to influence the keratin staining pattern in clinically normal buccal mucosa.

The spatial organisation of keratinocytes in acquired middle ear cholesteatoma resembles that of external auditory canal skin and pars flaccida

A scanning electron microscopic study of acquired middle ear cholesteatoma was undertaken in order to compare morphological findings with those of keratinising squamous epithelium of the tympanic membrane and external auditory meatus. Analysis of results confirms a highly ordered vertical architecture, with cell stacking, in the stratum corneum of cholesteatoma, similar to that found in the skin of the deep external auditory meatus and the pars flaccida of the tympanic membrane. These findings support the notion that acquired cholesteatoma originates by immigration of epithelium from the pars flaccida and external meatus rather than by metaplasia of middle ear mucosa.

Cell surface glycosylation patterns in psoriasis

Cell surface carbohydrates are excellent markers of cellular differentiation and maturation processes due to their great structural and antigenic diversity as well as their known biosynthetic precursor/product relationships. Using a panel of monoclonal antibodies with well-defined carbohydrate specificities we have studied the expression of biosynthetically related antigens in normal and psoriatic skin. Two "families" of carbohydrate structures were investigated. One series of structures based on N-acetyllactosamine chains (type 2 chain: N-acetyllactosamine and fucosylated derivates hereof of H, Lex, Ley and sialyl-Lex) and another based on the simple mucin type core structures (type 3 chain: Tn, T and sialylated derivates hereof as well as the fucosylated derivative, H). Previously we have found these carbohydrate structures define distinct cell layers in stratified squamous epithelia of mucosa of the cheek, esophagus and uterine cervix. In normal and uninvolved epidermis, N-acetyllactosamine and T carbohydrates were found in the spinous cell layer, whereas the fucosylated derivates, H structures, were found in the granular cell layers above. The fucosylated and sialylated derivate of N-acetyllactosamine, sialylated Lex, had the same distribution as N-acetyllactosamine and T structures. This sequential expression of carbohydrates is similar to our previous findings in mucosa. However, in contrast to mucosa, normal skin basal cells did not label. The glycosylation pattern in psoriatic epithelium was changed in two ways. 1) Some carbohydrates (types 2 and 3 chain H and T) were expressed at an earlier stage of cell maturation. 2) The biosynthetic precursors to T structures, Tn and sialyl-Tn, which are not expressed in normal skin, and are often considered cancer-associated antigens, appeared in psoriatic skin. The Tn-antigen was expressed on basal and lower spinous cells, whereas the sialyl-Tn was only found on basal cells above the dermal papillae. The findings in the present work support previous studies of changes in cell surface glycosylation in psoriatic epidermis and demonstrate the appearance of tumor-associated antigens in highly proliferative, but benign, stratified epithelium.

A topographical study of the circadian rhythm in labelling index of mouse gingival and floor-of-mouth epithelium, including changes in labelling activity with individual cell position on the epithelial ridges

A continuous strip of epithelium from the mandibular teeth to the ventral surface of the tongue of B6D2F-1 mice was examined autoradiographically after tritiated thymidine flash-labelling. Five areas were defined: area 1, the gingival sulcus epithelium adjacent to tooth enamel; area 2, the free gingival margin epithelium; area 3, the attached gingiva; area 4, the floor of mouth with undulating basement membrane; area 5, the floor of mouth with flat basement membrane. Data for the circadian variation in the proportion of DNA synthetic cells were recorded into a microcomputer, which enabled a large number of cells to be scored. The topographical position of each basal cell along the rete ridges and the incidence of labelling were noted. In each of the five areas a statistically significant circadian variation in labelling index (LI) was demonstrated, with a peak at 04.00-06.00 h and a trough at 20.00 h, although area 1 was slightly out of phase with the rest. The 24-h average LI values were almost double those obtained from a single flash-labelling at 10.00 h. The peak to trough ratio in LI was greatest in area 5 and fell towards area 1. Within the attached gingiva, cells deepest in the epithelial ridges had a larger peak to trough ratio than more superficial basal cells. For a group of mice labelled at 10.00 h the mean LI of the basal epithelial cells in areas 1-5 was 7.5 +/- (3.0)% (SD). Various aspects of the distribution of DNA synthesis in relation to topography were examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of withdrawal from the cell cycle in cultured human epidermal keratinocytes

In keratinizing epithelia one of the earliest changes in the process of terminal differentiation is cessation of replication or withdrawal from the cell cycle. In this report, we measured the loss of colony-forming ability, and confirmed that withdrawal from the cell cycle is a specific event that occurs during maturation of the keratinocyte in culture. In addition, the rate of withdrawal was assayed by labeling cultures for 24 h with [14C]dThd and then measuring the fraction of labeled cells that undergo repeated cycles of DNA synthesis. These additional cycles of replication were measured by feeding BrdUrd to the cultures and quantitating the distribution of 14C-labeled DNA in unsubstituted and BrdUrd-substituted DNA in CsCl density gradients. The results show that the fraction of 14C-labeled DNA undergoing replication decreases exponentially by 23% every 24 h. This cessation of replication could not be explained by a reduced level of replication in the entire culture since during each day of the experiment about 8% of the total DNA underwent replication. The exponential decrease in replication of 14C-labeled DNA represents withdrawal from the cell cycle. Since the doubling time for keratinocytes is approximately 24 h, these results suggest that following each cycle of replication, there is a probability of 0.23 that postreplicated cells will withdraw from the cell cycle.

Heterogeneity in epidermal basal keratinocytes: morphological and functional correlations

Two structurally distinct populations of basal keratinocytes, nonserrated and serrated, were observed in cynomolgus monkey and human palm epidermis. Anatomical location, fine structural features, and kinetic properties suggest that nonserrated cells represent a stem cell population and that serrated cells help anchor the epidermis to the dermis.