Biologic Structure and Function: Perspectives on Morphologic Approaches to the Study of the Granular Layer Keratinocyte

Cell differentiation in the embryonic periderm and in scaffolding epithelia of skin appendages

Terminal differentiation of epithelial cells is critical for the barrier function of the skin, the growth of skin appendages, such as hair and nails, and the development of the skin of amniotes. Here, we present the hypothesis that the differentiation of cells in the embryonic periderm shares characteristic features with the differentiation of epithelial cells that support the morphogenesis of cornified skin appendages during postnatal life. The periderm prevents aberrant fusion of adjacent epithelial sites during early skin development. It is shed off when keratinocytes of the epidermis form the cornified layer, the stratum corneum. A similar role is played by epithelia that ensheath cornifying skin appendages until they disintegrate to allow the separation of the mature part of the skin appendage from the adjacent tissue. These epithelia, exemplified by the inner root sheath of hair follicles and the epithelia close to the free edge of nails or claws, are referred to as scaffolding epithelia. The periderm and scaffolding epithelia are similar with regard to their transient functions in separating tissues and the conserved expression of trichohyalin and trichohyalin-like genes in mammals and birds. Thus, we propose that parts of the peridermal differentiation program were coopted to a new postnatal function during the evolution of cornified skin appendages in amniotes.

Immunolabeling for filaggrin and acidic keratins in the granular layer of mammalian epidermis indicates that an acidic-basic interaction is involved in cornification

The soft epidermis of mammals derives from the accumulation of keratohyaline granules in the granular layer, before maturing into corneocytes. Main proteins accumulated in the granular layer are pro-filaggrin and filaggrin that determine keratin clumping and later moisturization of the stratum corneum that remains flexible. This soft epidermis allows the high sensitivity of mammalian skin. Presence and thickness of the stratum granulosum varies among different species of mammals and even between different body regions of the same animal, from discontinuous to multilayered. These variations are evident using antibodies for filaggrin, a large protein that share common epitopes among placentals. Here we have utilized filaggrin antibodies (8959 and 466) and an acidic keratin antibody (AK2) for labeling placental, marsupial and monotreme epidermis. AK2 labeling appears mainly to detect K24 keratin, and less likely other acidic keratins. Immunoreactivity for filaggrin is absent in platypus, discontinuous in Echidna and in the tested marsupials. In placentals, it is inconstantly or hardly detected in the thin epidermis of bat, rodents, and lagomorphs with a narrow, mono-stratified and/or discontinuous granular layer. In contrast, where the granular layer is continuous or even stratified, both filaggrin and AK2 antibodies decorate granular cells. The ultrastructural analysis using the AK2 antibody on human epidermis reveals that a weak labeling is associated with keratohyalin granules and filamentous keratins of transitional keratinocytes and corneocytes. This observation suggests that basophilic filaggrin interacts with acidic keratins like K24 and determines keratin condensation into corneocytes of the stratum corneum.

Histopathological characterisation of trunk-dominant canine pemphigus foliaceus, and comparison with classic facial and insecticide-triggered forms

BACKGROUND

While the clinical features were described recently, the histopathological characterisation of trunk-dominant canine pemphigus foliaceus (PF) is lacking, and whether it differs from classic facial or insecticide-triggered PF is unknown.

HYPOTHESIS/OBJECTIVES

This study describes the histopathological findings of trunk-dominant PF, and compares the results to classic facial and insecticide-triggered PF.

ANIMALS

Skin biopsies from 103 dogs with clinically characterised trunk-dominant (n = 33), classic facial (n = 26) and insecticide-triggered PF (n = 44) were included.

MATERIALS AND METHODS

Histological sections, randomised and blinded, were scored for over 50 morphological parameters of pustules, epidermis, dermis, adnexa and crusts. Intact pustule area and width were measured by digital microscopy.

RESULTS

In trunk-dominant PF, 77 intact pustules were predominantly subcorneal (0.0019-1.940 mm2 area, 0.0470-4.2532 mm wide), and contained from one to over 100 acantholytic keratinocytes. Pustules had boat acantholytic cells, corneocytes, perinuclear eosinophilic rings, neutrophil rosettes, acantholytic cell necrosis, rafts, cling-ons and/or eosinophils. Peripustular epidermal spongiosis, necrosis and lymphocyte exocytosis occurred, as did follicular pustules. Mixed dermal inflammation often contained eosinophils. Trunk-dominant PF did not differ from the other PF groups except for few parameters, such as having fewer rafts (p = 0.003). Additional autoimmune inflammatory patterns occurred in all PF groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Trunk-dominant PF and other canine PF variants are histologically similar, which indicates shared pathomechanisms. The identification of common boat acantholytic cells and corneocyte separation has implications for the mechanisms of acantholysis. The diversity of histopathological and polyautoimmunity features support complicated immune mechanisms. Finally, results indicate that diagnostic biopsies cannot differentiate between these PF variants in dogs.

Targeting microbiome, drug metabolism, and drug delivery in oncology

Recent emerging scientific evidence shows a relationship between gut microbiota (GM) and immunomodulation. In the recently published "Hallmarks of Cancer", the microbiome has been reported to play a crucial role in cancer research, and perspectives for its clinical implementation to improve the effectiveness of pharmacotherapy were explored. Several studies have shown that GM can affect the outcomes of pharmacotherapy in cancer, suggesting that GM may affect anti-tumor immunity. Thus, studies on GM that analyze big data using computer-based analytical methods are required. In order to successfully deliver GM to an environment conducive to the proliferation of immune cells both within and outside the tumor microenvironment (TME), it is crucial to address a variety of challenges associated with distinct delivery methods, specifically those pertaining to oral, endoscopic, and intravenous delivery. Clinical trials are in progress to evaluate the effects of targeting GM and whether it can enhance immunity or act on the TME, thereby to improve the clinical outcomes for cancer patients.

The Discovery and Function of Filaggrin

Keratohyalin granules were discovered in the mid-19th century in cells that terminally differentiate to form the outer, cornified layer of the epidermis. The first indications of the composition of these structures emerged in the 1960s from a histochemical stain for histidine, followed by radioautographic evidence of a high incidence of histidine incorporation into newly synthesized proteins in cells containing the granules. Research during the next three decades revealed the structure and function of a major protein in these granules, which was initially called the ‘histidine-rich protein’. Steinert and Dale named the protein ‘filaggrin’ in 1981 because of its ability to aggregate keratin intermediate filaments. The human gene for the precursor, ‘profilaggrin,’ was reported in 1991 to encode 10, 11 or 12 nearly identical repeats. Remarkably, the mouse and rat genes encode up to 20 repeats. The lifetime of filaggrin is the time required for keratinocytes in the granular layer to move into the inner cornified layer. During this transition, filaggrin facilitates the collapse of corneocytes into ‘building blocks’ that become an impermeable surface barrier. The subsequent degradation of filaggrin is as remarkable as its synthesis, and the end-products aid in maintaining moisture in the cornified layer. It was apparent that ichthyosis vulgaris and atopic dermatitis were associated with the absence of this protein. McLean’s team in 2006 identified the cause of these diseases by discovering loss-of-function mutations in the profilaggrin gene, which led to dysfunction of the surface barrier. This story illustrates the complexity in maintaining a healthy, functional epidermis.

Cracking the Skin Barrier: Liquid-Liquid Phase Separation Shines under the Skin

Central to forming and sustaining the skin’s barrier, epidermal keratinocytes (KCs) fluxing to the skin surface undergo a rapid and enigmatic transformation into flat, enucleated squames. At the crux of this transformation are intracellular keratohyalin granules (KGs) that suddenly disappear as terminally differentiating KCs transition to the cornified skin surface. Defects in KGs have long been linked to skin barrier disorders. Through the biophysical lens of liquid-liquid phase separation (LLPS), these enigmatic KGs recently emerged as liquid-like membraneless organelles whose assembly and subsequent pH-triggered disassembly drive squame formation. To stimulate future efforts toward cracking the complex process of skin barrier formation, in this review, we integrate the key concepts and foundational work spanning the fields of LLPS and epidermal biology. We review the current progress in the skin and discuss implications in the broader context of membraneless organelles across stratifying epithelia. The discovery of environmentally sensitive LLPS dynamics in the skin points to new avenues for dissecting the skin barrier and for addressing skin barrier disorders. We argue that skin and its appendages offer outstanding models to uncover LLPS-driven mechanisms in tissue biology.

A unique mode of keratinocyte death requires intracellular acidification

The stratum corneum (SC), the outermost epidermal layer, consists of nonviable anuclear keratinocytes, called corneocytes, which function as a protective barrier. The exact modes of cell death executed by keratinocytes of the upper stratum granulosum (SG1 cells) remain largely unknown. Here, using intravital imaging combined with intracellular Ca²⁺- and pH-responsive fluorescent probes, we aimed to dissect the SG1 death process in vivo. We found that SG1 cell death was preceded by prolonged (∼60 min) Ca²⁺ elevation and rapid induction of intracellular acidification. Once such intracellular ionic changes were initiated, they became sustained, irreversibly committing the SG1 cells to corneocyte conversion. Time-lapse imaging of isolated murine SG1 cells revealed that intracellular acidification was essential for the degradation of keratohyalin granules and nuclear DNA, phenomena specific to SC corneocyte formation. Furthermore, intravital imaging showed that the number of SG1 cells exhibiting Ca²⁺ elevation and the timing of intracellular acidification were both tightly regulated by the transient receptor potential cation channel V3. The functional activity of this protein was confirmed in isolated SG1 cells using whole-cell patch-clamp analysis. These findings provide a theoretical framework for improved understanding of the unique molecular mechanisms underlying keratinocyte-specific death mode, namely corneoptosis.

Orchestrated control of filaggrin-actin scaffolds underpins cornification

Epidermal stratification critically depends on keratinocyte differentiation and programmed death by cornification, leading to formation of a protective skin barrier. Cornification is dynamically controlled by the protein filaggrin, rapidly released from keratohyalin granules (KHGs). However, the mechanisms of cornification largely remain elusive, partly due to limitations of the observation techniques employed to study filaggrin organization in keratinocytes. Moreover, while the abundance of keratins within KHGs has been well described, it is not clear whether actin also contributes to their formation or fate. We employed advanced (super-resolution) microscopy to examine filaggrin organization and dynamics in skin and human keratinocytes during differentiation. We found that filaggrin organization depends on the cytoplasmic actin cytoskeleton, including the role for α- and β-actin scaffolds. Filaggrin-containing KHGs displayed high mobility and migrated toward the nucleus during differentiation. Pharmacological disruption targeting actin networks resulted in granule disintegration and accelerated cornification. We identified the role of AKT serine/threonine kinase 1 (AKT1), which controls binding preference and function of heat shock protein B1 (HspB1), facilitating the switch from actin stabilization to filaggrin processing. Our results suggest an extended model of cornification in which filaggrin utilizes actins to effectively control keratinocyte differentiation and death, promoting epidermal stratification and formation of a fully functional skin barrier.

Ultrastructural Localization of Caspase-14 in Human Epidermis

Caspase-14 has been implicated in the formation of stratum corneum because of its specific expression and activation in terminally differentiating keratinocytes. However, its precise physiological role and its protein substrate are elusive. We studied the ultrastructural localization of caspase-14 in human epidermis to compare its distribution pattern with that of well-characterized differentiation markers. Immunogold cytochemistry confirmed that caspase-14 is nearly absent in basal and spinous layers. In the granular, layer nuclei and keratohyalin granules were labeled with increasing intensity towards the transitional layer. Particularly strong caspase-14 labeling was associated with areas known to be occupied by involucrin and loricrin, whereas F-granules, occupied by profilaggrin/filaggrin, were much less labeled. A high density of gold particles was also present at the forming cornified cell envelope, including desmosomes. In corneocytes, intense labeling was both cytoplasmic and associated with nuclear remnants and corneodesmosomes. These observations will allow focusing efforts of biochemical substrate screening on a subset of proteins localizing to distinct compartments of terminally differentiated keratinocytes.

Oxidative stress in aging human skin

Oxidative stress in skin plays a major role in the aging process. This is true for intrinsic aging and even more for extrinsic aging. Although the results are quite different in dermis and epidermis, extrinsic aging is driven to a large extent by oxidative stress caused by UV irradiation. In this review the overall effects of oxidative stress are discussed as well as the sources of ROS including the mitochondrial ETC, peroxisomal and ER localized proteins, the Fenton reaction, and such enzymes as cyclooxygenases, lipoxygenases, xanthine oxidases, and NADPH oxidases. Furthermore, the defense mechanisms against oxidative stress ranging from enzymes like superoxide dismutases, catalases, peroxiredoxins, and GSH peroxidases to organic compounds such as L-ascorbate, α-tocopherol, beta-carotene, uric acid, CoQ10, and glutathione are described in more detail. In addition the oxidative stress induced modifications caused to proteins, lipids and DNA are discussed. Finally age-related changes of the skin are also a topic of this review. They include a disruption of the epidermal calcium gradient in old skin with an accompanying change in the composition of the cornified envelope. This modified cornified envelope also leads to an altered anti-oxidative capacity and a reduced barrier function of the epidermis.

Distribution of caspase-14 in epidermis and hair follicles is evolutionarily conserved among mammals

Caspase-14, a member of the caspase family of cysteine proteases, is almost exclusively expressed in the epidermis. Studies on human and mouse cells and tissues have implicated caspase-14 in terminal differentiation of epidermal keratinocytes and in the formation of the stratum corneum. Here we investigated evolutionary aspects of the role of caspase-14 by analyzing its distribution in the epidermis and hair follicles of representative species of placental mammals, marsupials, and monotremes. Immunocytochemical staining showed that caspase-14 is consistently expressed in the granular and corneous layer of the epidermis of all mammalian species investigated. Ultrastructural analysis using gold-labeled anticaspase-14 antibodies revealed that caspase-14 is associated preferentially with keratin bundles and amorphous material of keratohyalin granules, but is also present in nuclei of transitional cells of the granular layer and in corneocytes. In hair follicles, caspase-14 was diffusely present in cornifying cells of the outer root sheath, in the companion layer, and, most abundantly, in the inner root sheath of all mammalian species here analyzed. In Henle and Huxley layers of the inner root sheath, labeling was seen in nuclei and, more diffusely, among trichohyalin granules of cornifying cells. In summary, the tissue expression pattern and the intracellular localization of caspase-14 are highly conserved among diverse mammalian species, suggesting that this enzyme is involved in a molecular process that appeared early in the evolution of mammalian skin. The association of caspase-14 with keratohyalin and trichohyalin granules may indicate a specific role of caspase-14 in the maturation of these keratinocyte-specific structures.

Immunocytochemical and autoradiographic studies on the process of keratinization in avian epidermis suggests absence of keratohyalin

The process of keratinization in apteric avian epidermis and in scutate scales of some avian species has been studied by autoradiography for histidine and immunohistochemistry for keratins and other epidermal proteins. Acidic or basic alpha-keratins are present in basal, spinosus, and transitional layers, but are not seen in the corneous layer. Keratinization-specific alpha-keratins (AE2-positive) are observed in the corneous layer of apteric epidermis but not in that of scutate scales, which contain mainly beta-keratin. Alpha-keratin bundles accumulate along the plasma membrane of transitional cells of apteric epidermis. In contrast to the situation in scutate scales, in the transitional layer and in the lowermost part of the corneous layer of apteric epidermis, filaggrin-like, loricrin-like, and transglutaminase immunoreactivities are present. The lack of isopeptide bond immunoreactivity suggests that undetectable isopeptide bonds are present in avian keratinocytes. Using immunogold ultrastructural immunocytochemistry a low but localized loricrin-like and, less, filaggrin-like labeling is seen over round-oval granules or vesicles among keratin bundles of upper spinosus and transitional keratinocytes of apteric epidermis. Filaggrin-and loricrin-labeling are absent in alpha-keratin bundles localized along the plasma membrane and in the corneous layer, formerly considered keratohyalin. Using ultrastructural autoradiography for tritiated histidine, occasional trace grains are seen among these alpha-keratin bundles. A different mechanism of redistribution of matrix and corneous cell envelope proteins probably operates in avian keratinocytes as compared to that of mammals. Keratin bundles are compacted around the lipid-core of apteric epidermis keratinocytes, which do not form complex chemico/mechanical-resistant corneous cell envelopes as in mammalian keratinocytes. These observations suggest that low amounts of matrix proteins are present among keratin bundles of avian keratinocytes and that keratohyalin granules are absent.

Synthesis of interkeratin matrix in differentiating lizard epidermis: An ultrastructural autoradiographic study after injection of tritiated proline and histidine

During epidermal differentiation in mammals, keratins and keratin-associated matrix proteins rich in histidine are synthesized to produce a corneous layer. Little is known about interkeratin proteins in nonmammalian vertebrates, especially in reptiles. Using ultrastructural autoradiography after injection of tritiated proline or histidine, the cytological process of synthesis of beta-keratin and interkeratin material was studied during differentiation of the epidermis of lizards. Proline is mainly incorporated in newly synthesized beta-keratin in beta-cells, and less in oberhautchen cells. Labeling is mainly seen among ribosomes within 30 min postinjection and appears in beta-keratin packets or long filaments 1-3 h later. Beta-keratin appears as an electron-pale matrix material that completely replaces alpha-keratin filaments in cells of the beta-layer. Tritiated histidine is mainly incorporated into keratohyalin-like granules of the clear layer, in dense keratin bundles of the oberhautchen layer, and also in dense keratin filaments of the alpha and lacunar layer. The detailed ultrastructural study shows that histidine-labeling is localized over a dense amorphous material associated with keratin filaments or in keratohyalin-like granules. Large keratohyalin-like granules take up labeled material at 5-22 h postinjection of tritiated histidine. This suggests that histidine is utilized for the synthesis of keratins and keratin-associated matrix material in alpha-keratinizing cells and in oberhautchen cells. As oberhautchen cells fuse with subjacent beta-cells to form a syncytium, two changes occur : incorporation of tritiated histidine, but uptake of proline increases. The incorporation of tritiated histidine in oberhautchen cells lowers after merging with cells of the beta-layer, whereas instead proline uptake increases. In beta-cells histidine-labeling is lower and randomly distributed over the cytoplasm and beta-keratin filaments. Thus, change in histidine uptake somehow indicates the transition from alpha- to beta-keratogenesis. This study indicates that a functional stratum corneum in the epidermis of amniotes originates only after the association of matrix and corneous cell envelope proteins with the original keratin scaffold of keratinocytes.

Adaptation to the land: The skin of reptiles in comparison to that of amphibians and endotherm amniotes

The adaptation to land from amphibians to amniotes was accompanied by drastic changes of the integument, some of which might be reconstructed by studying the formation of the stratum corneum during embryogenesis. As the first amniotes were reptiles, the present review focuses on past and recent information on the evolution of reptilian epidermis and the stratum corneum. We aim to generalize the discussion on the evolution of the skin in amniotes. Corneous cell envelopes were absent in fish, and first appeared in adult amphibian epidermis. Stem reptiles evolved a multilayered stratum corneum based on a programmed cell death, intensified the production of matrix proteins (e.g., HRPs), corneous cell envelope proteins (e.g., loricrine-like, sciellin-like, and transglutaminase), and complex lipids to limit water loss. Other proteins were later produced in association to the soft or hairy epidermis in therapsids (e.g., involucrin, profilaggrin-filaggrin, trichohyalin, trichocytic keratins), or to the hard keratin of hairs, quills, horns, claws (e.g., tyrosine-rich, glycine-rich, sulphur-rich matrix proteins). In sauropsids special proteins associated to hard keratinization in scales (e.g., scale beta-keratins, cytokeratin associated proteins) or feathers (feather beta-keratins and HRPs) were originated. The temporal deposition of beta-keratin in lepidosaurian reptiles originated a vertical stratified epidermis and an intraepidermal shedding layer. The evolutions of the horny layer in Therapsids (mammals) and Saurospids (reptiles and birds) are discussed. The study of the molecules involved in the dermo-epidermal interactions in reptilian skin and the molecular biology of epidermal proteins are among the most urgent future areas of research in the biology of reptilian skin.

The changes of epidermal calcium gradient and transitional cells after prolonged occlusion following tape stripping in the murine epidermis

Disruption of the epidermal permeability barrier causes an immediate loss of the calcium gradient, and barrier recovery is parallel with the restoration of the calcium gradient in the epidermis. Artificial restoration of the barrier function by occlusion with a water vapor-impermeable membrane abrogate the expected increase in lipid synthesis and retard the barrier recovery, as well as block the normalization of the epidermal calcium gradient. To clarify the long-term effects of occlusion after acute barrier perturbation, we studied the calcium distribution and epidermal keratinocytes response after occlusion with a water vapor-impermeable membrane immediately following tape stripping in the murine epidermis. Acute barrier disruption caused an immediate depletion of most calcium ions in the upper epidermis, obliterating the normal calcium gradient. When the skin barrier function was artificially corrected by occlusion, the return of calcium ions to the epidermis was blocked. After 2 h of air exposure or occlusion, the density of epidermal calcium precipitates remained negligible. The transitional cell layers appeared with occlusion, but not or negligibly with air exposure. By 6 h though, calcium precipitates could be seen, the density of the calcium precipitates with occlusion was more sparse than with air exposure. With the air exposure, the thickness of the stratum corneum had normalized and the calcium gradient nearly recovered to normal after 24 h. The longer the occlusion period, the greater was the increase of transitional cells. By 60 h of occlusion, the thickness of the stratum corneum had increased and the transitional cell layers had disappeared, in parallel with the calcium gradient which was almost normalized. These results show that prolonged occlusion of tape-stripped epidermis induced transitional cells and delayed the restoration of the epidermal calcium gradient, the stratum corneum was then restored, transitional cells having disappeared, in parallel with normalization of the epidermal calcium gradient.

Circumanal glands of the dog: a new classification and cell degeneration

BACKGROUND

The circumanal glands of the dog are thought to be a glandular tissue, but there is some controversy as to whether they should be classified as exocrine or endocrine. In this study, we examined the nature of the circumanal glands to determine whether they should be described as exocrine, endocrine, or something else altogether. In addition, we investigated the cell degeneration in lobules of the circumanal glands in relation to the apocrine glands.

METHODS

Light microscopic observations were made of paraffin sections stained with hematoxylin and eosin, and after immunohistochemical staining with antibodies against alpha-smooth muscle actin, keratin, filaggrin, and 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD). Samples were also examined by electron microscopy after fixation by aldehyde perfusion.

RESULTS

The lobules of circumanal glands could be divided into two types on the basis of the presence or absence of cysts. Four layers (I-IV) were detected in the lobules with cysts. The outermost layer (layer I or the basal layer) consisted of flattened cells that contained bundles of tonofilaments and were stained immunohistochemically with the antibody against keratin. Layer II (the polyhedral or "spinous" layer) consisted of polyhedral cells that contained bundles of tonofilaments. These cells were connected to adjacent cells by desmosomes, interdigitations, and gap junctions, and they were immunopositive for keratin. A small number of polyhedral cells were immunopositive for 3beta-HSD. Layer III (the granular layer) was composed of flattened cells that contained hematoxylin-stainable granules and were moderately immunopositive for filaggrin. The innermost layer (layer IV or the horny layer) consisted of keratin. Lobules without cysts consisted only of layer I (the basal layer) and layer II (the polyhedral layer). Lobules of the circumanal glands were not directly connected to apocrine glands. Polyhedral cells degenerated and were phagocytosed by basal cells at a periphery of lobules. Then, basal cells phagocytosing degenerated polyhedral cells escaped from lobules, moved into the walls of apocrine glands, and, finally, dropped into the lumen of apocrine glands.

CONCLUSIONS

Lobules of the circumanal glands have many characteristics of epidermis (a basal layer, a polyhedral or "spinous layer," a granular layer, and a horny layer) and they should not be classified as glandular tissue. The cysts in lobules can be interpreted as "closed hair canals." We suggest that steroid metabolism might occur in the polyhedral cells of the lobules.

Existence of trichohyalin-keratohyalin hybrid granules: co-localization of two major intermediate filament-associated proteins in non-follicular epithelia

Trichohyalin is a protein of relatively high molecular weight (approximately 200 kDa), associated with intermediate filaments, that was for many years thought to be expressed only in the inner root sheath and medulla of the hair follicle. We show here, however, that this protein is expressed in association with (pro)filaggrin in the granular layer of many non-follicular, keratinizing, stratified epithelia which also express keratins K6/K16, including those of the filiform papillae of dorsal tongue epithelia. In this epithelium, which elaborates morphologically heterogeneous keratohyalin granules in its upper cell layers, trichohyalin forms hybrid granules with filaggrin, the major intermediate filament associated protein found in keratohyalin granules, which is normally expressed in advanced epidermal differentiation. These two intermediate filament-associated proteins remain physically segregated in the hybrid granules, but they share the same fate, as they both become dispersed in transitional cells, and are undetectable in cornified cells. Trichohyalin was also detected in nail matrix epithelia, the epithelium of Hassal's corpuscles of the thymus, and newborn foreskin epidermis. It is essentially absent from normal trunk and scalp epidermis, but is expressed in a few scattered cells of the granular layer that are also filaggrin-positive. In addition, trichohyalin is expressed in the epidermis in a number of hyperplastic skin diseases. These findings demonstrate that trichohyalin is not peculiar to a small number of hair follicle cells, but is expressed in a number of normal and pathological epithelia where it is uniquely associated with filaggrin. In addition, since all these trichohyalin-expressing keratinocytes also synthesize keratins K6 and K16 (the markers for an "alternative" pathway of keratinocyte differentiation), this raises the possibility that the trichohyalin protein is specifically (or preferentially) involved in aggregating intermediate filaments containing the K6/K16 keratins.

Keratohyalin, trichohyalin and keratohyalin-trichohyalin hybrid granules: an overview

Filaggrin and trichohyalin are the two major intermediate filament associated proteins which interact with keratin filaments in the skin. These two proteins initially accumulate in cytoplasmic granules called keratohyalin or trichohyalin granules which provide prominent morphological hallmarks of differentiation in the epidermis and the inner root sheath of hair follicles, respectively. The contents of each of these granules are modified and subsequently released into the cytoplasm of the fully mature cells where they function in the role of aggregating keratin filament bundles. We are beginning to identify several important aspects relative to the unique biological functions of both filaggrin and trichohyalin during the late stages of keratinocyte differentiation. This overview summarizes recent work on these proteins and will also highlight the existence of novel cytoplasmic granules, keratohyalin-trichohyalin hybrid granules, in dorsal tongue epithelia.

Confluent and reticulated papillomatosis: a clinical, histopathological, and electron microscopic study

The clinical, histopathological, and electron microscopic features of ten patients with confluent and reticulated papillomatosis (CRP) of Gougerot and Carteaud were studied. Histopathologically, hyperkeratosis, decreased granular layer, irregular papillomatosis, and hypermelanosis of the basal layer were present. In an electron microscopic study, we found transitional cells between the stratum granulosum and stratum corneum that were increased in CRP. This finding supported the suggestion that this is a defect of keratinization.

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.

Interaction of filaggrin with keratin filaments during advanced stages of normal human epidermal differentiation and in ichthyosis vulgaris

Filaggrin is a histidine-rich, basic protein whose name was first proposed based on its ability to aggregate intermediate filaments in vitro. Based on this in vitro observation, it has generally been assumed that filaggrin functions in vivo as a matrix protein which causes keratin filaments to become densely packed in the terminally differentiated cornified cells. Inconsistent with this view however, is the well-known observation that keratin aggregation appears to proceed normally in the affected epidermis of ichthyosis vulgaris patients despite a greatly reduced quantity of filaggrin. To address this issue, we used immuno-electron microscopy to localize filaggrin and its cross-reactive precursor, profilaggrin, in human and mouse epidermis, as well as in ichthyosis vulgaris epidermis. We found that the localization of filaggrin in lower cornified cells correlates precisely with the formation of aggregated keratin filaments, and the disappearance of filaggrin in upper cornified cells correlates precisely with the loosening of keratin filaments. Furthermore, we showed that, even in ichthyosis vulgaris, small amounts of filaggrin/profilaggrin are present as electron-dense deposits associated with keratin filaments in the granular cells, and that the localization of this small amount of antigen again correlates with the aggregation state of keratin filaments. These data strongly suggest that filaggrin is indeed involved in filament aggregation in vivo.

Electron microscopical demonstration of thiols and disulphides in the porcine epidermis

This study describes the electron microscopical distribution of free thiols and disulphides in the epidermis of the domestic pig and the wild boar, as compared to light microscopical histochemistry. With the silver methenamine method, silver labelling of thiols was clearly achieved on the keratohyalin and cytofilament accumulations in the cells of the living epidermis and the plasma membrane of granular cells. To a certain extent, the envelope and cytoplasm of young corneocytes reacted equally intensively. Disulphides were very abundant in the filaments, keratohyalin granules, and cell envelope of granular cells, and, particularly, in the envelope (marginal band) of corneal cells; the latter structure being distinctly delineated from the background. As a specific feature, the viable epidermis of the wild boar stained strongly for disulphides. The results obtained are discussed in view of actual concepts of epidermal keratinization and corneal cell function.

Modulation of epidermal terminal differentiation in patients after long-term topical corticosteroids

The expression of the various markers for terminal epidermal differentiation in atrophic skin of patients after long-term topical corticosteroids (TCS) was studied by electron microscopy, immunofluorescence using antibody to profilaggrin/filaggrin (PF/FG), immunoperoxidase staining using antibody to involucrin, and oil red O stain for neural lipids of the stratum corneum. Thirty-nine patients were subdivided into two groups: (A) 19 patients suffering from rebound phenomenon after stopping TCS and (B) 20 patients without rebound phenomenon. Biopsy specimens were taken before ending the use of TCS in both groups. In group A, both the morphological markers (including the different epidermal strata, keratohyalin granules, lamellar granules, and cornified cell envelopes) and the molecular markers (including involucrin, PF/FG, and neutral lipids) of terminal epidermal differentiation were significantly suppressed. On the other hand, the differentiational markers in the atrophic skin of patients without rebound phenomena were only slightly altered. These results suggest that potent TCS not only has antiproliferative actions but also inhibits the differentiation of epidermis, resulting in structural defects in the epidermis, especially the stratum corneum.

The perinuclear factor, a rheumatoid arthritis-specific autoantigen, is not present in keratohyalin granules of cultured buccal mucosa cells

Rheumatoid arthritis patients have antibodies in their serum directed against the perinuclear factor, a protein component present in keratohyalin granules in the cytoplasm of human buccal mucosa cells. The anti-perinuclear factor (APF) can only be detected by an indirect immunofluorescence test performed on fresh buccal mucosa cells from 'selected donors'. To obtain a more reliable antigen source and to gain more insight into the origin and nature of the perinuclear factor we attempted to culture perinuclear factor-containing buccal mucosa cells. Here we describe the successful culturing of such cells, which, however, did not contain keratohyalin granules nor the perinuclear factor. By adding the phorbol ester 12-o-tetradecanoylphorbol-13-acetate (TPA) we were able to induce keratohyalin granules in both cultured primary buccal mucosa cells and a squamous carcinoma cell line of the cheek (SqCC/Y1). These induced keratohyalin granules do contain the protein profilaggrin, which in vivo, in fresh buccal mucosa cells, co-localizes with the perinuclear factor. However, we were not able to demonstrate the presence of the perinuclear factor, not even after induction of terminal differentiation of the cultured cells nor after Epstein-Barr virus infection. Our results suggest that the perinuclear factor, in contrast to profilaggrin, is not an integral component of buccal mucosa cells.

Bicomponent keratohyalin in normal human ridged skin

Up to now, bicomponent keratohyalin has only been described for rat epithelium and human intraepidermal sweat ducts and fetal nail organ cells. In normal human interductal epidermis, the keratohyalin appears homogeneous, osmiophilic and stellate in shape. Under pathological conditions, bicomponent keratohyalin has been observed in different palmoplantar keratoses and has therefore been thought to be associated with abnormal keratosis. We studied the keratinization process in normal human plantar epidermis, in which keratohyalin was found to exhibit several morphological differences as compared to that seen in non-ridged skin. The most striking feature was seen in upper granular cells, where the keratohyalin granules consisted of two components of differing electron density. The electron-dense component formed the main part of the composite granule and was found in the cytoplasm of lower and upper granular cells. The less-electron-dense component was attached to the main component and appeared in the cytoplasm of upper granular cells, forming the convex contact zone. No intranuclear osmiophilic inclusions were present. The respective electron densities of the two keratohyalin components of ridged skin were obviously different to that of the bicomponent keratohyalin granules seen in the epidermal sweat-duct cells of the same specimen. These findings indicate the presence of at least two different types of keratohyalin proteins in normal human ridged skin. They can be distinguished at the electron-microscope level and differ from the keratohyalin of human non-ridged skin as well as from bicomponent keratohyalin granules derived from human epidermal sweat-duct cells or from rat epithelium.