Trends in electron microscopy of skin

Calcium Signaling in the Photodamaged Skin: In Vivo Experiments and Mathematical Modeling

The epidermis forms an essential barrier against a variety of insults. The overall goal of this study was to shed light not only on the effects of accidental epidermal injury, but also on the mechanisms that support laser skin resurfacing with intra-epidermal focal laser-induced photodamage, a widespread medical practice used to treat a range of skin conditions. To this end, we selectively photodamaged a single keratinocyte with intense, focused and pulsed laser radiation, triggering Ca2+ waves in the epidermis of live anesthetized mice with ubiquitous expression of a genetically encoded Ca2+ indicator. Waves expanded radially and rapidly, reaching up to eight orders of bystander cells that remained activated for tens of minutes, without displaying oscillations of the cytosolic free Ca2+ concentration ([Formula: see text]). By combining in vivo pharmacological dissection with mathematical modeling, we demonstrate that Ca2+ wave propagation depended primarily on the release of ATP, a prime damage-associated molecular patterns (DAMPs), from the hit cell. Increments of the [Formula: see text] in bystander cells were chiefly due to Ca2+ release from the endoplasmic reticulum (ER), downstream of ATP binding to P2Y purinoceptors. ATP-dependent ATP release though connexin hemichannels (HCs) affected wave propagation at larger distances, where the extracellular ATP concentration was reduced by the combined effect of passive diffusion and hydrolysis due to the action of ectonucleotidases, whereas pannexin channels had no role. Bifurcation analysis suggests basal keratinocytes have too few P2Y receptors (P2YRs) and/or phospholipase C (PLC) to transduce elevated extracellular ATP levels into inositol trisphosphate (IP3) production rates sufficiently large to sustain [Formula: see text] oscillations.

Central role of alpha9 acetylcholine receptor in coordinating keratinocyte adhesion and motility at the initiation of epithelialization

Epithelialization, a major component of wound healing, depends on keratinocyte adhesion and migration. Initiation of migration relies upon the ability of keratinocytes to free themselves from neighboring cells and basement membrane. The local cytotransmitter acetylcholine (ACh) controls keratinocyte adhesion and locomotion through different classes of ACh receptors (AChR). In this study, we explored signaling pathways downstream of the alpha9 AChR subtype that had been shown to control cell shape and cytoplasm mobility. Inactivation of alpha9 signaling by pharmacologic antagonism and RNA interference in keratinocyte cultures and null mutation in knockout mice delayed wound re-epithelialization in vitro and in vivo, respectively, and diminished the extent of colony scattering and cell outgrowth from the megacolony. Although keratinocytes at the leading edge elongated, produced filopodia and moved out, most of them remained anchored to the substrate by long cytoplasmic processes that stretched during their migration instead of retracting the uropod. Since the velocity of keratinocyte migration was not altered, we investigated the role of alpha9 in assembly/disassembly of the cell-cell and cell-matrix adhesion complexes. Stimulation of alpha9 upregulated in a time-dependent fashion phosphorylation of the adhesion molecules comprising focal adhesions (FAK, paxillin) and intercellular junctions (beta-catenin, desmoglein 3) as well as cytokeratins. Stimulation of alpha9 was associated with activation of phospholipase C, Src, EGF receptor kinase, protein kinase C, Rac and Rho, whereas inhibition of this receptor interfered with phosphorylation of adhesion and cytoskeletal proteins, and also altered cell-cell cohesion. We conclude that signaling through alpha9 AChR is critical for completion of the very early stages of epithelialization. By activating alpha9 AChR, ACh can control the dynamics and strength of cell-cell cohesion, disabling of a trailing uropod and disassembly and reassembly of focal adhesions, thus facilitating crawling locomotion.

Cole disease: hypopigmentation with punctate keratosis of the palms and soles

Cole disease is an uncommon disorder characterized by distinctive cutaneous hypopigmentation and punctate keratosis of the palms and soles. It is a congenital skin disease with an autosomal dominant inheritance pattern. We report two patients from a family with 15 members, 5 of whom were affected. One of the patients had both types of lesions since birth, while in the other they arose in the first months of life. We studied the pedigree, histopathology, immunohistochemistry, and electron microscopy findings of the hypopigmented macules with the patients' normal skin used as a control. The pedigree showed involvement of both genders, with a Mendelian autosomal dominant inheritance pattern with phenotypic variability in the family. Immunohistochemistry showed a reduction in the melanin pigment in the keratinocytes and normal pigmentation in the melanocytes. Ultrastructural studies showed a strong contrast between the large number of melanosomes in the body and dendrites of the melanocytes, in contrast with the small number of these organelles in the neighboring keratinocytes. These findings suggest that this disease is a primary congenital disorder of the transfer mechanisms of the melanosomes from melanocytes to keratinocytes in hypopigmented lesions, associated with abnormal epidermopoiesis in the punctate hyperkeratosis.

Neuronal nicotinic receptors in non-neuronal cells: new mediators of tobacco toxicity?

The nicotinic acetylcholine receptors are prototypic ionotropic receptors that mediate fast synaptic transmission. However, also non-excitable cells, and particularly the tegumental cells that line external and internal body surfaces, express acetylcholine receptors of neuronal type sensitive to nicotine. Bronchial epithelial cells, endothelial cells of blood vessels and skin keratinocytes express neuronal nicotinic receptors composed of alpha(3), alpha(5), beta(2) and beta(4) subunits, similar to those expressed in sympathetic ganglia, and neuronal nicotinic receptors composed of alpha(7) subunits. Neuronal nicotinic receptors in tegumental cells are involved in modulating cell shape and motility, and therefore in maintaining the integrity of the surfaces lined by those cells. Neuronal nicotinic receptors in non-neuronal tissues may modulate other functions, including cell proliferation and differentiation. Acetylcholine is synthesized, secreted and degraded by a variety of cells, including the tegumental cells that express neuronal nicotinic receptors. Thus, acetylcholine may function as a local "hormone" that is able to modulate cell functions that require fast adaptation to new conditions. The presence of neuronal nicotinic receptors sensitive to nicotine in tissues known to be involved in tobacco toxicity, like bronchi and blood vessels, raises the possibility that they mediate some of the toxic effects of smoking.

Bikunin, a serine protease inhibitor, is present on the cell boundary of epidermis

Bikunin, which is an inhibitor of serine proteases, is widely distributed in human tissues, including liver, kidney, and mucous membranes of the stomach and colon. The aim of this study was to clarify whether bikunin is expressed in human epidermis and its appendages. Immunoblot analysis using a specific polyclonal antibody to bikunin revealed that a single 43 kDa protein is present in the cell lysate from the human keratinocyte cell line HaCaT. Immunohistochemically, dotted reaction products stained with anti-bikunin antibody were localized on the cell boundary in both basal and spinous cell layers, except on the cell boundary of the basal cells facing the basal membrane. There were no reaction products in the granular-horny cell layers. Reaction products stained with anti-bikunin antibody were also observed on the hair bulb cells and eccrine sweat gland cells, but not on apocrine sweat glands. Also, reaction products were observed on the luminal surface of the renal proximal tubules and in the cytoplasm of these cells. In immunoelectron microscopy, gold particles were observed on the cell membranes close to the desmosomal structures. Reverse transcription-polymerase chain reaction and northern blot analyses showed that mRNA specific for bikunin was expressed in HaCaT cells and human epidermal keratinocytes obtained from suction blisters, and was contained in a commercially available human keratinocyte cDNA preparation. These findings indicate that bikunin is expressed in keratinocytes and may play an important part in regulating keratinocytes in either mitosis or inflammation.

Morphology of lipid alterations in the epidermis: a review

An overview of the morphological correlates of mammalian epidermal lipids and the cutaneous permeability barrier is provided. The following features are discussed: 1) ultrastructural characterization of the progressive lipid alterations accompanying normal stratum corneum (SC) formation; 2) effects of drugs/inhibitors that interfere with specific aspects of epidermal lipid metabolism; 3) lipid alterations in certain human skin disorders; 4) effects of selected topical emollients on SC lipid organization; and 5) potential pitfalls in interpretation of ultrastructural data in terms of cutaneous function and dysfunction.

A nicotinic acetylcholine receptor regulating cell adhesion and motility is expressed in human keratinocytes

Acetylcholine is synthesized and released by human epidermal keratinocytes and modulates the adhesion and motility of these cells. To understand the molecular basis of the effects of acetylcholine on keratinocytes, we investigated the presence, pharmacology, structure, and function of nicotinic acetylcholine receptors in human epidermal keratinocytes. Patch-clamp studies indicated that keratinocytes express acetylcholine receptors with ion gating and pharmacologic properties similar to those observed so far only in neurons, and containing the alpha 3 subunit. Specific binding of the receptor-specific ligand 125I-kappa-bungarotoxin revealed approximately 5500 binding sites per cell on undifferentiated keratinocytes in cell cultures and approximately 35,400 binding sites per cell on mature keratinocytes freshly isolated from human neonatal foreskins. Antibody binding and polymerase chain reaction experiments demonstrated the presence of alpha 3, beta 2, and beta 4 nicotinic receptor subunits. Binding of subunit-specific antibodies indicated that nicotinic receptors were associated with the suprabasal keratinocytes in epidermis and localized to the cell membranes of differentiated keratinocytes in cell cultures. Acetylcholine and the nicotinic agonist nicotine increased cell-substrate and cell-cell adherence of cultured keratinocytes and stimulated their lateral migration. The specific antagonists kappa-bungarotoxin and mecamylamine caused cell detachment and abolished migration. Thus, a nicotinic receptor expressed in keratinocytes may mediate acetylcholine control of keratinocyte adhesion and motility.

Epidermolysis bullosa herpetiformis (Dowling-Meara type) exhibits ultrastructural derangement of tonofilaments and desmosomes

Ultrastructural and immunohistochemical studies of clinically intact skin obtained from three severe neonatal cases of epidermolysis bullosa herpetiformis (Dowling-Meara type) demonstrated disorders in the assembly of keratin intermediate filaments and desmosomes of the keratinocytes. During mitosis, K5- and K14-positive and K1- and K10-negative tonofilaments were disrupted and formed spherical bodies associated with intracytoplasmic desmosomes by invagination of the desmosomes and the adjacent plasma membrane. During the invagination process, destructive changes in the internalized membrane were noted. These were accompanied by gradual loss of reactivity with a monoclonal antibody ZK31, which detected plasma membrane adjacent to the attachment plaques of desmosomes. However, the reactivity of the attachment plaques of the internalized desmosomes for desmoplakins and desmoglein did not decline during the process of internalization. In the suprabasal layers of the epidermis, filamentous substructures and K1 and K10 appeared at the periphery of the spherical bodies. Simultaneously, the desmosomes that were sparsely located in the lower epidermis, increased in number as cell differentiation progressed. Thus, the keratinocytes attained an almost normal appearance with respect to tonofilaments and desmosomes by the time they reached the upper layer of the epidermis. These findings may be relevant to the mechanism responsible for the clinical appearance of the herpetiform blisters in epidermolysis bullosa herpetiformis, which are also characterized by spontaneous involution during childhood or when exposed to high ambient temperatures.

Histologic investigation of hyperphonated canine vocal cords

Vocal cord injury and its effect on the larynx are topics of considerable importance, yet a clear understanding of acute vocal cord injury from excessive phonation remains elusive. The inability to develop an animal model for vocal abuse has hampered research in this area. This project describes the development of a canine model for acute vocal cord injury from excessive phonation. The method of phonation consisted of a humidified, temperature-controlled, constant flow of air delivered subglottally by an external compressor. A small group of dogs underwent phonation for 2 and 4 hours. Vocal cord injury is described using histologic techniques. Damage to the vocal cords, other than hemorrhage (rare in dogs) and edema, proved to be too elusive for routine microscopy. Scanning and transmission electron microscopy showed extensive surface damage with destruction and loss of the surface microridges and premature desquamation of the vocal cord squamous epithelium. Injury extended to the lamina propria, where the collagen fibers were detached from the basement membrane.

Cantharide acantholysis: endogenous protease activation leading to desmosomal plaque dissolution

Using a method which allowed us to study the morphological consequences of the expression and the inhibition of proteases in living tissues, we demonstrated that the primary detectable cellular event in cantharide acantholysis is the dissolution of the dense plaque, leading to the detachment of tonofilaments from desmosomes. This process is inhibited by neutral serine protease inhibitors. This suggests that the desmosome-tonofilament complex, more precisely the desmosomal dense plaque, is the primary target of activated proteases during cantharide acantholysis, and can be disrupted by a specific epidermal protease-anti protease system. Cantharide acantholysis may be useful model for studying desmosomal turnover.

The epidermal permeability barrier

The permeability barrier of the skin which prevents transcutaneous water loss and penetration of harmful drugs from the environment is localized in the horny layer of the epidermis. Multiple lipid bilayers obstructing the intercellular space of the stratum corneum fulfill this function. In contrast to cellular membranes consisting predominantly of phospholipids, these lamellae contain mostly ceramides, cholesterol and free fatty acids. The lamellae are derived from the contents of lamellar granules (LGs) which are synthesized in the viable epidermal layers by the keratinocytes. LGs display stacks of small disks each of which represents a flattened vesicle or liposome. Prior to terminal differentiation, the disks are exocytosed into the intercellular space and fused to form uninterrupted sheetlike lamellae. The singular lipid composition of LG-disks and of stratum corneum-lamellae reflects the multistage process of barrier formation. It also renders these structures well suited to provide for a barrier function.

Acylglucosylceramide causes flattening and stacking of liposomes. An analogy for assembly of the epidermal permeability barrier

When 5% of 1-(3'-O-acyl)-beta-D-glucosyl-N-(omega-hydroxyacyl)sphingosine, isolated from pig epidermis, was added to distearoylphosphatidylcholine and cholesterol (10:1), the lipid mixture formed liposomes in phosphate buffer which were flattened and aggregated like stacks of coins. Other glycolipids from pig epidermis did not cause this phenomenon. This supports the hypothesis that the acylglucosylceramide is responsible for assembly of the lamellar granules found in epidermal cells.

Hyperkeratinization, corneocyte cohesion, and alpha hydroxy acids

Hyperkeratinization is a primary or fundamental event in a majority of today's skin disorders. Hyperkeratinization is usually the result of decreased desquamation due to increased corneocyte cohesion. Strength of corneocyte cohesion is determined by strength of intercellular bonding. Intercellular bonding is weakened by water and diminished by retinoids and alpha hydroxy acids (AHAs). Conversely, bonding is strengthened or enhanced by dehydration, vitamin A deficiency, and some alpha acetoxy acids (AAAs). Agents that control or modify keratinization can be useful in treatment of many skin disorders.

Osmium ferricyanide fixation improves microfilament preservation and membrane visualization in a variety of animal cell types

Using a fixation formula which includes adding potassium ferricyanide (K3Fe(CN)6) to the osmium step and an en bloc aqueous uranyl acetate step before dehydration we have looked at cells from mammals, birds, amphibia, algae, and higher plants and we have collaborated in fixing cells of teleost fish. In every cell type except the algae and higher plants the final EM image was improved by the OsFeCN-uranium method. The most common improvement was an increase in the membrane contrast but more significantly, some cells show improved preservation of microfilaments. We conclude that the OsFeCN adds contrast to all classes of membrane and does not destroy microfilaments to the extent that osmium alone does. Adding uranyl acetate to the cells may protect delicate filamentous structures from collapse during dehydration and embedding. We have preliminary evidence in PtK1 cells that addition of tannic acid after OsFeCN may function in a similar manner. This method is recommended for any animal cell type where improved visualization of membranes and filaments is required.

Glycolipids in mammalian epidermis: structure and function in the water barrier

In the epidermis of terrestrial vertebrates, lipid lamellae between the horny cells are thought to form a barrier to water loss. The lipids are extruded from living cells after assembly in lamellar granules. This assembly might be promoted by recently identified 1-(3'-O-acyl)-beta-D-glucosyl-N-(omega-hydroxyacyl)sphingosines, which have 30- and 32-carbon hydroxy acids as amides and linoleic acid esterified to glucose. Such a role for these molecules could explain the effects of essential fatty acid deficiency, in which the lamellar granules fail to assemble and the barrier to water diffusion is lost.

Cellular junctions in human oesophageal epithelium

The appearances of this intercellular junctions in normal and in inflamed oesophageal epithelium are described. Typical hemidesmosomes occur along the abluminat aspect of the basal cells. Desmosomes are most frequent in the prickle cell layer, where desmosome fields may occur, and in the lower functional cell layer. Their fate in the upper layers is not known. Gap junctions are few in the basal cell layer and rare in the most superficial layers. They are maximal in the prickle cell layer. Tight junctions only occur in inflamed tissue between the most superficial cells usually as part of a lateral intercellular junctional complex that also contains belt desmosomes. They may represent attempts by a non-keratinised epithelium to increase its defences against toxic luminal contents.

Effect of pemphigus antibodies on desmosomal structure in vitro

Changes in desmosomes were noted in nonacantholytic regions of skin explants cultured in the presence of intercellular antibodies. These changes included dissociation of desmosomes, as well as a "selective loss" of desmosomal components in one or both of neighboring cells.

The light and electron microscopic distribution of acid phosphatase activity in human normal oesophageal epithelium

Acid phosphatase activity in human normal oesophageal epithelium was studied with light and electron microscopic techniques. The maximum activity was found to be in the prickle and lower functional layers. Electron microscopic examination revealed activity to be localized in GERL, lysosomes and membrane coating granules. These last structures probably secreted their content into the intercellular space in the central part of the functional layer. Thick sections (0.5 micron) with tilting showed GERL to consist of anastomosing tubules.

The electron microscopy of normal human oesophageal epithelium

Oesophageal biopsies were studied with the electron microscope. Three layers were identified, as in the light microscopy of the oesophageal epithelium: basal, prickle and funtional cell layers. A continuous basement membrane separated the lamina propria from the basal cells. The basal cell membrane carried hemidesmosomes, desmosomes and microvillous processes. Their cytoplasm contained the usual organelles plus free ribosomes and tonofirbrils. Prickle cells contained glycogen rosettes and many tonofilaments, and their cell membrane many microvillous and demosomal processes, in places elaborated into desmosome fields. In both these layers there was a wide intercellular space containing some particulate and membranous debris. The flattened cells of the functional layer had fewer desmosomes and microvilli but abundant glycogen and tonofilaments, and a narrow intercellular space. Membrane coating granules first reaching a maximum in the functional cell layer appeared in the upper prickle cell layer and few persisted into the surface cells. The apical cell membrane of the most superficial cells was thickened and had few small microvillous processes, which were covered with a filamentous "fuzzy" coat. No keratohyaline granules were present. Papillae of lamina propria contained capillaries, some with a fenestrated endothelium.

An ultrastructural study of osmiophilia in normal human oesophageal epithelium

Oesophageal biopsies were obtained from patients with normal oesophagi during fibre-optic endoscopy for upper gastro-intestinal symptoms. They were studied with the prolonged osmication technique. The forming face of the Golgi apparatus was demonstrated in the basal and lower prickle cells. These cells also showed osmium deposition in their mitochondria, endoplasmic reticulum, perinuclear cisternae and lysosomes. The capillary endothelial cells also showed osmium deposition in their Golgi apparatus and endoplasmic reticulum.

Ultrastructural demonstration of cell coat on the cell surfaces of normal human oesophageal epithelium

The cell coat in human oesophageal biopsies was studied with Alcian Blue, Ruthenium Red, Safranin O, colloidal iron and the ferrocyanide--osmium tetroxide techniques. Alcianophilic material was found on the cell surface of the basal, prickle cell and functional layers, being most abundant on the superficial cells where it appeared as a continuous coat. In the deeper layers, it tended to have a particulate distribution. Some membrane-coating granules were alcianophilic. Ruthenium Red had a particulate distribution over all cell surfaces. Intercellular debris was also stained. Safranin O produced no staining. Colloidal iron stained the cell coat in a particulate manner. The ferrocyanide--osmium technique showed a uniform filamentous cell coat. The oesophageal epithelial cell coats are, in part, acid mucosubstances which, on the surface cells, may have a protective function.