Ultrastructure of the stratum corneum

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.

Topical Treatment for Cutaneous Leishmaniasis: Dermato-Pharmacokinetic Lead Optimization of Benzoxaboroles

Cutaneous leishmaniasis (CL) is caused by several species of the protozoan parasite Leishmania, affecting an estimated 10 million people worldwide. Previously reported strategies for the development of topical CL treatments have focused primarily on drug permeation and formulation optimization as the means to increase treatment efficacy. Our approach aims to identify compounds with antileishmanial activity and properties consistent with topical administration. Of the test compounds, five benzoxaboroles showed potent activity (50% effective concentration [EC50] < 5 μM) against intracellular amastigotes of at least one Leishmania species and acceptable activity (20 μM < EC50 < 30 μM) against two more species. Benzoxaborole compounds were further prioritized on the basis of the in vitro evaluation of progression criteria related to skin permeation, such as the partition coefficient and solubility. An MDCKII-hMDR1 cell assay showed overall good permeability and no significant interaction with the P-glycoprotein transporter for all substrates except LSH002 and LSH031. The benzoxaboroles were degraded, to some extent, by skin enzymes but had stability superior to that of para-hydroxybenzoate compounds, which are known skin esterase substrates. Evaluation of permeation through reconstructed human epidermis showed LSH002 to be the most permeant, followed by LSH003 and LSH001. Skin disposition studies following finite drug formulation application to mouse skin demonstrated the highest permeation for LSH001, followed by LSH003 and LSH002, with a significantly larger amount of LSH001 than the other compounds being retained in skin. Finally, the efficacy of the leads (LSH001, LSH002, and LSH003) against Leishmania major was tested in vivo LSH001 suppressed lesion growth upon topical application, and LSH003 reduced the lesion size following oral administration.

Skin permeation: The years of enlightenment

Considerable advances in our understanding of the mechanisms by which drugs permeate the skin barrier have been made over the past 60 years. The key publications, which have influenced the field of skin permeation research are highlighted in the present review. The methodologies commonly employed for estimation of skin permeability are discussed as are the mechanisms proposed for skin permeation. The principal findings from the commonly employed in vitro and in vivo models are considered as well as the applications of enhancers and surfactants for optimization of skin delivery. As these studies presaged the emergence of transdermal drug delivery research in the 1970s, early approaches to model and predict dermal and transdermal absorption are also outlined. The published work on skin permeability in this period embodies the fundamental literature sources for consultation by scientists new to and currently engaged in transdermal drug delivery.

A light and electron microscopy study of normal human stratum corneum with particular reference to the intercellular space

Intercellular and skin-surface substances, and exfoliating corneocytes, were clearly visualized by both light and electron microscopy. The intercellular space constituted an essential part of the normal human stratum corneum, in the basal and middle zones of which this space was filled with substances producing a compact appearance. The intercellular constituents were a nonhomogeneous substance, intact, single and "compound" lamellar granules and an intensely stained, membrane-like material that in some parts, but not in others, had a lamellar pattern. The artifacts produced by ultrathin sectioning for electron microscopy were too small to provide sufficient explanation for the porous appearance of the superficial zone. More important factors seemed to be enlargement of the intercellular space with decrease in the number of desmosomes and alterations of the intercellular substances, with decrease in the amount of nonhomogeneous substance and transformation of the single and "compound" lamellar granules into single and "compound" vesicular bodies. The hypothesis is advanced that the single and "compound" vesicular bodies together with the decreased amount of nonhomogeneous substance may contribute to maintain the patency of the intercellular space in the superficial zone (stratum disjunctum), thereby facilitating absorption of surface-applied agents into the stratum corneum by some shunt mechanism, while the content of the intercellular space in the basal and middle zones (stratum compactum) forms the principal barrier to free diffusion.

Autosomal-dominant lamellar ichthyosis: ultrastructural characteristics of a new type of congenital ichthyosis

Recently, autosomal-dominant lamellar ichthyosis (ADLI) has been shown to be a new genetic trait with clinical and histologic features similar to those of autosomal-recessive lamellar ichthyosis. In two patients affected with ADLI, the malpighian keratinocytes showed ultrastructural signs of increased cellular metabolism. The tonofilaments and keratohyaline granules were regular in structure and number. However, as a distinctive ultrastructural feature, a prominent transforming zone was found between the granular and horny layers. Moreover, a normal keratin pattern and only a limited number of lipid inclusions were observed in the stratum corneum. Thus, ADLI can be distinguished from the autosomal-recessive forms of lamellar ichthyosis, permitting a correct diagnosis when genetic counselling has to be given in sporadic cases.

Ultrastructure of the fibrous substance in the keratinocytes of the epidermis in healthy individuals

The present findings in the epidermis of healthy individuals suggest that the fibrous substance in the keratinocytes is subjected to an individual differentiation as the cells move from stratum basale through strata spinosum and intermedium to stratum coreum. In the basal cells the fibrous substance consists of opaque tonofilaments, mostly aggregated in dense tonofibrils, but also occurring randomly scattered and in loose tonofibrils (fibrous substance of basal-cell type). In most cells the fibrous substance undergoes differentiation: a) in stratum spinosum into compact tonofibrils with no individual tonofilaments visible (fibrous substance of spinous-cell type), b) in stratum inter-medium into compact tonofibrils with intensely stained regions or keratohyalin (fibrous substance of intermediate-cell type), and c) in stratum corneum into tonofibrils with a keratin pattern (fibrous substance of horny-cell type). There are also keratinocytes in which the fibrous substance undergoes only an incomplete differentiation, with the appearance of horny cells with a fibrous substance of spinous-cell type, intermediate-cell type or horny-cell type with keratohyalin. Finally, there are keratinocytes in which the fibrous substance does not undergo any differentiation at all, but which display a fibrous substance of basal-cell type throughout the entire epidermis.

Alterations of clinically normal skin in early eruptive guttate psoriasis. A light- and electron-microscopic study

In clinically normal skin of early eruptive guttate psoriasis, in patients with psoriasis for the first time, the epidermal changes were restricted to small tissue areas showing a slight hyperplasia and a condensed stratum corneum but no parakeratosis. The center (zone 1) of the small tissue areas was characterized by (1) exoserosis with spongiotic dilatation of the intercellular space in the non-cornified epidermis, (2) derangement of keratinocytes with respect to shape, orientation, and number of desmosomes and fibrils, (3) dyskeratotic keratinocytes of two types, A and B, (4) agranulosis restricted to the dyskeratotic deratinocytes of type A, (5) exocytosis of mononuclear cells and Langerhans' cells into the entire non-cornified epidermis, (6) large gaps in the basement membrane between the dermis and epidermis. No polymorphonuclear leukocytes were seen. The periphery (zones 2) of the small tissue areas showed slight changes. The alterations of the upper dermis were slight: besides a very mild general inflammatory reaction with capillary dilatation and perivascular cell infiltrate there was also a focal accumulation of inflammatory cells immediately beneath the epidermis, with cells seemingly infiltrating the stratum basale. It is postulated that the epidermal and dermal changes as shown here represent primary posoriatic changes.