What is the biological basis of pattern formation of skin lesions?

Segmental vitiligo distribution follows the underlying arterial blood supply territory: a hypothesis based on anatomo-clinical, pathological and physio-pathological studies

Background

Segmental vitiligo (SV) is a subset of vitiligo typically characterized by its unilateral distribution. The pathogenesis of SV remains unclear, and until now the two main patterns proposed for SV have lacked biological support. This calls for a new approach.

Objectives

Use data obtained from anatomo-clinical, pathological, and physio-pathological studies to formulate a new hypothesis on segmental vitiligo distribution and its pathogenesis.

Methods

Using transparent templates of local arterial blood supply, we evaluated anatomical correspondence (AC) in 140 SV lesions according to the number of SV lesions that fit within the corresponding arterial blood-supply areas. SV lesions were graded as 1 (moderate: AC < 50%), 2 (good: AC > 50%), or 3 (excellent: AC of all lesions). To support this anatomical investigation, we searched for complementary assessments according to the activity of SV lesions. Arterial and periarterial network impairment and inflammatory infiltration were histologically studied using nerve growth factor (NGF) and CD4 and CD8 monoclonal antibodies. Increased blood flow of the underlying arteries was also investigated using thermography and ultrasonography.

Results

We recruited 140 patients with a sex ratio of 0.8 and mean age 26.13 years. Localizations: head and neck 84.28%; trunk 6.42%; upper limb 5%; genital areas 2.14%; lower limb 1.42%. The AC of each SV lesion with the underlying artery blood supply territory was rated as 72% excellent; 16% good; and 12% moderate. Histologically (40 patients), we found some periarterial network impairments. Thermal asymmetry was significantly associated with active SV (p < 0.001).

Conclusion

We hypothesized that SV distribution corresponds to the underlying artery blood-supply territory.

Relationship of psoriatic arthritis with nail and scalp involvement in Turkish psoriasis patients: Multicentered cross-sectional study

Psoriasis is a common multisystem inflammatory disease, and arthritis is an essential component of the disorder, requiring early diagnosis and prompt treatment for successful management. In this study, we aimed to investigate the relationship between nail and scalp involvement and other covariates with psoriatic arthritis (PsA). This cross-sectional study, conducted from June 2021 through December 2021, included 763 patients from 11 different centers in Turkey. The severity of involvement was evaluated using psoriasis area severity index (PASI), nail psoriasis severity index (NAPSI), and psoriasis scalp severity index (PSSI) scores. Predictors for PsA were evaluated using univariate and multivariate logistic regression models. PsA (n = 155, 21.5%) was significantly more common in patients having a family history of psoriasis (43.2% vs 30.9%, P = .004), nail involvement (68.4% vs 52.3%, P < .001), and coexistence of nail and scalp involvement (53.7% vs 39.6%, P = .002). Furthermore, patients with PsA had considerably higher PASI (7 vs 5.6, P = .006), NAPSI (5 vs 2, P < .001), and PSSI scores (7 vs 4, P = .002) and longer disease duration (months) (126 vs 108, P = .009). In multivariate analysis, female gender [OR: 3.01, 95% CI (1.861-4.880), P < .001], nail involvement [OR: 2.06, 95% CI (1.293-3.302), P = .002)], and body mass index (BMI) [OR: 1.06, 95% CI (1.017-1.100), P = .005] were identified as independent predictors for PsA. Female gender, nail involvement, and high BMI are significant predictors for PsA and warrant detailed rheumatological assessment. Notably, being female is the strongest predictor of increased risk of PsA in our survey. Scalp involvement appears not to be associated with PsA. Also, the presence of PsA seems related to a more severe skin involvement phenotype.

Pathogenesis of annular lesions: Why a ring presentation?

Annular lesions represent a distinct morphology which characterizes many well-known dermatologic conditions. Little is definitively known regarding the pathogenesis of annular lesions, however there a few well-regarded hypotheses. Lesions that clear centrally while enlarging peripherally may result from a local central tissue anergy, or tolerance. The central area in lesions due to dermatophyte infections or subacute cutaneous lupus erythematous may have a central immunity to the antigen that trigged the lesion. The peripheral spread of inflammatory mediators may also contribute to lesions that expand centrifugally. In a highly active immune response, some of the inflammatory mediators may spread to adjacent tissue, which can propagate the inflammatory reaction. The additional hypotheses regarding pathogenesis are disease specific with individual mechanisms having been proposed. This chapter will describe both general and disease specific mechanisms that may contribute to the formation of annular lesions.

The hair follicle-psoriasis axis: Shared regulatory mechanisms and therapeutic targets

It has long been known that there is a special affinity of psoriasis for the scalp: Here, it occurs most frequently, lesions terminate sharply in frontal skin beyond the hair line and are difficult to treat. Yet, surprisingly, scalp psoriasis only rarely causes alopecia, even though the pilosebaceous unit clearly is affected. Here, we systematically explore the peculiar, insufficiently investigated connection between psoriasis and growing (anagen) terminal scalp hair follicles (HFs), with emphasis on shared regulatory mechanism and therapeutic targets. Interestingly, several drugs and stressors that can trigger/aggravate psoriasis can inhibit hair growth (e.g. beta-blockers, chloroquine, carbamazepine, interferon-alpha, perceived stress). Instead, several anti-psoriatic agents can stimulate hair growth (e.g. cyclosporine, glucocorticoids, dithranol, UV irradiation), while skin/HF trauma (Köbner phenomenon/depilation) favours the development of psoriatic lesions and induces anagen in "quiescent" (telogen) HFs. On this basis, we propose two interconnected working models: (a) the existence of a bidirectional "hair follicle-psoriasis axis," along which keratinocytes of anagen scalp HFs secrete signals that favour the development and maintenance of psoriatic scalp lesions and respond to signals from these lesions, and (b) that anagen induction and psoriatic lesions share molecular "switch-on" mechanisms, which invite pharmacological targeting, once identified. Therefore, we advocate a novel, cross-fertilizing and integrative approach to psoriasis and hair research that systematically characterizes the "HF-psoriasis axis," focused on identification and therapeutic targeting of selected, shared signalling pathways in the future management of both, psoriasis and hair growth disorders.

Linear patterns of the skin and their dermatoses

Knowledge about the linear patterns of the skin is a key competence of dermatologists. Four major groups of linear patterns can be distinguished: Langer lines, dermatomes, Blaschko lines and exogenous patterns. Langer lines run in the direction of the underlying collagen fibers (least skin tension) and play an important diagnostic role for some exanthematous skin diseases. In the thoracodorsal region, the distribution of the Langer lines gives rise to what is referred to as a 'Christmas tree pattern'. A dermatome is an area of skin that is supplied by a single spinal nerve. Disorders with a neuronal origin follow this pattern of distribution. The lines of Blaschko delineate the lines of migration of epidermal cells during embryogenesis. Exogenous linear patterns are caused by external factors. The present CME article will highlight important skin disorders that primarily present in the form of one of the aforementioned patterns. In addition, we will also address skin conditions that may secondarily follow with these patterns (or distinctly not do so) as the result of various mechanisms such as the Koebner phenomenon, reverse Koebner phenomenon, and Wolf's isotopic response.

Analysis of symmetricity in the three different (sagittal, transverse and frontal) planes in generalized nonsegmental vitiligo

BACKGROUND

Nonsegmental vitiligo is defined as being "often symmetrical", however, no work has tackled the point as to how valid it is to depend upon the concept of symmetricity in generalized nonsegmental vitiligo.

AIMS

To investigate vitiligo symmetry, taking into account sites of predilection, the clinical characteristics of patients were studied.

METHODS

This multicentric study included 712 nonsegmental vitiligo patients with 2876 examined lesions. Three models were drawn for each patient. Sagittal, transverse and frontal planes were drawn to divide the body into right/left, upper/lower and anterior/posterior halves respectively. Patients were examined by Wood's light and analyzed for symmetry.

RESULTS

Bilateral involvement was present in 78% (P < 0.001). Studying the similarity of clinical involvement in the upper and lower body parts revealed that such similarity was present in 38%, with a significant positive association in some areas. Studying clinical similarity in the anteroposterior distribution pattern revealed a significant positive association in 11%.

LIMITATIONS

Relatively low number of patients.

CONCLUSIONS

We found significant bilateral symmetry in the lesions of 78% of vitiligo patients. Our work could aid in drawing the anticipated vitiligo map in patients with active disease, helping in increasing our understanding of the clinical behaviour of this disease.

Toward Predicting the Spatio-Temporal Dynamics of Alopecia Areata Lesions Using Partial Differential Equation Analysis

Hair loss in the autoimmune disease, alopecia areata (AA), is characterized by the appearance of circularly spreading alopecic lesions in seemingly healthy skin. The distinct spatial patterns of AA lesions form because the immune system attacks hair follicle cells that are in the process of producing hair shaft, catapults the mini-organs that produce hair from a state of growth (anagen) into an apoptosis-driven regression state (catagen), and causes major hair follicle dystrophy along with rapid hair shaft shedding. In this paper, we develop a model of partial differential equations (PDEs) to describe the spatio-temporal dynamics of immune system components that clinical and experimental studies show are primarily involved in the disease development. Global linear stability analysis reveals there is a most unstable mode giving rise to a pattern. The most unstable mode indicates a spatial scale consistent with results of the humanized AA mouse model of Gilhar et al. (Autoimmun Rev 15(7):726-735, 2016) for experimentally induced AA lesions. Numerical simulations of the PDE system confirm our analytic findings and illustrate the formation of a pattern that is characteristic of the spatio-temporal AA dynamics. We apply marginal linear stability analysis to examine and predict the pattern propagation.

Linear Lichen Planus: Continuum From Skin to Mucosa

Lichen planus (LP) is a chronic inflammatory disorder that most commonly affects middle-aged individuals. LP involves the skin and/or mucous membranes, including oral, vulvovaginal, esophageal, laryngeal, and conjunctival mucosa. Linear LP, a rare distinct variant of LP, is characterised by pruritic eruption of lichenoid and violaceous papules in a linear distribution. We report an unusual presentation of linear LP in a 14-year-old child that extended from the left anterior nares to upper lip and into the oral mucosa up to the upper gum margin.

Self-assembled multi-ring formations of glutamine and a possible link to erythema gyratum repens

In the body L-glutamine is abundant and required for the proliferation of cells. Indeed human physiology is dependent upon having and maintaining the correct glutamine levels for a range of functions including neurological signalling and a healthy immune system. However, during tumourigenesis cell proliferation demands elevated levels of glutamine, which can ultimately lead to muscle atrophy. In some cases the skin provides the first indications of the underlying disease and erupts in a wave of complicated pattern formations. One such skin marker is erythema gyratum repens. We investigated the pattern formations associated with concentrations of glutamine in aqueous solutions at levels higher than that of a normal biological functionality. We find remarkable similarities between the patterns of erythema gyratum repens and the unusual self-assembled patterns of glutamine. The findings may lead to new therapeutics and understanding for those working in oncology and toxicology. Utilising the formations associated with glutamine could also assist in bio-functionalising micro and nanoparticles for high efficacy.

Classification of segmental vitiligo on the trunk

BACKGROUND

Segmental vitiligo is characterized by a unilateral and localized distribution. A classification for segmental vitiligo on the face has been proposed, while this is still unavailable for lesions on the trunk.

OBJECTIVES

To design a classification for segmental vitiligo lesions on the trunk by evaluating their distribution pattern.

METHODS

In this retrospective observational study, the distribution pattern of 106 segmental vitiligo lesions on the trunk was analysed and classified into recurring subtypes.

RESULTS

Segmental vitiligo was more frequently observed on the ventral side (85·8%) than the lateral side (52·8%) or the back (36·8%) of the trunk. Based on recurring similarities in distribution patterns, lesions were categorized into six distinct subtypes: types 1, 2 and 3 involved the upper part of the trunk, type 4 and 5 the middle part and type 6 the lower part of the trunk. In total, 67·9% of all segmental vitiligo lesions fitted into these subtypes. The most frequent type of lesion was subtype 3 (22·6%), which showed a characteristic V-shaped pattern on the upper trunk, followed by subtype 5 (17·9%), with a band-like pattern on the lateral side, and subtype 6 (13·2%), with a rectangular depigmentation on the lower abdomen.

CONCLUSIONS

Based on our observations we suggest a new classification into six subtypes for segmental vitiligo lesions on the trunk, allowing categorization of the majority of segmental vitiligo patterns on the trunk. This may have important prognostic and diagnostic significance in early developing lesions.

Cells or signals: which moves to drive skin pattern formation?

During its development, the skin produces an array of evenly spaced hair follicles. How the location of each follicle is determined to produce this pattern has been the subject of study and speculation for several decades. A central unresolved issue is the extent to which movement of scattered, precommitted follicle cells might play a role in this process. Xavier et al. now report the identification of subpopulations of dermal cells in developing sheep skin which are positive for Delta1 expression, suggesting that these cells may represent precommitted dermal papilla cells and that dermal Notch pathway signalling plays a role in hair follicle patterning.

Linear lichen planus in children--case report

Lichen planus is an uncommon disease in children, and only 2 to 3% of affected patients are under twenty years of age. This dermatosis may appear in several clinical forms, which vary according to the morphology and distribution of lesions. In less than 0.2% of all lichen planus cases, the lesions are distributed along the lines of Blaschko, and is a variant called linear lichen planus. This is a case report of a patient aged two years and eight months, who presented keratotic violaceous papules, affecting the abdomen, buttocks and right thigh, distributed along the lines of Blaschko. Histopathological examination confirmed a diagnosis of linear lichen planus.

What causes alopecia areata?

The pathobiology of alopecia areata (AA), one of the most frequent autoimmune diseases and a major unsolved clinical problem, has intrigued dermatologists, hair biologists and immunologists for decades. Simultaneously, both affected patients and the physicians who take care of them are increasingly frustrated that there is still no fully satisfactory treatment. Much of this frustration results from the fact that the pathobiology of AA remains unclear, and no single AA pathogenesis concept can claim to be universally accepted. In fact, some investigators still harbour doubts whether this even is an autoimmune disease, and the relative importance of CD8(+) T cells, CD4(+) T cells and NKGD2(+) NK or NKT cells and the exact role of genetic factors in AA pathogenesis remain bones of contention. Also, is AA one disease, a spectrum of distinct disease entities or only a response pattern of normal hair follicles to immunologically mediated damage? During the past decade, substantial progress has been made in basic AA-related research, in the development of new models for translationally relevant AA research and in the identification of new therapeutic agents and targets for future AA management. This calls for a re-evaluation and public debate of currently prevalent AA pathobiology concepts. The present Controversies feature takes on this challenge, hoping to attract more skin biologists, immunologists and professional autoimmunity experts to this biologically fascinating and clinically important model disease.

Patterning skin by planar cell polarity: the multi-talented hair designer

In mammals, the skin can form complex global and local patterns to meet diverse functional requirements in different parts of the body. To date, the fundamental principles that underlie skin patterning remain poorly understood because of the involvement of multiple interacting processes. Genes involved in the planar cell polarity (PCP) signalling pathway, which is capable of polarizing cells within the planar plane of an epithelium, can control the orientation and differentiation of hair follicles, underlining their involvement in skin pattern formation. Here, we summarize recent progress that has been made to understand the PCP signalling pathway and its function in mammalian skin, including its role in hair follicle morphogenesis, ciliogenesis and wound healing. We argue that dissecting PCP signalling in the context of hair follicle formation might reveal many as-yet-undiscovered functions for PCP in the development, homeostasis and regeneration of skin.

Patterned disorders in dermatology

The skin gives us an opportunity to study pathologies unapparent in other systems such as patterned disorders. Among the best-identified patterns of skin disorders are the well-known lines of Blaschko, but other types of skin-patterned lesions have also been recognized. This short review will describe and discuss these different patterns and their pathophysiologic mechanisms, such as somatic mosaicism and X-chromosome associated mosaicism. Cutaneous patterned disorders are thought to be associated usually with inherited diseases per se, but in fact they are also reported in so-called acquired diseases. These cases suggest the existence of an underlying defect in a gene closely associated with the disease pathogenesis. The study of these acquired patterned disorders in the future may help us to understand the biologic foundations and pathogenesis of common human diseases.

The cycling hair follicle as an ideal systems biology research model

In the postgenomic era, systems biology has rapidly emerged as an exciting field predicted to enhance the molecular understanding of complex biological systems by the use of quantitative experimental and mathematical approaches. Systems biology studies how the components of a biological system (e.g. genes, transcripts, proteins, metabolites) interact to bring about defined biological function or dysfunction. Living systems may be divided into five dimensions of complexity: (i) molecular; (ii) structural; (iii) temporal; (iv) abstraction and emergence; and (v) algorithmic. Understanding the details of these dimensions in living systems is the challenge that systems biology aims to address. Here, we argue that the hair follicle (HF), one of the signature features of mammals, is a perfect and clinically relevant model for systems biology research. The HF represents a stem cell-rich, essentially autonomous mini-organ, whose cyclic transformations follow a hypothetical intrafollicular "hair cycle clock" (HCC). This prototypic neuroectodermal-mesodermal interaction system, at the cross-roads of systems and chronobiology, encompasses various levels of complexity as it is subject to both intrafollicular and extrafollicular inputs (e.g. intracutaneous timing mechanisms with neural and systemic stimuli). Exploring how the cycling HF addresses the five dimensions of living systems, we argue that a systems biology approach to the study of hair growth and cycling, in man and mice, has great translational medicine potential. Namely, the easily accessible human HF invites preclinical and clinical testing of novel hypotheses generated with this approach.

Patterns in naevoid skin disease: development, disease and modelling

The aetiology of pattern-formation in human naevoid skin disease remains unknown. However, it is likely that the majority of previously proposed mechanisms - those that simply rely on passive clonal trafficking in embryogenesis - are incomplete. A more comprehensive explanation for pattern-formation in naevi invokes the principle of self-organization. We define two types of patterning: anatomical and functional. Anatomical patterning is where the abnormal clone is limited to regions of pathologic skin, while functional patterning is where the abnormal clone and pathologic skin are spatially uncorrelated. From a theoretical perspective self-organized naevoid patterns may be either secondary to local interactions between normal and aberrant genotypes or due to the interaction between aberrant genotypes and the presence of normal embryonic patterning cues. The latter possibility suggests the critical observation and analysis of patterns in naevoid skin disease may lead to unique insights into key aspects of early human embryogenesis.

Evolution in health and medicine Sackler colloquium: Somatic evolutionary genomics: mutations during development cause highly variable genetic mosaicism with risk of cancer and neurodegeneration

Somatic mutations must happen often during development because of the large number of cell divisions to expand from a single-cell zygote to a full organism. A mutation in development carries forward to all descendant cells, causing genetic mosaicism. Widespread genetic mosaicism may influence diseases that derive from a few genetically altered cells, such as cancer. I show how to predict the expected amount of mosaicism and the variation in mosaicism between individuals. I then calculate the predicted risk of cancer derived from developmental mutations. The calculations show that a significant fraction of cancer in later life likely arises from developmental mutations in early life. In addition, much of the variation in the risk of cancer between individuals may arise from variation in the degree of genetic mosaicism set in early life. I also suggest that certain types of neurodegeneration, such as amyotrophic lateral sclerosis (ALS), may derive from a small focus of genetically altered cells. If so, then the risk of ALS would be influenced by developmental mutations and the consequent variation in genetic mosaicism. New technologies promise the ability to measure genetic mosaicism by sampling a large number of cellular genomes within an individual. The sampling of many genomes within an individual will eventually allow one to reconstruct the cell lineage history of genetic change in a single body. Somatic evolutionary genomics will follow from this technology, providing new insight into the origin and progression of disease with increasing age.