Histopathology of Pseudoxanthoma Elasticum and Related Disorders: Histological Hallmarks and Diagnostic Clues

Among ectopic mineralization disorders, pseudoxanthoma elasticum (PXE)—a rare genodermatosis associated with ocular and cardiovascular manifestations—is considered a paradigm disease. The symptoms of PXE are the result of mineralization and fragmentation of elastic fibers, the exact pathophysiology of which is incompletely understood. Though molecular analysis of the causal gene, ABCC6, has a high mutation uptake, a skin biopsy has until now been considered the golden standard to confirm the clinical diagnosis. Although the histological hallmarks of PXE are rather specific, several other diseases—particularly those affecting the skin—can present with clinical and/or histological characteristics identical to or highly resemblant of PXE. In this paper, we will summarize the histopathological features of PXE together with those of disorders that are most frequently considered in the differential diagnosis of PXE.


Introduction
Ectopic calci�cation is a pathologic deposition of calcium salts or inappropriate biomineralization in so tissues [1], resulting in the formation of osseous material in so tissues including the lungs, eyes, arteries, and skin. Ectopic mineral deposits in the extracellular matrix (ECM) of the cell can result in fragmentation of connective tissue components, such as the elastic �bers.
e paradigm of ectopic mineralization disorders is pseudoxanthoma elasticum (PXE, OMIM# 264800). is rare hereditary connective tissue disorder affects the ECM in different organs and-because of its molecular and etiopathological characteristics-has a unique position among the connective tissue diseases [2]. It is characterized by dystrophic mineralization and fragmentation of elastic �bers and causes dermal (papular lesions in �exural areas), ocular (angioid streaks, subretinal neovascularization, and haemorrhage), and vascular symptoms (coronary and peripheral vascular disease) (Figure 1). PXE is caused by mutations in the ABCC6 (ATP-binding cassette subfamily C member 6; OMIM# 603234) gene, encoding a transmembrane transporter protein, the substrate of which is currently unknown. Moreover, the exact relation between the ABCC6 transporter and the elastic �ber abnormalities remains unclear.
ough the pathophysiology of PXE is still ill-de�ned, two main concepts have been proposed to explain the consequences of ABCC6 mutations, respectively, coined as the metabolic and cellular hypothesis. e metabolic hypothesis results from the observation that ABCC6 is expressed primarily in the liver and kidney, and to much lesser extent in the affected organs, and suggests that a decrease of ABCC6 transport activity in the liver results in altered plasma levels of one or more substrates [2]. e cellular hypothesis of PXE is based on the observation that PXE �broblasts are suffering from a mild chronic oxidative stress due to imbalance between the production and degradation of oxidative stress species as a consequence of ABCC6 de�ciency [3].
Several other dermatological or system diseases with skin manifestations can have similar clinical and/or histological features as PXE. ese include among others the PXE-like syndrome with coagulation de�ciency, elastosis perforans serpiginosa, and haemoglobinopathies such as thalassemia and sickle cell disease. In this paper, the histological hallmarks of PXE and PXE-related disorders will be discussed, both in humans and murine models, focussing on those characteristics which can differentiate them from one another. For each disease, the histopathological clues will be shortly summarized and-if available-immunohistochemical �ndings relevant to the pathogenesis of disease will be considered.

Pseudoxanthoma Elasticum
2.1. Histopathology in Humans. e primary histological feature of PXE is degeneration of elastic �bers that undergo progressive mineralization and fragmentation resulting in a histological image pattern known as elastorrhexis [4,5]. ese alterations can be observed by light microscopy (LM) and electron microscopy (EM) in the main affected organs of PXE patients including skin, retina and blood vessels but also in other tissues which contain elastin. e latter include the urinary system (kidney, bladder), the gastrointestinal tract (oesophagus, intestines), and the pulmonary system (trachea, lung). e alterations in these systems, although oen widespread and associated with collagen abnormalities, are, however, usually very small which could account for their lack of clinical signi�cance. Although variation can be noted in the histological �ndings in the different affected tissues, the chain of degenerative events is always alike. One of the intruiging questions was whether the primum movens was the calci�cation or fragmentation. Calci�ed �bers are prone to degradation, while fragmented �bers become more easily calci�ed. EM studies have reported calci�cation in elastic �bers that appear normal as the �rst pathological sign in young individuals, allowing to put forward a pathological cascade as follows [6][7][8]. Initially, mineralization of the elastic �ber is seen as a central core of electron density on EM, with core density increasing as mineralization continues. Prior to fragmentation, the elastic �bers will develop holes, where the central portion of the core disappears or spontaneously fades. Finally, the �bers become maximally calci�ed, followed by fragmentation.
A series of studies have described abnormalities of other ECM components, such as collagens and proteoglycans. Collagen �owers, a sign of abnormal collagen �brillogenesis, are-although commonly found in PXE-also highly aspeci�c. Abnormal amounts of proteoglycans are localized nearby and within mineralized elastic �bers and abnormal amounts of GAGs, as well as alterations in their synthesis and deposition have been detected in PXE patients [14][15][16][17]. Moreover, PXE cells have been shown to produce proteoglycan species with altered properties, such as stronger polyanion properties, increased hydrodynamic size, abnormal hydrophobic actions, and different content and distribution of heparan sulphate, indicating an abnormal proteoglycan metabolism [17,18]. In urine of PXE patients and carriers, both decreased and increased concentration of GAGs has been observed [18,19]. Although no straightforward explanation exists for these discrepant �ndings, Maccari et al. found three distinguishing differences of urinary GAGs in PXE: the chondroitin sulphate/heparin sulphate ratio (which is lower), the 4-sulphated/6-sulphated chondroitin sulphate ratio (which is lower), and the high degree of chondroitin sulphate sulfation [4]. Baccarani-Contri et al. showed that elastic �bers have enhanced expression of normal constitutive proteins (e.g., vitronectine), but also accumulated aberrant matrix proteins known for their high affinity for calcium and involvement in mineralization processes (e.g., alkaline phosphatase, bone sialoprotein, osteonectin) [16].

Histopathology of the Skin.
Elastic �bers in the middermis are typically polymorphous, mineralized and fragmented, while those in the papillary dermis and deep dermal layers have a normal morphology. Light microscopy using von Kossa or Alizarin Red calcium stains reveal middermal clumps of calci�ed and fragmented elastic �bers ( Figure  2(a)). By EM, two types of mineralization can be noted: �ne deposits in the center of the �ber and bulky precipitates deforming and breaking the �bers (Figures 2(b) and 2(c)) [20][21][22]. In these dermal mineralized areas, deposits of thread-like material, collagen �owers, and collagen �brils of irregular diameter are present in most patients. Fibroblasts are oen numerous, with hypertrophy and dilatation of the endoplasmatic reticulum [20]. Near the mineralized areas, macrophages are abundant. Interestingly, ultrastructural elastic tissue alterations can be observed in both lesional and clinically noninvolved skin, while the other ECM changes are only seen in clinically involved skin in vicinity of aberrant elastic �bers [23]. In rare cases, the dermal calci�cation can lead to ossi�cation [24]. Interestingly, this has always been noted in those patients with signi�cant cutis laxa. �f note, the dermal EM alterations are certainly not speci�c for PXE� they can also be seen in other inherited diseases of the ECM and normal skin aging [23]. erefore, LM evaluation remains essential for the diagnosis of PXE. In this regard, the quality of the skin biopsy-particularly that it is a full-thickness specimen-as well as its location is essential for a reliable result. Indeed, contrary to EM �ndings, LM aberrations in nonlesional skin can be very mild or absent.

2.1.2.
Histopathology of the Eye. PXE typically affects elastic �bers in Bruch�s Membrane (BrM), a thin connective tissue layer separating the retina from the choriocapillaris. e changes in BrM are apparently identical to those seen in the middermis, with similar calcium deposits onto and clumping of fragmented elastic �bers. As a result, the barrier membrane will no longer have a smooth surface, but will start to exhibit breaks, making it possible for choroid vessels to grow towards the inner retina ( Figure 3). e start of this degradation is characterized by discontinuities in the middle elastic layer and loss of RPE pigment granules. In a next stage, full thickness breaks in combination with atrophy of the overlying RPE and photoreceptor cells are observed plus ruptures of the underlying choroid. In some cases, herniation of choroidal �brillar collagen tissue and choriocapillaris into the breaks in BrM can be seen, separating this membrane.

Histopathology of the Cardiovascular System.
In PXE patients, both the myocardium and pericardium show elastic �ber mineralization and collagen �owers, resembling the mid-dermal alterations [4]. �imilar �ndings can be observed in nearly all small and medium-sized bloodvessels, including intestinal vessels, although it has been observed that alterations are not homogeneously distributed in the vessel wall. Alterations are most prominent close to the adventitia [4].
In contrast to the dermis and retina, vascular elastic �bers tend to form aggregates of thin strands of amorphous elastin, which replaces the internal elastic lamina, for example, in the * * aorta. ese degenerative changes may be accompanied by various degrees of intima thickening due to a patchy proliferation pattern of the �broelastic components [25]. ese vascular changes show great comparison with Mönckebergtype arteriosclerosis and the idiopathic type of generalized arterial calci�cation in infancy (IACA). e former is a type of focal calci�c arteriosclerosis in the elderly, while IACA is a dramatic variant of arterial myoelastic degenerative tissue calci�cations, caused by mutations in the ENPP1 gene (OMIM# 173335) and leading to intrauterine death or lethality within the �rst months of life. �hile recent advances in deep phenotyping of the vascular implications of PXE have led to the suggestion of a unique vasculopathy in PXE, it remains interesting that recently ENPP1 mutations have been found in classic PXE patients, while some IACI patients were discovered to harbor ABCC6 mutations. is underlines that these disorders are in fact part of the same spectrum of diseases [26,27]. Also the venous system, more speci�cally the vena cava, can be affected at the ultrastructural level in PXE [4].

Histopathology of the Murine Model
. e murine PXE model is a transgenic mouse model generated by targeted ablation of the mouse Abcc6 gene [28]. Light and electron microscopy observations of Accc6-/-mice revealed that KO mice spontaneously developed calci�cation of elastic �bers in blood vessel walls and in Bruch's Membrane in the eye, while no clear abnormalities were seen in the dermal ECM, possibly due to the different distribution of elastic �bers in murine dermis compared to humans ( Figure 4) [28]. Mineralization can affect both elastic structures and collagen �bers [29]. Calci�cation of blood vessels was most prominent in small arteries in the cortex of the kidney [28]. Klement et al. also found profound mineralization of several tissues, including skin in the Abcc6 null mice [29]. Interestingly, calci�cation in the murine model started in the connective tissue sheet around the whiskers, and this feature has been used as a biomarker for calci�cation in murine PXE since ( Figure 4).

Immunohistochemical Pathogenetic Clues in PXE.
So tissue mineralization is a complex process, the exact mechanisms of which have not yet been completely elucidated. However, a signi�cant overlap has been observed in several studies with osteogenesis and several proteins involved in bone metabolism have been implicated in so tissue calci�cation. Besides the many propagators of calcium precipitation, ectopic mineralization tends to be more frequently due to a disruption of calci�cation inhibitors. e latter include the endogenous inhibitor matrix Gla protein (MGP), the inducible inhibitor osteopontin (OPN), and the systemic circulating inhibitor fetuin-A [30][31][32].
In PXE patients, the role of so tissue calci�cation inhibitors has been studied by establishing serum concentration of osteocalcin (OC), fetuin-A and MGP. It was observed that the total amount of OC was decreased in patients [33]. PXE patients also have a signi�cantly lower serum level of both MGP and fetuin-A than controls [34][35][36]. Dermal �broblasts of PXE patients produce less of the -carboxylated form of MGP compared to controls, suggesting that these cells also play a role in ectopic calci�cation in PXE [34]. Immunohistochemistry on PXE dermis revealed signi�cant middermal staining of carboxylated and uncarboxylated MGP, as well as osteocalcin and fetuin-A ( Figure 5).
In the murine knockout model for PXE, a reduction in fetuin-A serum levels was found compared to control, albeit that there was considerable variability between the individual mice [37,38]. On IHC of the whiskers in abcc6-/-mice, MGP colocalized with mineralization but in its inactive or undercarboxylated form [38].

PXE-Like Disease with ��a���ati�� De��ie���
In PXE-like disease with coagulation de�ciency (OMIM# 610842), patients initially present with clinical features, which cannot be distinguished from classic PXE. However, the natural history of this disease is quite different with progression of the skin lesions towards thick and leathery skin folds beyond the �exural areas ( Figure 6) [39]. Contrary to PXE, ophthalmological and cardiovascular manifestations usually remain mild. In addition, these patients have a de�ciency of the vitamin-K-(VK-) dependent coagulation factors (factor II, VII, IX, and X), though most oen this remains asymptomatic and is a coincidental �nding. e disease is caused by loss-of-function mutations in the GGCX gene (OMIM# 137167), encoding a gamma-carboxylase which in F 5: Immunohistochemical �ndings in PXE, showing excessive middermal staining of carboxylated (a, ×20) and uncarboxylated (b, ×20) MGP, as well as osteocalcin (c, ×20) and fetuin-A (d, ×20).
the liver is responsible for the activation of VK-dependent coagulation factors. In peripheral tissues, it activates several inhibitors of mineralization, including MGP. e histopathology of this PXE-like disease is indistinguishable from classic PXE on LM. However, EM reveals several speci�c characteristics, which can be used in the differential diagnosis, particularly in young individuals in whom the clinical presentation may still be identical to classic PXE.
3.1. EM Histopathology. e overall appearance of the elastic �ber network has a rather "ragged" aspect, as if someone pulled at it [39]. Although mineralization is most prominent in the middermis and does not affect all elastic �bers, the morphology on EM is different from classic PXE. Indeed, they are oen made of aggregates of distinct strands of elastin and con�ned to the periphery of the elastic �ber, whereas PXE mostly affects the elastic �ber core. Also, electrondense crystal-like bodies can be seen in the central core of mineralized �bers in PXE-like patients, whereas in PXE this feature is not encountered (Figure 7).

Immunohistochemical
Findings. e immunohistochemical staining for carboxylated and uncarboxylated MGP, as well as for osteocalcin and fetuin-A, were similar to what was observed in PXE. On EM, uncarboxylated MGP was more abundant and located in the core of elastic �bers, whereas carboxylated MGP resided at the border of the mineralized areas.

PXE Phenocopies Associated with Haemoglobinopathies
Patients with inherited haemoglobinopathies, most oen thalassaemias but also sickle cell disease, may have elastic tissue changes and clinical features closely resembling-if not identical to-PXE. e skin, eye, and cardiovascular symptoms are indeed indistinguishable from PXE-except for their age of onset which is usually later in life-despite that none of these patients carry ABCC6 mutations ( Figure  8) [40][41][42][43][44][45][46][47][48][49]. ese clinical �ndings are not anecdotal in haemoglobinopathy patients: out of a cohort of 100 patients with major or intermediate beta-thalassaemia, 26 patients had angioid streaks and/or skin lesions [46]. While one hypothesis might be that this is related to the pathophysiology of the primary haemoglobinopathy (and as such "acquired"), the identical histological �ndings in these patients suggest that either a pathway is involved which is independent of ABCC6 or that the largely unknown ABCC6 pathway is affected more downstream. As a result, a haemoglobin electrophoresis should be considered in the standard workup of every patient presenting with clinical and/or histological features resembling PXE. F 7: E� �ndings in the PXE-like disease with coagulation factor de�ciency. e elastic �ber network displays a ragged aspect (a, asterisk), while mineralization can be observed at the periphery of affected elastic �bers (b, asterisk).

Dermatological Diseases with (Near-) Identical Skin Lesions
Disorders with a phenotype showing signi�cant resemblance to PXE and resulting from elastic �ber abnormalities are usually within the group of heritable or acquired skin diseases. Indeed, although elastic �ber fragmentation has been described in eye diseases such as conjunctivochalasis (with �ber fragmentation in the conjunctiva) or in primary open-angle glaucoma (�ber loss and fragmentation in the lamina cribrosa), these entities are clinically completely different from the ophthalmological features in PXE [50,51]. �imilarly, elastic �ber fragmentation has been described in certain tumor types, such as the papillary �broelastoma, but these disorders lie beyond the scope of this paper [51].  Figure 9) [53,54]. e disease typically affects women in late adulthood. With less than 40 cases reported, it has been suggested to be rather underdiagnosed [55]. ough its pathogenesis is not completely understood, the etiopathogenic factors considered include intrinsic skin aging, ultraviolet radiation, and abnormal elastogenesis [54,55]. PXE-PDE may appear as multiple, asymptomatic or pruritic, yellow or skin-colored, nonfollicular, cobblestone-appearing papules that coalesce into large plaques distributed symmetrically over the supraclavicular, lateral, and posterior regions of the neck, the �exor aspect of the forearms, the axillae, the lower part of the abdomen, and the inframammary folds [54,56]. Histopathologic examination of the affected skin shows an atrophic epidermis and band-like loss of elastic tissue in the papillary dermis (Figure 9(b)) [57]. Clumping and fragmentation of elastic �bers may also be seen [56]. In addition, melanophages can be observed in the papillary dermis, possibly as a consequence of subclinical junctional photodamage. Immunohistochemical staining may show loss of �brillin 1 and 2, and micro�bril-associated glycoprotein 1 and 2 in the papillary dermis [58].  �brosis and variable loss of dermal elastic tissue. e lesions are asymptomatic and gradually increase in number. ere is some clinical resemblance to PXE, but as in PXE-PDE there are no systemic complications such as angioid streaks in fundo [59][60][61][62]. e etiology is unknown, but in view of its late onset in life, the condition may be the result of age-related changes in dermal collagen [63]. Lightmicroscopy examination most prominently reveals dermal �brosis consisting of areas of thickened collagen bundles in the papillary and midreticular dermis as well as loss of elastic tissue in the papillar and reticular dermis. e presence of �brosis is the main feature to distinguish WFPN from PXE-PDE. Also complete absence of oxytalan �bers, an EC� component, has been noted [60,[62][63][64]. Electron microscopy may show a decrease in elastic tissue [65]. Remaining elastin �bers appear smaller, fragmented, and cribriform [62].

Late-Onset Focal Dermal Elastosis.
Late-onset focal dermal elastosis (LOFDE) is a rare disorder of elastic tissue, characterized by a PXE-like yellowish papular eruption, with local accumulation of elastic �bers in the mid-and deep reticular dermis [66,67]. e disease occurs mainly in elderly people and typically affects the sides of the neck and �exural areas, closely mimicking PXE from the clinical point of view [66]. e elderly onset of this disease suggests that LOFDE may be a disorder of aging. Occurrence of the lesions on the neck, antecubital and popliteal fossae is potentially signi�cant� it has been hypothesized that elastic �ber turnover may be accelerated in these areas, which are naturally subjected to mechanical stress [67,68]. Isolated �broblasts from lesional skin show increased collagen and elastin messenger RNA suggesting that the disorder may be the result of increased elastin synthesis rather than a reduction in the degradation process [69].
e most prominent histopathologic feature is a local accumulation of normal-appearing elastic tissue or thick, interlacing elastic �bers in the mid-and deep reticular dermis without fragmentation, calci�cation, or phagocytosis of elastic �bers [69][70][71]. A hydroxyproline assay of lesional skin may show a modest increase in collagen content and a more substantial increase in elastin [72].

Buschke-Ollendorff
Syndrome. e Buschke-Ollendorf syndrome (OMIM# 166700) is an autosomal dominant disease, characterized by disseminated connective tissue nevi and osteopoikilosis [81]. As these two main features may occur separately, it can be difficult to distinguish the skin lesions from �broelastolytic elastosis, PXE, or papular elastorrhexis. e presenting symptom may be disseminated lesions on the trunk, upper arms, and thighs. e skeletal features, fascicular streaks, or condensations of the epiphysis and metaphysis of the long bones, are usually an incidental �nding which o�en occur later than the skin features. In some patients, only one of the cardinal features is present [82]. e disease is caused by loss-of-function mutations in the LEMD3 gene (OMIM# 607844), encoding an inner nuclear membrane protein [83].
Histologically, the lesions may either be elastin-rich (elastoma) or collagen-rich (dermato�brosis lenticularis dissminata, Figure 11). For the elastomae, dermal elastic �bers may be increased in number and size, with hypertrophy on light microscopical exam. Electron microscopy can reveal altered electrolucent elastic �bers and decrease of micro�brillar components [84].

Related Dermatological Diseases
In this section, some differential diagnosis, which can occur in PXE patients or have a similar histological phenotype, will be discussed.

Elastosis Perforans Serpiginosa.
Elastosis perforans serpiginosa (EPS; OMIM# 130100) is a rare skin condition of unknown aetiology characterized by hyperkeratotic papules and plaques, transepidermal elimination of abnormal elastic �bers, and focal dermal elastosis [67,85]. Patient ages range from 5 to 89 years, but 90% of patients with EPS are younger than age 30, with approximately 75% of affected being male [86]. EPS has been postulated to be a focal irritation in the dermis, which may induce formation of epidermal and follicular channels to extrude the irritating agent. e disease can also occur in patients with an underlying connective tissue disorder, such as PXE or Ehlers-Danlossyndrome type IV (the vascular subtype) [87]. EPS can typically be induced by D-Penicillamine, used to treat Wilson disease. In the latter, patients develop skin lesions that may be highly resemblant of PXE, which is sometimes referred to as pseudo-PXE [88]. In these cases, discontinuation of the drug usually leads to improvement of the lesions.
Symptoms of EPS can usually appear in the neck and on the face and, less frequently, the upper and lower extremities or trunk with pruritic, erythematous or skincolored hyperkeratotic papules and plaques with central scaling, hypopigmentation, and atrophy. Papules are typically distributed in a serpiginous pattern (Figure 12(a)).  e most prominent histopathological feature of EPS is the presence of narrow transepidermal or perifollicular perforating canals that extend upward in a straight or corkscrew pattern from the dermis and contain a mixture of degenerated eosinophilic elastic �bers, basophilic debris, and in�ammatory cells (Figures 12(b) and 12(c)). Signi�cantly increased amount and thickness of papillary dermal elastic tissue is also noticed [86]. A chronic in�ammatory in�ltrate with granuloma-forming multinucleated giant cells may be present [89]. e epidermis may be acanthotic and hyperkeratotic. LM and EM of Penicillamine-induced EPS reveal a characteristic "lumpy-bumpy" or "bramble-bush" appearance of elastic �bers and elastic tissue changes are observed in both lesional and nonlesional skin, which distinguishes it from other types of EPS [90][91][92][93][94]. Transepidermal elimination may also be seen in some forms of acquired PXE; however, the mineralization of elastic �bers characteristic of PXE is absent in EPS.

Papular
Elastorrhexis. Papular elastorrhexis (PE) is a sporadic dermatosis, mostly occurring in adolescence, characterized by asymptomatic �rm, nonfollicular, 1-5 mm diameter, well-demarcated white papules distributed evenly over the trunk including the chest, abdomen, back, shoulders, and upper extremities [85,[95][96][97][98]. ough the cause and pathogenesis of PE is mostly unknown, recent studies suggest that PE can occur in both acquired and familial forms [99]. It is regarded as a variant of elastic tissue nevi though controversy exists [98]. PE causes substantial fragmentation or nearly complete loss of elastic tissue in the reticular dermis [99][100][101][102]. A perivascular lymphohistiocytic in�ltrate in the super�cial and deep dermis has been reported. Collagen bundles can be thickened and homogenized, or normal [99].

Upper and Middermal Elastolysis.
Upper and middermal elastolysis are rare disorders, which are both characterized by a signi�cant loss of dermal elastic �bers. ough they may be highly resemblant clinically, the main differences can be noted histologically in the location of the absent elastic �bers but also in severity of ultrastructural damages [103].

Upper Dermal Elastolysis.
Upper dermal elastolysis is characterized by an eruption of many small papules on the neck, shoulder, upper chest, and upper back region and selective loss of elastic tissue in the papillary dermis [103,104]. It has been suggested that lysis of elastic tissue may be a primary process in activation of dermal phagocytes that recognize fragments of elastic �bers as foreign. Alternatively, activation of dermal phagocytes may be a primary in�ammatory process that leads to elastolysis and engulfment of both normalappearing and degraded elastic �bers [103].
Histopathology is characterized by complete loss of elastic �bers in the upper dermis including the papillary dermis, whereas middermal �bers are intact. Electron microscopy reveals a loose assembly of elastic �brils and electron-dense substance aggregated between the loosely bound subunits. Elastophagocytosis of normal-appearing and abnormal �bers is also observed [103].

Middermal Elastolysis.
Middermal elastolysis typically occurs in healthy young or middle-aged women and is characterized by focal loss of elastic tissue in the midreticular dermis. e disease manifests clinically as multiple, discrete, wrinkled, �esh-colored, so� papules, some with a central umbilication, were present on the abdomen and lower back [104,105]. Many cases appear to be induced or worsened by ultraviolet light exposure [106]. On histological examination, a selective absence of elastic �bers ( Figure 14) in the middermis can be observed (Figure 13). Additionally, perivascular lymphocytic, monocytic, or neutrophilic in�ltrates and phagocytosis of elastin by multinucleated giant cells can be seen [107][108][109][110][111][112]. Electron microscopy studies have reported phagocytosis of degenerated elastic �bers by macrophages [107,111,[113][114][115], as well as loose assembly of skeleton �brils and irregular aggregations of dense substance [107,110,[115][116][117]. Immunohistochemistry shows absent immunoreactivity of elastin. Stainings for MMP-9 can be positive in epidermal keratinocytes and in large multinucleated cells in the affected dermis [118].

Linear Focal Elastosis.
Linear focal elastosis (LFE) is a disorder of elastic �bers characterized by palpable, hypertrophic, yellowish, linear striae distributed horizontally over the mid and lower back, thighs, arms and breasts, and an increase in abnormal elastic tissue [119][120][121]. Reports on occurrence show that men are more prone than women. Although the pathogenesis of LFE remains unknown, its association with striae distensae may suggest involvement of a process analogous though not identical to keloidal repair [120].

Elastoderma.
Elastoderma is an exceedingly rare condition presenting with an acquired localized laxity of skin resembling cutis laxa, with an abundance of pleomorphic elastic tissue in the dermis [127,128]. It has been suggested that the abundance of elastic tissue results from increased synthesis, as evidenced by the presence of active �broblasts with prominent rough endoplasmic reticulum [127][128][129]. It has been speculated that elastoderma may be a localized disorder of elastin synthesis analogous to localized scleroderma (Morphea) resulting from excessive collagen distribution [67].
Histologic examination of specimens from the affected area can reveal increased masses of intertwined thin, elastic �bers without calci�cation in the papillary and upper reticular dermis [127]. Electron microscopic examination demonstrates irregular deposition of elastic material at the periphery of elastic tissue �bers, with grape-like globular structures [127].
6.6. Calcinosis Cutis. Calcinosis cutis refers to a group of disorders featuring pathologic calci�cation, generally calcium phosphate deposits, of the skin and so tissues [130]. Calcinosis cutis can occur in a variety of systemic and localized conditions and is classi�ed into 4 ma�or types according to etiology: dystrophic, metastatic, iatrogenic, and idiopathic; among these, dystrophic calcinosis cutis is most common. A few rare types have been variably classi�ed as dystrophic or idiopathic [119]. ese include calcinosis cutis circumscripta, calcinosis cutis universalis, tumoral calcinosis, and transplant-associated calcinosis cutis. Calcinosis cutis circumscripta tends to arise in second half of life, calcinosis cutis universalis occurs in the second decade of life, and the tumoral calcinosis usually arises in the �rst or second decade of life. e pathogenesis of calcinosis cutis is not completely understood, although metabolic and physical factors are pivotal in the development of most cases of calcinosis. On skin biopsy, granules and deposits of calcium are seen in the dermis, with or without a surrounding foreign-body giant cell reaction ( Figure 15). Alternatively, massive calcium deposits may be located in the subcutaneous tissue. In areas of necrosis, calcium deposition is frequently found within the walls of small-and medium-sized blood vessels [130].

Conclusion and Summary
e histological examination of the skin is currently still the golden standard to make a diagnosis of PXE. Because of the high mutation uptake of molecular analysis of the ABCC6 gene, a skin biopsy is currently not always performed. However, in the absence of either two causal ABCC6 mutations or of unequivocal systemic features compatible with PXE, a histological evaluation of the skin lesions still remains the most important diagnostic step towards a de�nite diagnosis. Indeed, the histological features-either on LM, EM or both-are capable of distinguishing most of the disorders in the differential diagnosis of cutaneous PXE, even when clinically highly resemblant or identical. Table  1 summarizes the most important differential diagnoses of PXE and their histopathological clues. In those patients-for example, suffering from a haemoglobinopathy-where clinical and histological �ndings are identical to ABCC6-related PXE, additional blood examinations can be performed to con�rm the diagnosis.