Increased 68 Ga-FAPI Uptake in Capillary Hemangiomas in an Adult Patient.

ABSTRACT: A 67-year-old woman was enrolled in our 68 Ga-FAPI PET/CT tumor clinical trial due to her lung adenocarcinoma. The PET/CT scan additionally revealed increased uptake of FAPI in the tongue. Combined with the patient's medical history and the contrast-enhanced CT of the maxillofacial region, it was suspected to be a hemangioma. Subsequently, the patient underwent surgery and was diagnosed with capillary hemangioma of the left side of her tongue.

Propranolol inhibits the angiogenic capacity of hemangioma endothelia via blocking ß-adrenoceptor in mast cell.

BACKGROUND: Propranolol, a non-selective blocker of the ß-adrenoceptor (AR), is a first-line treatment for infantile hemangioma (IH). Mast cells have been implicated in the pathophysiology of propranolol-treated hemangioma. However, the function of mast cells remains unclear. METHODS: HMC-1s (Human mast cell line) having been treated with propranolol for 24 h were centrifuged, washed with PBS twice, and maintained in cell culture medium for another 24 h. The supernatants with propranolol which were named as propranolol-treated HMC-1s supernatants were obtained. The expression of cytokines and mediators was examined among HMC-1s dealt with propranolol. HemECs (hemangioma endothelial cells) were co-cultured with propranolol-treated HMC-1s supernatants, and their proliferation and apoptosis were investigated. The autophagic-related protein was examined in HemECs using immunoblot. RESULTS: In propranolol-treated HMC-1s, the expressions of ADRB1 (ß1-AR) and ADRB2 (ß2-AR) were reduced by 70% and 60%, respectively, and that of cytokines and mediators were reduced. The proliferation was decreased, but apoptosis and autophagy were induced in HemECs treated with propranolol-treated HMC-1s supernatants. However, propranolol can work well in shRNA-ADRB1 or shRNA-ADRB2 transfected HMC-1s. CONCLUSIONS: Propranolol inhibit the proliferation of HemECs and promote their apoptosis and autophagy through acting on both ß1 and ß2 adrenoceptor in mast cell. IMPACT: Treated with propranolol, ß1, and ß2 adrenoceptor on human mast cell expression was reduced significantly. After hemangioma endothelial cell treated with the supernatants from propranolol-treated human mast cell, its proliferation was decreased, but apoptosis and autophagy were significantly induced. Propranolol can work well in shRNA-ADRB1 or shRNA-ADRB2 transfected HMC-1s. Mast cells may have a role in the action of propranolol in infantile hemangioma through both ß1 and ß2 adrenoceptors to inhibit the angiogenic capacity of hemangioma endothelial cells.

Critical role of Aquaporin-1 and telocytes in infantile hemangioma response to propranolol beta blockade.

Propranolol, a nonselective ß-adrenergic receptor (ADRB) antagonist, is the first-line therapy for severe infantile hemangiomas (IH). Since the incidental discovery of propranolol efficacy in IH, preclinical and clinical investigations have shown evidence of adjuvant propranolol response in some malignant tumors. However, the mechanism for propranolol antitumor effect is still largely unknown, owing to the absence of a tumor model responsive to propranolol at nontoxic concentrations. Immunodeficient mice engrafted with different human tumor cell lines were treated with anti-VEGF bevacizumab to create a model sensitive to propranolol. Proteomics analysis was used to reveal propranolol-mediated protein alteration correlating with tumor growth inhibition, and Aquaporin-1 (AQP1), a water channel modulated in tumor cell migration and invasion, was identified. IH tissues and cells were then functionally investigated. Our functional protein association networks analysis and knockdown of ADRB2 and AQP1 indicated that propranolol treatment and AQP1 down-regulation trigger the same pathway, suggesting that AQP1 is a major driver of beta-blocker antitumor response. Examining AQP1 in human hemangioma samples, we found it exclusively in a perivascular layer, so far unrecognized in IH, made of telocytes (TCs). Functional in vitro studies showed that AQP1-positive TCs play a critical role in IH response to propranolol and that modulation of AQP1 in IH-TC by propranolol or shAQP1 decreases capillary-like tube formation in a Matrigel-based angiogenesis assay. We conclude that IH sensitivity to propranolol may rely, at least in part, on a cross talk between lesional vascular cells and stromal TCs.

Expression of (pro)renin receptor and its effect on endothelial cell proliferation in infantile hemangioma.

BACKGROUND: Propranolol is the preferred treatment for problematic proliferating infantile hemangioma (IH) by targeting the renin-angiotensin system (RAS) expressed by IH endothelium. (Pro)renin receptor (PRR) is a major component of the RAS associated with the canonical wnt signaling pathway. We proposed that activation of PRR by renin causes proliferation of IH. METHODS: The expression of PRR in IH tissue samples was investigated using immunohistochemical (IHC) staining and NanoString analysis. NanoString analysis was also used to confirm transcriptional expression of PRR in CD34-sorted proliferating IH-derived primary cell lines. MTT assay was utilized to determine the effect of exogenous renin on the number of viable IH cells. RT-qPCR was used to determine the effect of renin on the stem cell gene expression. RESULTS: NanoString analysis and IHC staining confirmed transcriptional and translational expression of PRR, which was localized to the non-endothelial and the endothelial IH cell populations. MTT assay demonstrated an increased number of viable IH cells by administration of renin and the effect was negated by the wnt receptor blocker dickkopf-1. CONCLUSION: Our results present a model for renin-induced increased proliferation of IH cells through PRR acting via the wnt signaling pathway, which may account for accumulation of cells in IH during the proliferative phase of the tumor.

ß-Adrenoceptor Blockade for Infantile Hemangioma Therapy: Do ß3-Adrenoceptors Play a Role?

Infantile hemangiomas (IH) are frequent (4-5% of the childhood population) benign vascular tumors that involve accumulation, proliferation, and differentiation of aberrant vascular cells. Typically, IH are innocuous and spontaneously disappear, but they represent a potential risk for harmful effects in the body (e.g., permanent disfigurement) and health (e.g., ulcerations) in some patients. From a serendipitous discovery, the nonselective ß-adrenoceptor blocker propranolol (which blocks ß1-adrenoceptors, ß2-adrenoceptors, and ß3-adrenoceptors) emerged as an alternative therapy to treat this pathology and it quickly became a first-line treatment for IH. Nevertheless, its specific mechanisms of action remain thus far unknown. In this respect, several studies have suggested that ß1-adrenoceptors and ß2-adrenoceptors play a role in proliferative and angiogenic mechanisms. However, current basic research studies suggest that ß3-adrenoceptors could be also involved. Notably, ß3-adrenoceptors stimulate multiple intracellular pathways related to vascular function (e.g., blood flow, angiogenesis, etc.). This review compiles some lines of evidence suggesting that ß3-adrenoceptors may: (1) play a role in the pathophysiology of IH and (2) represent a potential therapeutic target for IH treatment. Hence, clinical evidence is mandatory to decide whether incorporation of ß3-adrenoceptor blockers into the therapeutic armamentarium may increase effectiveness in the treatment of IH and other vascular anomalies.

NDRG1 and FOXO1 regulate endothelial cell proliferation in infantile haemangioma.

The etiopathogenesis of infantile haemangioma has not been well understood, and it is accepted that angiogenic mediator dysregulation is the main contributor to the abnormal haemangioma capillary formation. The role of NDRG1, a hypoxia-inducible protein; FOXOs, which are tumor suppressor proteins; and the mTOR complex 2 pathway in infantile haemangioma have not been studied yet. The purpose of this study was to investigate NDRG1 and FOXO1 expression in the infantile haemangioma and the correlation of these proteins with proliferation and involution. Primary endothelial cells were obtained, with parental agreement, from 12 infantile haemangioma patients during surgery; 6 patients had proliferating infantile haemangiomas and 6 had involuting IHs. We compared the infantile haemangioma tissues and primary endothelial cells with human vein endothelial cells using microarrays, real-time PCR, Western blotting and immunohistochemical staining. Our data indicated that FOXO1 expression was downregulated in proliferating infantile haemangioma tissue. We found that the expression of NDRG1, a molecule upstream of the FOXO1 pathway, increased during haemangioma proliferation. NDRG1 knockdown decreased haemangioma endothelial cell proliferation and downregulated c-MYC oncoprotein levels. Our findings suggest that NDRG1 positively regulates haemangioma proliferation. FOXO1 dysregulation plays an important role in infantile haemangiomas pathogenesis.

Analysis of Follicle-Stimulating Hormone Receptor in Infantile Hemangioma.

INTRODUCTION: The life cycle of infantile hemangioma (IH) and secretion of follicle-stimulating hormone (FSH) are identical. We previously have shown that IH contains the FSH receptor (FSHR). The purpose of this study was to identify which cell type(s) in IH expresses FSHR. METHODS: Human proliferating IH tissues obtained during a clinically indicated surgical procedure were used. Paraffin sections and isolated cell populations (endothelial, pericyte, stem cell) were subjected to immunofluorescence for FSHR. Tissues were costained with DAPI, anti-α smooth muscle actin, or biotinylated Ulex Europaeus Agglutinin I to identify nuclei, pericytes, and endothelial cells, respectively. Whole tissue and purified single cell populations underwent polymerase chain reaction (PCR) for FSHR. Positive control specimens (ovary, sertoli cells) and negative control tissues (skin/subcutis, hepatic cells) were included. RESULTS: Immunofluorescence of 9 IHs demonstrated that FSHR was enriched in pericytes compared with endothelial cells. Follicle-stimulating hormone receptor was expressed in 6 of 6 whole tissue IHs along with the positive control via PCR. Follicle-stimulating hormone receptor was not present in the negative control samples. Four of 5 sets of pericytes expressed FSHR by PCR. Neither IH endothelial cells, IH stem cells, nor negative control cells exhibited FSHR by PCR. CONCLUSIONS: Because the secretion of FSH correlates with the growth pattern of IH, FSH might be involved in the disease process. Follicle-stimulating hormone receptor is enriched in the pericytes of IH, suggesting that this cell type may be involved in the pathogenesis of the tumor.

Salivary levels of angiopoietin-2 in infants with infantile haemangiomas treated with and without systemic propranolol.

Infantile haemangiomas (IHs) with functional or cosmetic concerns necessitate systemic treatment for which propranolol is the preferred treatment. However, the mechanism of action is unknown. Mouse models suggest the angiopoietin-2 (Ang2)/Tie-2 system is implicated. Ang2 can promote endothelial growth or induce apoptosis depending on the presence of vascular endothelial growth factor. This pilot study investigates the saliva Ang2 levels in infants with IH treated with and without systemic propranolol. Patients with clinically confirmed IHs were recruited from an academic paediatric dermatology centre. Treatment was based on clinical evaluation. Saliva samples were collected over 6 months. An enzyme-linked immunosorbent assay determined Ang2 levels. Ang2 levels were detectable in 45% of samples. However, by the late time point, only 28% had detectable levels. There were no changes of Ang2 over time, and there were no differences in Ang2 levels between groups. However, Ang2 levels were correlated with baseline size and changes in size from baseline. Ang2 is detectable in saliva of affected infants, but does not decrease with propranolol treatment. However, Ang2 levels are positively correlated with size and changes in size. Thus, Ang2 is not the primary factor in the mechanism of propranolol resulting in IH reduction.

M1 Macrophage-Induced Endothelial-to-Mesenchymal Transition Promotes Infantile Hemangioma Regression.

Infantile hemangiomas are benign tumors of vascular endothelial cells (ECs), characterized by three distinct stages: proliferating phase, involuting phase, and involuted phase. The mechanisms that trigger involution of hemangioma into fibro-fatty tissue remain unknown. We report a novel mechanism by which M1-polarized macrophages induce endothelial-to-mesenchymal transition (EndMT) and promote hemangioma regression. M1- but not M2-polarized macrophages induced EndMT in ECs. Tumor necrosis factor-α and, to a lesser extent, IL-1ß and interferon-γ were the most potent cytokines produced by the M1 macrophages that induce in vitro EndMT. Western blot analysis and gene expression profiling showed that ECs treated with M1 macrophages, tumor necrosis factor-α, or IL-1ß decreased the expression of endothelial markers, whereas mesenchymal markers increased concomitantly. Immunohistochemical staining of patient samples revealed that a significant perivascular infiltration of M1, but not M2, macrophages coincides with endothelial expression of the critical EndMT transcription factors Snail/Slug in involuting hemangiomas. Most strikingly, M1 macrophage-treated ECs isolated from patient hemangiomas (HemECs) but not untreated HemECs readily differentiated into adipocytes on adipogenic induction. Thus, in vitro EndMT and adipogenesis of HemECs have, in part, recapitulated the natural history of hemangioma regression. In conclusion, our findings indicate that EndMT induced by M1 macrophages promotes infantile hemangioma regression and may lead to novel therapeutic treatments for this vascular tumor.

The expression of renin-angiotensin-aldosterone axis components in infantile hemangioma tissue and the impact of propranolol treatment.

BackgroundPropranolol's mechanism of action for controlling infantile hemangioma (IH) remains unclear. We hypothesize that this nonselective beta antagonist downregulates renin-angiotensin-aldosterone (RAA) axis components, preventing angiogenic substrate induction of IH.MethodsIH tissue and serum were collected from children with propranolol-treated or -untreated IH during surgery. Normal skin and serum from demographically matched children were used as controls. Real-time PCR and western blot quantified RAA components in proliferative (n=10), involuting (n=10), propranolol-treated (n=12) IH, and normal specimens (n=11). Serum was analyzed by enzyme-linked immunosorbent assay (ELISA).ResultsThere were significantly greater messenger RNA (mRNA) levels of angiotensinogen (AGT) in proliferating IH, but not in involuting or treated IH, when compared with controls (P<0.05). Angiotensin-converting enzyme (ACE) and angiotensin II receptor 1 (AGTR1) mRNA expression was higher in all IH specimens when comparedwith controls (P<0.05). ACE and AGTR1 protein expression was greater in proliferating IH tissue compared with that in controls and in involuting and treated IH tissue (P<0.05). ELISA showed no significant difference in ACE serum levels but did show a significant reduction in renin in involuting compared with proliferating IH (P<0.05).ConclusionsThe protein and mRNA expression of several RAA pathway constituents is elevated in IH tissue when compared with that in normal tissue. The action of propranolol on IH may be the result of reductions in ACE and AGTR1.

Solitary Pulmonary Capillary Hemangioma of Adult Cases: Clinicopathologic Characteristics as an Unrecognized Entity.

Solitary pulmonary capillary hemangioma (SPCH) is a rare disease, first described about autopsy cases in 2000 and about surgically resected cases in 2006. To date, only 9 surgically resected cases have been published in English. Here, we report 7 original cases with surgery (median age, 54 y; 4 females, 2 never-smokers). All patients were asymptomatic, and all nodules were detected by computed tomography (CT). The median (range) size of nodule was 11 (8 to 16) mm. Six of 7 cases showed the part-solid nodule appearance and 1 showed pure ground-glass nodule appearance in CT findings. The growth speed was very slow. No abnormal uptake of fluorine-18 fluorodeoxyglucose was observed in systemic positron-emission CT in all 3 cases we examined. No patients died from SPCH. Histologically, SPCH manifested as a solitary lesion composed of densely proliferating and dilated capillaries without cytologic atypia within the alveolar septa. In addition, capillaries of SPCH spread into the vascular lumen and involved the walls of bronchioles with protrusion into the lumen. Immunohistochemically, capillaries of SPCH uniformly expressed endothelial markers, such as CD31, CD34, and Factor VIII; and α-smooth muscle actin positive cells were also observed. To be accurately diagnosed, especially in intraoperative frozen sections, SPCH should be conceived as an entity that presents as a solitary nodule in CT. We propose that SPCH is an unrecognized benign capillary proliferative disease.

Pulmonary Veno-Occlusive Disease: A Newly Recognized Cause of Severe Pulmonary Hypertension in Dogs.

Pulmonary hypertension is a well-known though poorly characterized disease in veterinary medicine. In humans, pulmonary veno-occlusive disease (PVOD) is a rare cause of severe pulmonary hypertension with a mean survival time of 2 years without lung transplantation. Eleven adult dogs (5 males, 6 females; median age 10.5 years, representing various breeds) were examined following the development of severe respiratory signs. Lungs of affected animals were evaluated morphologically and with immunohistochemistry for alpha smooth muscle actin, desmin, CD31, CD3, CD20, and CD204. All dogs had pulmonary lesions consistent with PVOD, consisting of occlusive remodeling of small- to medium-sized pulmonary veins, foci of pulmonary capillary hemangiomatosis (PCH), and accumulation of hemosiderophages; 6 of 11 dogs had substantial pulmonary arterial medial and intimal thickening. Ultrastructural examination and immunohistochemistry showed that smooth muscle cells contributed to the venous occlusion. Increased expression of CD31 was evident in regions of PCH indicating increased numbers of endothelial cells in these foci. Spindle cells strongly expressing alpha smooth muscle actin and desmin co-localized with foci of PCH; similar cells were present but less intensely labeled elsewhere in non-PCH alveoli. B cells and macrophages, detected by immunohistochemistry, were not co-localized with the venous lesions of canine PVOD; small numbers of CD3-positive T cells were occasionally in and around the wall of remodeled veins. These findings indicate a condition in dogs with clinically severe respiratory disease and pathologic features resembling human PVOD, including foci of pulmonary venous remodeling and PCH.

The embryo-placental CD15-positive "vasculogenic zones" as a source of propranolol-sensitive pediatric vascular tumors.

OBJECTIVE: Propranolol-induced involution is a unique biological feature of some pediatric vascular tumors, for instance infantile hemangioma (IH), cerebral cavernoma or chorioangioma. Currently, the cellular origin of these distinct tumors is unclear. In this study, we tested the hypothesis that propranolol-responsive vascular tumors are derived from common vessel-forming CD15 + progenitor cells which occur in early gestation. The aim of this study was to identify the tumor-relevant CD15 + progenitors at the early stages of embryo-placental development. MATERIALS AND METHODS: Human embryo-placental units of 4-8 weeks gestation and pediatric vascular tumors were tested for expression of the tumor-relevant markers CD15, CD31 and CD34. RESULTS: Placental vessel-forming progenitors were characterized by immunostaining for CD15, CD31, and CD34. In embryonic tissue, a discontinuous CD15+/CD31+/CD34 + progenitors was detected in immature vessels of the skin, neural tube, spinal and cerebral meninges. Similarly, vessels in IH and chorioangioma exhibited a co-expression of CD15, CD31, and CD34. In contrast, the majority of embryonic vessels presented a CD31+/CD34+, but CD15-negative immunophenotypic pattern. DISCUSSION: Our results suggest the existence of a CD15+ "vasculogenic zones" in the embryo-placental unit as well as in IH and chorioangioma. A site-specific correlation between normal embryo-placental and tumoral vessel-forming CD15 + progenitors was demonstrated. CONCLUSION: Hence, site- and stage-specific CD15 + progenitors of vascular wall could be considered as propronalol-sensitive targets and source of pre- and postnatal vascular tumors. We propose, that the CD15+ "vasculogenic zones" are a site-specific reserve of multi-lineage progenitors that could be recruited in pre- and postnatal emergency situations.

[Expression and significance of the proteins in TSP-1 and NF-κB signal pathways of infantile capillary hemangioma].

Objective: To explore the significance of the relative proteins in thrombin sensitive protein(TSP-1) and nuclear factor kappa B (NF-κB) signal pathways by detecting their expressions in different phases of hemangioma and the relations between them and microvessel density(MVD). Methods: Forty cases with hemangioma were randomly selected and divided into proliferative group(n =21) and involutional group(n =19) according to Mulliken standard.The expressions of TSP-1,CD36,p59fyn,Caspase-3,p38MAPK,NF-κBp65,p-IκBα,p-IKKß,VEGF and CD34 in endothelial cells were detected by immunohistochemistry.The relevance between the proteins in TSP-1 and NF-κB signal pathways and MVD were compared and analyzed respectively. Results: The expressions of TSP-1,CD36,p59fyn,Caspase-3,p38MAPK in proliferative hemangioma were all lower than those in involutional hemangioma (F=32.582,47.575,11.645,18.824,13.140;P =0.000,0.000,0.002,0.000,0.001).Conversely, the expressions of NF-κBp65,p-IκBα,p-IKKß,VEGF in proliferative hemangioma were higher than those in involutional hemangioma(F =7.807,6.323,5.988,4.216;P =0.009,0.018,0.021,0.049).Meanwhile,MVD were different between the two groups(F =7.256,P =0.012).The expressions of TSP-1, CD36, p59fyn and MVD showed a negative relationship(rs =-0.420,-0.519,-0.388;P =0.021,0.003,0.034). Conversely, the expressions of NF-κBp65,pvIκBα,VEGF and MVD showed a positive relationship(rs =0.666,0.611,0.515;P =0.000,0.000,0.004).The expression of TSP-1 and NF-κBp65 showed a negative relationship (rs =-0.413,P =0.023). Conclusions: TSP-1 signal pathway and NF-κBp65 signal pathway may be interactive in evolution of infantile hemangioma, which has a potential role by effecting the progress of angiogenesis.

CD105 expression in oral capillary hemangiomas and cavernous hemangiomas.

Capillary hemangioma (capillary lobular hemangioma) and cavernous hemangioma (venous malformation) are relatively common oral tumors/malformations and are characterized by increased numbers of normal and abnormal blood vessels. However, the causes of these lesions are not well understood. CD105 (endoglin) is predominantly expressed in proliferating blood endothelial cells (ECs). We analyzed expressions of CD105, CD34, von Willebrand factor, Ki-67, cyclooxygenase-2 (COX-2), and vascular endothelial growth factor (VEGF)-A in 31 capillary hemangiomas and 34 cavernous hemangiomas. Staining scores were calculated as the product of the proportion score and intensity score. Morphologically normal oral mucosa specimens (n = 10) were simultaneously evaluated as normal controls. As compared with cavernous hemangiomas and normal controls, capillary hemangiomas had higher staining scores for CD105, VEGF-A, and COX-2. The Ki-67 labeling index was significantly higher in capillary hemangiomas than in cavernous hemangiomas and normal controls (P < 0.01). These findings suggest that the biological characteristics of capillary and cavernous hemangiomas are quite different. The ECs of capillary hemangiomas actively proliferated and were generally regulated by VEGF-A. In contrast, the ECs of cavernous hemangiomas lacked proliferative activity. These results suggest that angiogenesis and vasodilatation of pre-existing blood vessels are important in the development of capillary hemangioma and cavernous hemangioma, respectively.

Effects of dosing protocol on distribution of propranolol in periocular tissues after topical ocular instillation.

PURPOSE: Our previous study demonstrated that topical ocular instillation can deliver effective concentrations of propranolol in the periocular tissues, and may be superior to oral propranolol in the treatment of periocular capillary hemangiomas. The objective of this study was to investigate the effects of dosing protocol on the distribution of propranolol in the periocular tissues and plasma after topical ocular instillation. METHODS: Each rabbit received propranolol 0.5% ophthalmic solution using one of the following dosing protocols: three drops of 50 µL, one drop of 50 µL, or one drop of 25 µL. The periocular tissues (e.g. eyelids and extraocular muscles) and blood were collected and assayed for propranolol at 1 h after dosing. RESULTS: Decreasing the concentration of eye drops (1% to 0.5%), decreasing the number of eye drops during dosing (three drops to one drop), or decreasing the instilled volume (50 µL to 25 µL) generally lowered the concentration of propranolol in the periocular tissues. Nevertheless, therapeutic levels of propranolol (> 0.4 µg/g) were delivered to the periocular tissues at 1 h after dosing using any of the three protocols examined. Dose-dependent concentrations in some periocular tissues and plasma were observed over the dose range of 0.125 mg to 1.5 mg. The plasma concentration of propranolol was not measurable when a single 25 µL of propranolol 0.5% was instilled in the eye. CONCLUSIONS: The dosing protocol of topical ocular instillation can be tailored to achieve the desired therapeutic concentrations of propranolol in the periocular tissues while minimizing systemic exposure.

Educational paper: Pathogenesis of infantile haemangioma, an update 2014 (part I).

UNLABELLED: Infantile haemangioma (IH) is the most frequent childhood tumour. Although it is benign and self-limiting, severe complications can arise due to localisation and fast tumour growth. Management and therapy of IH has changed greatly after 2008 with propranolol. However, the pathogenesis remains elusive. This update provides an overview of all possible mechanisms currently considered. We discuss the possibility that several mechanisms act together, although local hypoxia seems to be important. Clinically, in about half of the cases, an IH is preceded by an anaemic macula (local ischaemia) or a so-called precursor lesion. Laboratory findings indicate stabilisation and an increased transcription activity of hypoxia-inducible factor 1 alpha (HIF1α), leading to up-regulation of its downstream target genes (such as vascular endothelial growth factor (VEGF)), which normally occurs in cases of hypoxia. CONCLUSION: Three main hypotheses have been proposed, namely (1) the theory of tissue hypoxia, (2) the theory of embolization of placental endothelial cells and (3) the theory of increased angiogenic and vasculogenic activity.