Intravascular papillary endothelial hyperplasia in the skin and subcutaneous tissue

Elshinawy W, Mohamed Ahmed E, A. Raafat M, Husien Abdo W, A. Abdel Wahab M, E. Elimam S, Alsagheer Alhewy M. Masson's Tumor as an Uncommon Cause of Neck Mass: A Case Presentation

BACKGROUND

Masson's tumor, commonly referred to as intravascular papillary endothelial hyperplasia (IPEH), is an uncommon growth of endothelial cells within a vessel wall that is frequently assumed to indicate an abnormal resolution of thrombosis. IPEH is most typically found in the extremities however it is rare for IPEH to appear as a neck tumor. The issue with IPEH is that it could clinically, radiologically, and pathologically imitate some malignant neoplasms such as angiosarcomas creating a diagnostic challenge.

CASE REPORT

We describe a 21-year-old male patient who presented with right anterolateral neck swelling for 12 months. Ultrasound revealed a 9.0 × 8.0 cm well-defined echogenic hyper-vascular lesion. The contrast computed tomography (CT) scan of the neck revealed an oval, well-defined subcutaneous mass, measuring 9 × 4.5 cm, situated over and separable from the right sternocleidomastoid muscle with no significant enhancement in the post-contract study. T1-weighted and T2-weighted MRI revealed a 10 × 9 × 7 cm well-defined subcutaneous lobulated lesion superficial to the sternocleidomastoid expanding upward to the Rt. side of the cheek and below to the suprasternal region, eliciting an intermediate signal in T1 and a heterogenous bright signal (mostly fluid) in T2 with low signal foci within the mass. The decision had been reached to entirely excise the lesion surgically with safety margins for histological evaluation. Histological examination indicated thrombosed variable-sized ectatic vascular spaces with papillary formations related to the thrombus, covered with a single layer of flat endothelium, and no features suggestive of malignancy. There was no recurrence at 18 months follow-up post-surgery.

CONCLUSION

Masson's tumor is a benign intravascular disease with an unclear origin and no confirmed inheritance pattern. Presentation of Masson's tumor as a neck mass is incredibly uncommon. Masson's tumor lacks a distinct or distinguishing clinical and radiological appearance. Histopathologic examination is the sole definitive way for diagnosing the disease and the only tool for distinguishing it from angiosarcoma. Surgical excision is the best treatment for IPEH. Recurrence is extremely rare.

Extravascular papillary endothelial hyperplasia mimicking soft tissue sarcoma in 2 cats: a potential diagnostic pitfall

Papillary endothelial hyperplasia (PEH) is a rare soft tissue lesion arising from excessive reactive endothelial cell proliferation described in humans, dogs, and horses. PEH is considered a diagnostic challenge in humans, in which it is frequently misdiagnosed as angiosarcoma. We describe here PEH that developed at injection sites in 2 cats that were initially misdiagnosed as feline injection-site sarcoma by cytology and as subcutaneous angiosarcoma by histopathology. Morphologic features included sharp demarcation from surrounding tissues, and a layered microscopic architecture with an outer fibrous capsule from which emerged fibrovascular stalks covered by a monolayer of factor VIII-related antigen and CD31-positive flat-to-plump endothelial cells. Both lesions had a cystic core containing abundant erythrocytes and fibrin. PEH lesions did not recur in either case. Immunohistochemistry for α-smooth muscle actin and desmin demonstrated that the capsule was devoid of smooth muscle cells, excluding an intravascular origin. PEH in these cats was hypothesized to have developed extravascularly following trauma related to injection. We wish to provide awareness of PEH in domestic cats and of the risk of misdiagnoses leading to overtreatment.

Intravascular papillary endothelial hyperplasia of the cavernous sinus-A rare occurrence

Intravascular papillary endothelial hyperplasia (IPEH) is a benign, reactive proliferation of endothelial cells within areas of thrombus. It most commonly occurs in the skin and subcutaneous tissue, with less than 40 cases reported in intracranial location. Ours is the first case report from India of IPEH in the cavernous sinus. A 45-year-old male presented with headache and decreased visual acuity. Magnetic resonance imaging (MRI) revealed a well-defined, homogeneously contrast-enhancing mass in the cavernous sinus. Postoperatively, examination of tissue showed organizing thrombus within the vessel wall along with delicate, acellular, pseudopapillary cores. The fibrin-rich connective tissue of these showed blue staining with Masson's trichrome stain. These were lined by a single layer of endothelial cells without evidence of atypia; no nuclear pleomorphism, hyperchromasia, atypical mitosis, or areas of necrosis were seen. Hence, it was diagnosed to be IPEH. Follow-up MRI revealed no residual disease and the patient is disease-free at 8 months. Recognition of this entity by pathologic examination is important to rule out angiosarcoma. Gross total resection is curative. Residual lesions have the potential for recurrence, requiring adjuvant therapy.

Vulvar intravascular papillary endothelial hyperplasia or Masson's tumor: A case report

Masson's tumor, also known as intravascular papillary endothelial hyperplasia (IPEH), is a rare and benign vascular disease in which there is a reactive hyperplasia of intravascular endothelial cells. This tumor is most commonly found in soft tissues in the head, neck and upper extremities. We report a rare case of IPEH on the vulva. A Hispanic woman in her mid-thirties presented with a painful and pruritic left vulvar mass. On physical examination, a pedunculated mass was visualized on the left labia majora. Under pathologic examination, it was concluded the lesion was IPEH and it was surgically excised. This is a rare case of IPEH located on the vulva. However, despite this rarity, a simple local excision could be used to manage IPEH.

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Masson's tumor, also known as intravascular papillary endothelial hyperplasia (IPEH), is a rare and benign vascular disease.

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IPEH is most commonly found in soft tissues in the head, neck and upper extremities.

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The tumor is best treated with complete surgical resection.

Masson's Tumor of the Hand: An Uncommon Histopathological Entity

Masson's tumor or Masson's hemangioma, more precisely termed intravascular papillary endothelial hyperplasia (IPEH), is an uncommon benign vascular lesion of the skin and subcutaneous tissues which can be frequently confused with angiosarcoma. Although relatively rare, its accurate diagnosis is essential since it can clinically be similar to both benign and malignant lesions. We present a 39-year-old man with a round bulging arising from the left palm side of the hand with gradual growth in the last 5 months and on and off tenderness. The microscopic section demonstrated the papillary proliferation of endothelial cells in favor of Masson's hemangioma, which was sufficiently treated with excision alone.

Intravascular Papillary Endothelial Hyperplasia in the Coronary Artery: An Unusual Cause of Massive Myocardial Infarction in Hypoplastic Left Heart Syndrome

Background: Intravascular papillary endothelial hyperplasia (IPEH) is a benign vascular lesion that usually involves the head and neck or extremities. Involvement of the coronary arterial system is unreported. Case: A 1-month-old patient born with hypoplastic left heart syndrome died from a massive myocardial infarction shortly after first stage palliation with Norwood/Sano. Autopsy demonstrated a massive univentricular hemorrhagic infarction with complete occlusion of the left main coronary artery and its intramural branches by intraluminal papillary endothelial hyperplasia (IPEH). Immunostaining with CD34 and CD31 confirmed the diagnosis. The inferior and superior mesenteric artery branches also had IPEH. Conclusion: IPEH can involve the coronary vasculature, can be multifocal and can occur in the newborn.

Sinusoidal hemangioma and intravascular papillary endothelial hyperplasia: Interrelated processes that share a histogenetic piecemeal angiogenic mechanism

Sinusoidal hemangioma, characterized by interconnecting thin-walled vascular spaces, may present papillae/pseudo-papillae and zones that resemble intravascular papillary endothelial hyperplasia (IPEH). Our objectives are to explore the existence of zones in IPEH with sinusoidal hemangioma characteristics, the mechanism of papillary and septa formation in sinusoidal hemangioma and the comparison of this mechanism with that in IPEH. For these purposes, specimens of 4 cases of each entity were selected and studied by serial histologic sections and by immunochemistry and immunofluorescence procedures. The results showed a) zones with characteristics of sinusoidal hemangioma in IPEH cases, b) presence in both entities of papillae with a cover formed by a monolayer of CD34+ and CD31+ endothelial cells (ECs) and a core formed by either type I collagen and αSMA+ cells (presenting a pericyte/smooth muscle cell aspect) or thrombotic components, and c) a similar piecemeal angiogenic mechanism in papillary formation, including sprouting of intimal ECs toward the vessel wall itself or intravascular thrombi, formation of vascular loops that encircle and separate vessel wall or thrombus components, and parietal or thrombotic papillae development. The major differences between both entities were the number, arrangement and substrate of papillae: myriad, densely grouped, parietal and thrombotic papillae in IPEH, and a linear arrangement of predominant parietal papillae in sinusoidal hemangioma, originating septa (segmentation). In conclusion, sinusoidal hemangioma and IPEH are interrelated processes, which share morphologic findings and a piecemeal angiogenic mechanism, combining sprouting and intussusceptive angiogenesis, and leading to papillary formation and vessel segmentation.

Oral Intravascular Papillary Endothelial Hyperplasia Associated with an Organizing Thrombus: Case Report and Immunohistochemical Analysis

Intravascular papillary endothelial hyperplasia (IPEH) is a benign lesion of the skin and mucosa of vascular origin characterized by reactive proliferation of endothelial cells. A 76-year-old woman was referred presenting a painless nodule on the lip. Intraoral examination revealed bluish submucosal nodular proliferation, measuring 10 × 5 × 5 mm, affecting the lower labial mucosa. The lesion had a firm consistency and it was not fixed to the adjacent tissues. The main differential diagnoses were mucocele/mucus retention cyst, sialolith, or salivary gland neoplasia. An incisional biopsy was performed and during the intraoperative procedure an encapsulated red-bluish nodular mass was observed. Microscopic analysis revealed papillary endothelial proliferation in the center of the lesion and fibrin admixed with inflammatory cells in organization peripherally. There was no nuclear atypia, mitotic figures, or necrosis. The endothelial cells were CD34 positive, with low Ki-67 proliferation index (4%). α-SMA highlighted the vessel walls, whereas negativity for D2-40 excluded lymphatic origin. Final diagnosis was IPEH associated with an organizing thrombus. Dentists should be aware about this rare benign vascular lesion, whose final diagnosis is achieved only after histopathology analysis. Surgical removal is the treatment of choice and no recurrence is expected.

Contributions of Dr. Juan Rosai to the pathology of cutaneous vascular proliferations: A review of selected lesions

Vascular proliferations in the skin have been thoroughly studied over the past 35 years, and a great deal of knowledge has been accrued regarding their pathobiological features. Dr. Juan Rosai has been a consistent contributor to the literature on this topic throughout most of his career, and this article reviews selected cutaneous endothelial lesions that he helped to characterize. They include histiocytoid-epithelioid hemangioma, targetoid-hobnail hemangioma, acquired tufted hemangioma, glomeruloid hemangioma, spindle cell hemangioma, retiform hemangioendothelioma, and angiosarcoma. Two potential simulants of angiosarcoma-papillary intravascular endothelial hyperplasia and cutaneous rudimentary meningocele-meningothelial hamartoma-are also considered.

Intravascular papillary endothelial hyperplasia: An unusual histopathological entity

Intravascular papillary endothelial hyperplasia (Masson's tumor) is a benign lesion of the skin and subcutaneous tissue consisting of a reactive proliferation of endothelial cells with papillary formations related to a thrombus. It poses a diagnostic challenge as the clinical signs and symptoms are nonspecific and may mimic a soft tissue sarcoma. The diagnosis is based on histopathology. Here we report two cases of Masson's hemangioma occurring on the upper lip and on the left hand.

Orbital intravascular papillary endothelial hyperplasia in a Nigerian child: a case report and review of the literature

INTRODUCTION

Intravascular papillary endothelial hyperplasia is a reactive proliferative lesion of endothelial cells in blood vessels. It typically presents as a painless, reddish purple lesion in the sites affected. The orbit remains an uncommon site of affectation of this relatively common disease. It is noteworthy that this is the first reported case, to the best of our knowledge, of orbital intravascular papillary endothelial hyperplasia in a Nigerian child.

CASE PRESENTATION

The case reported here is an orbital intravascular papillary endothelial hyperplasia causing non-axial proptosis and loss of vision in a 14-year-old Nigerian boy. We describe the clinical and histological findings of intravascular papillary endothelial hyperplasia in the orbit of this 14-year-old boy. The key distinguishing features are discussed and relevant literature is reviewed.

CONCLUSION

Although unusual in presentation, intravascular papillary endothelial proliferation should be considered in the list of differentials of proptosis due to mass lesion in young Nigerians and, possibly, Africans.

Intravascular papillary endothelial hyperplasia of the calf in an infant: MR features with histological correlation

Intravascular papillary endothelial hyperplasia (IPEH), also known as Masson tumour, is a rare nonneoplastic vascular lesion caused by the abnormal proliferation of endothelial cells. IPEH presenting as a soft-tissue mass may simulate a sarcomatous tumour clinically and radiographically. We report an infant with IPEH affecting the right calf. The MRI appearance is described, and the literature is briefly reviewed. Knowledge of the MR characteristics of IPEH may facilitate its diagnosis and obviate an invasive procedure.

Intravascular papillary endothelial hyperplasia: report of 2 cases and immunohistochemical study

Intravascular papillary endothelial hyperplasia (IPEH) is a benign, nonneoplastic, vascular lesion. The main significance of the lesion lies in the fact that it may be mistaken for angiosarcoma. Oral lesions are uncommon and the present paper reports 2 cases of oral IPEH, in different sites. Histologically, the tissue was characterized by papillary fronds lined by proliferating endothelium. Immunohistochemically (IHC), the lesion was positive for CD34, smooth muscle actin (SMA), type I and IV collagen, vimentin, and laminin, but it was negative for CD105. Local excision was the treatment of choice. No recurrence was observed during a 1-year and 6-month follow-up period, respectively. The clinical, histological, and immunohistochemical characteristics are discussed.

Papillary Endothelial Hyperplasia of the Adrenal Gland: Report of a Case and Review of the Literature

Papillary endothelial hyperplasia (PEH) is a benign vascular proliferative process most frequently seen in the skin and integument, but may involve any of the visceral organs. It is a rare entity, with less than 30 cases of visceral PEH described in the literature. Adrenal papillary endothelial hyperplasia is an exceedingly rare process and is the basis of this review. A 66-year-old female was referred for evaluation of an asymptomatic 6 cm right adrenal mass. Computed tomography indicated that the lesion was solid and hypervascular. After appropriate workup, the patient underwent laparoscopic adrenalectomy. The pathologic analysis was consistent with adrenal PEH. The patient recovered without incident and is doing well at 1 year follow-up. A review of the world's literature on papillary endothelial hyperplasia (PEH), and in particular adrenal PEH, yields five previous reports of this entity, and no comprehensive review. A compilation of the now six patients with adrenal PEH reveals several common features: five of six patients were female and mean age was 64 years. The disease radiologically mimics adrenal cortical carcinoma mandating a surgical oncological technique. Pathologic differentiation from angiosarcoma can be a difficult task requiring evaluation by an experienced pathologist.

Extravascular papillary endothelial hyperplasia of the palm masquerading as an angiosarcoma

Cutaneous extravascular papillary endothelial hyperplasia (PEH) is a rare lesion presenting as a cutaneous mass and histologically mimicking angiosarcoma. Herein, we describe a patient who presented with a palm tumor that had developed after trauma. Histologically, the lesion was ulcerated and showed proliferation of atypical epithelioid cells with nuclear enlargement and mitosis. Vascular proliferation was evident, and these were positive for CD31 and CD34, with approximately 20% positivity for MIB-1. We diagnosed this case as an extravascular papillary endothelial hyperplasia because there was no evidence of invasion into the surrounding tissue and elastica staining proved it did not have a vascular structure. To our knowledge, cutaneous extravascular PEH has not yet been reported in the published work.

Papillary endothelial hyperplasia and dilated lymphatic vessels in bullous pilomatricoma

This is a report of papillary endothelial hyperplasia in a 9-year-old girl with a pilomatricoma showing bullous appearance. Histologically, papillary proliferation of endothelial cells was found within dilated lymphatic endothelium-lined vascular channels overlying a pilomatricoma. The endothelial cells covering the papillae were of a lymphatic endothelial cell nature proved by immunohistochemistry and electron microscopy. Abundant fibrous long-spacing collagen was observed in the connective tissue and fibroblasts within papillae.

Intravascular papillary endothelial hyperplasia (Masson's hemangioma) of the liver: a new hepatic lesion

Intravascular papillary endothelial hyperplasia (Masson's hemangioma) is a disease characterized by exuberant endothelial proliferation within the lumen of medium-sized veins. In 1923, Masson regarded this disease as a neoplasm inducing endothelial proliferation, however, now it is considered to be a reactive vascular proliferation following traumatic vascular stasis. The lesion has a propensity to occur in the head, neck, fingers, and trunk. Occurrence within the abdominal cavity is known to be very rare, and especially in the liver, there has been no reported case up to date. The authors have experienced intravascular papillary endothelial hyperplasia of the liver in a 69-yr-old woman, and report the case with a review of the literature.

Intravascular Papillary Endothelial Hyperplasia of the Central Nervous System-Four Case Reports-

Four rare cases of intracranial intravascular papillary endothelial hyperplasia (IPEH) manifesting as cranial nerve disturbances occurred in 16-, 18-, 24-, and 28-year-old females. Magnetic resonance imaging showed all lesions as isointense with strong enhancement on T1-weighted images, and as hyperintense on T2-weighted images. All lesions were removed via craniotomies. Histological examination found vascular structures and papillary spaces lined with endothelial cells showing immunoreactivity for CD31. Complete removal was curative in two cases, whereas incomplete removal resulted in cure in one case and residual deficits in one case. Iatrogenic deficits should be avoided in IPEH treatment by surgery. Differentiation from neoplasm such as angiosarcoma depends on histological characteristics.

Intravascular papillary endothelial hyperplasia arising from the upper lip

We report a rare case of intravascular papillary endothelial hyperplasia (IPEH) arising from the upper lip. Pathologically, it consisted of a few lobulated masses lined by an incomplete fibrous capsule of variable thickness which was separated from the surrounding tissue and which partially formed papillary structures bearing fibrous stalks and a single layer of endothelium. The capillary formation was poorly defined, and mitotic figures were frequently observed. Immunohistochemically, the endothelial cells were positive for factor VIII related antigen and vimentin, and many cells were positive for PCNA, not only in the solid proliferating area but also in the papillary proliferating area. This case represents IPEH with high proliferative activity.

Recurrent intravascular papillary endothelial hyperplasia developing from a pyogenic granuloma

Intravascular papillary endothelial hyperplasia (IPEH) is a rare benign vascular tumour. Many histological features are similar to those of low-grade angiosarcoma, a common, but more serious condition. Clinical and histological differentiation is important to avoid overtreatment of this benign condition. We report on a 34-year-old woman who developed recurrent IPEH at the site of a previously excised pyogenic granuloma.

Cutaneous vascular proliferation. Part II. Hyperplasias and benign neoplasms

This second part of our review about vascular proliferations summarizes the clinicopathologic features of the cutaneous vascular hyperplasias and benign neoplasms. Hyperplasias comprise a heterogeneous group of vascular proliferations that eventually show a tendency to regression. Angiolymphoid hyperplasia with eosinophilia is included within the group of hyperplasias because of its historical denomination and its reactive nature, probably as a consequence of an arteriovenous shunt, although usually the lesions do not regress. Pyogenic granuloma, bacillary angiomatosis, intravascular papillary endothelial hyperplasia, and pseudo-Kaposi's sarcoma qualify as vascular hyperplasias because they regress when the stimulus that initiated them is removed. Benign neoplasms form a large group of hemangiomas with distinctive clinicopathologic characteristics, although some of them are of recent description and may produce diagnostic difficulties. We classified cutaneous benign vascular neoplasms according to their cell lineage of differentiation, for example, endothelial, glomus cell, and pericytic differentiation. Subsequent categories are established according to the size of the involved vessels (capillaries, venules and arterioles, or veins and arteries) or the nature of the proliferating vessels (blood or lymphatic vessels). Capillary and cavernous hemangiomas have been the terms classically used to name the most common variants of benign vascular neoplasms (i.e., infantile hemangiomas), but they are not the most appropriate denominations for these lesions. First, these names are not contrasting terms. Furthermore, most of the socalled "cavernous" hemangiomas are not hemangiomas (neoplasms) at all, but venous malformations. The most important conceptual issue is that, at any point in time, a particular hemangioma has its own histopathologic pattern throughout the depth of the lesion. For these reasons, we classified hemangiomas into superficial and deep categories. Some of the lesions reviewed have been recently described in the literature, and they may histopathologically mimic lesions of Kaposi's sarcoma; these include targetoid hemosiderotic hemangioma, microvenular hemangioma, tufted hemangioma, glomeruloid hemangioma, kaposiform hemangioendothelioma, spindle-cell hemangioendothelioma, and benign lymphangioendothelioma. In each of these lesions, we update and emphasize those clinical and histopathologic features that are helpful for differential diagnosis with lesions of authentic Kaposi's sarcoma in any of its three stages of development (patch, plaque, or nodule).

Intravascular papillary endothelial hyperplasia of the oral soft tissues: report of 18 cases and review of the literature

PURPOSE

Intravascular papillary endothelial hyperplasia (IPEH) is a benign, nonneoplastic, vascular lesion, that is characterized histologically by papillary fronds lined by proliferating endothelium: it is thought to represent an unusual form of organizing thrombus. It may develop in a distended vessel or in association with other vascular lesions. The main significance of IPEH is its microscopic resemblance to angiosarcoma and possible misinterpretation as such.

METHODS

We report the clinicopathologic features of 18 examples of IPEH of the oral soft tissues and lips, and review such features of 55 previously reported oral cases.

RESULTS

The lips and tongue were most frequently involved and a female predilection was evident. Histologically, the pure form of IPEH was more commonly encountered.

CONCLUSION

The differential diagnosis and possible histogenesis of the lesion are discussed.

Immunohistochemistry of intravascular papillary endothelial hyperplasia

Intravascular papillary endothelial hyperplasia is an interesting endothelial proliferation, the nature of which has aroused some controversy. Five cases were studied by light microscopy and by immunohistochemistry using antibodies to Factor VIII-related antigen (FVIII-rAg), ferritin, alpha 1-antitrypsin, alpha 1-antichymotrypsin and vimentin and were compared with conventional intravascular organizing thrombi. The results show a similar progression of the immunophenotype of the endothelial cells in both entities: they are initially positive for ferritin, then acquire vimentin positivity and only display FVIII-rAg positivity in advanced ("mature") lesions. This suggests that intravascular endothelial hyperplasia is closely related to organizing thrombi and is probably a peculiar form thereof.

Intraoral Masson's vegetant intravascular hemangioendothelioma

Two cases of Masson's vegetant intravascular hemangioendothelioma are described. The intraoral appearance of these reported lesions are rare. Due to histologic similarities to angiosarcoma and the benign progression of vegetant intravascular hemangioendothelioma, a thorough discussion of these diseases is presented.

Intravascular papillary endothelial hyperplasia in the hand

Intravascular papillary endothelial hyperplasia is a rare benign tumor of vascular origin. The hand is one of its common locations. The case reported showed bony erosion that has not been described in the hand.

Monoclonal antibodies to blood group isoantigens: an alternative marker to factor VIII related antigen for benign and malignant vascular endothelial cells

Monoclonal antibodies to A, B and H blood group isoantigens (BGI) were used in an indirect immunoperoxidase technique to compare the distribution of BGI expression as a marker of vascular endothelial cells with that of coagulation factor VIII related antigen (F VIII-RAg), a well established marker for vascular endothelium. Formalin-fixed and paraffin-embedded material from 70 specimens representing a wide range of reactive and benign or malignant neoplastic states of the vascular endothelium was used to provide adjacent serial tissue sections to compare directly the tissue expression of BGI and of F VIII-RAg. In reactive and benign neoplastic conditions of the vascular endothelium, cell membrane expression of the BGI appropriate to the patient's blood group was readily detected. In the more cellular and proliferative areas of some haemangiomas, pyogenic granulomas and cases of angiolymphoid hyperplasia with eosinophilia, BGI proved a superior marker of the endothelial cells compared with F VIII-RAg. The malignant endothelial cells of 27 specimens of angiosarcoma showed deletion of BGI expression in 24 and of F VIII-RAg in 19. Expression of neither antigen showed any significant correlation with the degree of tumour differentiation. Some cases of lymphangioma, uniformly negative for F VIII-RAg staining, showed strong expression of BGI. The possible use of BGI expression as a marker of vascular endothelium is discussed in the light of the known tissue expression of F VIII-RAg.

Central intravascular papillary endothelial hyperplasia of the mandible

Light and electron microscopic findings in papillary endothelial hyperplasia of the mandible in a 49-year-old female are reported. The endothelial cell-lined papillary projection into a cystic lumen was examined by light microscopy and characteristic features of the endothelial cells were found by electron microscopy. Factor VIII-related antigen was demonstrated in the endothelial cells by the immunoperoxidase technique.