Multifocal vascular tumors in fowl induced by a newly isolated retrovirus

The morphological features of a spontaneous, multifocal vascular neoplasm of chickens are described. Histologically, the tumor was characterized by areas consisting of freely anastomosing vascular channels with prominent papillary appearance and lined by bland-looking endothelial cells, which alternate with areas resembling cavernous hemangioma. Occasionally solid areas composed of plump, pleomorphic cells were also present. Although there was no clear evidence for metastatic spread, some tumors were obviously invasive. Electron microscopy and immunohistochemistry confirmed the endothelial nature of neoplastic cells, demonstrating in particular pinocytic vesicles, well developed junctional complexes, fragmented basal lamina, occasional Weibel-Palade bodies, and patchy factor VIII-related antigen immunoreactivity. The overall appearance of the tumor was that of a cavernous hemangioma with prominent papillary endothelial hyperplasia. Previously we have shown that the tumor was induced by a newly isolated strain of avian hemangioma retrovirus and in this study we demonstrated typical type C retrovirus particles in the tumor by electron microscopy. It is suggested that this retrovirus-induced avian tumor may serve as a useful model for the study of transformed endothelia and other vascular tumorigenesis.     

Angiosarcoma of the thyroid. An immunohistochemical and ultrastructural study of a case in a Chinese patient

A case of angiosarcoma (malignant hemangioendothelioma) developing in a chronic goitrous thyroid gland of an elderly Chinese woman is described. Histologically it showed the same classical appearance of angiosarcoma occurring in the skin and soft tissue. The endothelial origin of this tumor was confirmed by demonstrating Factor VIII-related antigen in the neoplastic cells with the immunoperoxidase technique and Weibel-Palade bodies by electron microscopic study. Because of its extreme rarity outside the European Alpine regions, many authorities are reluctant to accept it as a distinct entity and merely consider it as a variant of an undifferentiated carcinoma. Our report not only provides additional evidence that angiosarcoma of the thyroid gland is a specific condition of endothelial origin but also documents the first case among Chinese.     

外周血中循环血管内皮细胞及血管内皮前体细胞的扩增与鉴定

目的证实外周血中循环血管内皮细胞(circulating endothelial cells,CECs)及血管内皮前体细胞(circulating endothelial precursors,CEPs)的存在,以及内皮前体细胞的增殖分化能力.方法应用流式细胞法检测57例肿瘤患者及15例正常人外周血中的CECs/CEPs.选择经造血干细胞动员的肿瘤患者,分离单个核细胞(mononuclear cells,MNCs),在VEGF、IGF及bFGF条件下培养血管内皮细胞,并应用透射电镜法对培养细胞进行鉴定.结果肿瘤患者外周血中CECs/CEPs含量分别为0.378±0.047%、0.059±0.013%,高于正常对照组.培养细胞透射电镜下可见Weibel-Palade小体,为血管内皮细胞的特异性标志.结论外周血中存在CECs/CEPs,在肿瘤患者体内存在血管内皮及其前体细胞的动员,循环血管内皮干/祖细胞可进一步分化成熟.     

肿瘤患者循环血管内皮干/祖细胞的检测及体外诱导分化

目的　证实外周血中循环血管内皮细胞 (CECs)及血管内皮前体细胞 (CEPs)的存在 ,检测内皮前体细胞的增殖分化能力。方法　应用流式细胞术检测 5 7例肿瘤患者及 15例正常人外周血中的CECs和CEPs ,应用ELISA方法检测血清VEGF水平。选择经造血干细胞动员的肿瘤患者 ,分离单个核细胞 (MNCs) ,在VEGF、IGF及bFGF条件下培养血管内皮细胞 ,并应用透射电镜法对培养细胞进行鉴定。结果　肿瘤患者外周血中CECs、CEPs含量分别为 0 .3 78%± 0 .0 47%、0 .0 5 9%± 0 .0 13 % ,高于正常对照组 ,且与血清VEGF水平相关。透射电镜下培养细胞可见Weibel-Palade小体 ,为血管内皮细胞的特异性标志。结论　外周血中存在CECs和CEPs ,在肿瘤患者体内存在血管内皮及其前体细胞的动员 ,循环血管内皮干 /祖细胞可进一步分化成熟     

Expression of the multidrug-resistance P-glycoprotein (Pgp, MDR-1) by endothelial cells of the neovasculature in central nervous system tumors

To elucidate the expression of theMDR1 gene products P-glycoprotein (Pgp) in endothelial cells on newly formed blood microvessels in brain tumors, 30 brain tumors were examined by immunohistochemistry using an anti-Pgp monoclonal antibody, JSB-1. Positive reactions for JSB-1 were detected in endothelial cells in newly formed microvessels in all 16 cases of glioma but not in the 4 meningiomas. Although endothelial cells in newly formed microvessels of all 10 metastatic carcinomas showed positive reactions, negative reactions were seen in those of the primary carcinomas. Compared with reactions of the endothelial cells of normal cerebral capillaries, weak reactions were found in the endothelial cells forming glomeruloid proliferation in newly formed microvessels in the eight glioblastomas and at the border of the surrounding cerebral tissue of the metastatic carcinomas. Since the endothelial cells showing glomeruloid proliferation also had a high proliferative cell nuclear antigen labeling index, the present findings demonstrate a negative relationship between positive reactions for Pgp and the proliferative activities of endothelial cells in cerebral capillaries.     

Cathepsin B immunohistochemical staining in tumor and endothelial cells is a new prognostic factor for survival in patients with brain tumors.

The cysteine endopeptidase, cathepsin (Cat) B, and its endogenous inhibitor, stefin A, were found relevant for cancer progression of many neoplasms, including human brain tumors. Histological sections of 100 primary brain tumors, 27 benign and 73 malignant, were stained immunohistochemically for Cat B and stefin A. The immunohistochemical staining of Cat B in tumor cells, endothelial cells, and macrophages was scored separately from 0-12. The score in tumor and endothelial cells was significantly higher in malignant tumors compared with benign tumors (P<0.000). A significant correlation between immunostaining of Cat B (scored together for tumor and endothelial cells) and clinical parameters, such as duration of symptoms, Karnofsky score, psycho-organic symptoms, and histological score was demonstrated. Univariate survival analysis indicated that total Cat B score above 8 was a significant predictor for shorter overall survival (P = 0.003). In glioblastoma multiforme, intense Cat B staining of endothelial cells was a significant predictor for shorter survival (P = 0.003). Stefin A immunostaining was weak and detected only in a few benign and some malignant tumors, suggesting that this inhibitor alone is not sufficient in balancing proteolytic activity of Cat B. We conclude that specific immunostaining of Cat B in tumor and endothelial cells can be used to predict the risk of death in patients with primary tumors of the central nervous system.     

P-glycoprotein expression in brain tumors

Summary Overexpression of P-glycoprotein (P-gp) in cancer cells can result in resistance to several chemotherapy agents (multidrug resistance) including doxorubicin and vincristine. The drugs to which resistance develops also penetrate the blood brain barrier poorly. P-gp expression in brain capillary endothelial cells suggests that P-gp may restrict drug entry into brain tumors and thus be another mechanism of drug resistance. To seek evidence for either of these roles in the drug resistance of brain tumors, we examined the location of expression of P-gp in 49 brain tumors, using an anti-P-gp mouse monoclonal antibody and immunohistochemistry. P-gp expression was observed in tumor cells of two glioblastomas and a meningeal sarcoma but not in low-grade primary or metastatic tumors. In low-grade primary tumors, P-gp was present in all vascular endothelial cells. In the vascular endothelial cells of anaplastic primary brain tumors and brain metastases, P-gp expression was heterogeneous or absent. These findings are consistent with a role for P-gp in the resistance of some brain tumors to chemotherapy agents.     

Protein delivery of caspase-3 induces cell death in malignant C6 glioma,primary astrocytes and immortalized and primary brain capillary endothelial cells

Most brain tumors consist of transformed glia cells and are highly vascularized by capillary endothelial cells. The aim of the present study therefore was to deliver pro-apoptotic caspase-3 into malignant C6 glioma and immortalized rBCEC4 brain endothelial cells to induce cell death. Both cell lines were transfected with a reporter protein (-galactosidase) using lipid-mediated gene transfer (FuGENE6^TM) or using the novel protein delivery reagent BioPORTER^TM. -Galactosidase protein was successfully delivered into both cells, the protein expression peaked around day 2 and was transient. Delivery of caspase-3 induced TUNEL-positive cell death of both cell types. As a control, caspase-3 was also delivered to non-neoplastic primary astrocytes and endothelial cells and induced cell death. In conclusion BioPORTER^TM-protein delivery of pro-apoptotic molecules may provide a potent tool to cause death of the cells in brain tumors, however, this method is limited due to its toxicity to non-malignant cells.     

Distribution of collagen Type IV in brain tumors: An immunohistochemical study

Summary One hundred-twenty seven human brain tumors were examined by an immunoperoxidase technique for the expression of collagen Type IV, a major constituent of basement membrane. The parenchymal components were negative for the marker protein in all tumors except for neurilemmomas which were positively stained. In every case, the antibody to collagen Type IV showed distinct staining of the vascular pattern. In gliomas, capillaries increased in number and the vascular staining increased in intensity. Fine branching capillaries and endothelial glomeruloid proliferations characteristic each of oligodendrogliomas and glioblastomas could be distinctly illustrated. In two ependymomas, marked capillary proliferation was noted in periventricular areas. Fibrillar staining was observed between the tumor cells in seven of 34 meningiomas. Pericapillary lamellar deposition of collagen Type IV suggests a vascular origin of psammoma bodies. In some malignant tumors, pial-glial membranes were disrupted and the Virchow-Robin spaces were filled with malignant cells. Collagen Type IV was absent around the stromal cells of hemangioblastomas, suggesting that these stromal cells were unrelated histogenetically with endothelial cells. Collage Type IV may be useful in the differential diagnosis between meningiomas and neurilemmonas.     

Immunohistochemical study of tight junction-related protein in neovasculature in astrocytic tumor

To clarify whether the neovasculature of brain tumors preserves blood-brain barrier (BBB) functions, we studied the expression of a tight junction—related protein, Zo-1, using immunohistochemistry. Twenty-six astrocytic tumors were examined using an anti-Zo-1 Mab, and Zo-1 expression was compared with the expression of vasicular endothelial growth factor (VEGF), vascular endothelial growth factor receptor (VEGFR) (flt-1), and antiproliferative cell nuclear antigen (PCNA). A positive reaction for Zo-1 was seen in the endothelial cells in micro-blood vessels in all astrocytic tumors. The reactions for Zo-1 in the endothelial cells forming glomeruloid proliferations in newly formed micro-blood vessels in high-grade tumors were weaker than those in the endothelial cells of normal cerebral capillaries. Although there is a negative correlation between positive immunoreactions for BBB-related proteins and the expression of VEGF of the endothelial cells in micro-blood vessels, the proliferative activity of tumor cells, and histological grades, the present findings suggest that the endothelial cells of the neovasculature of high-grade tumors preserve partial BBB function at the cellular level. Because of the ease of immunohistochemical procedures compared with electron microscopic examination, the immunohistochemical detection of Zo-1 should provide a useful marker for tight junctions.     

Atypical teratoid/rhabdoid tumor of the brain: cytopathologic characteristics and differential diagnosis

BACKGROUND: Atypical teratoid/rhabdoid tumor (AT/RT) is a highly aggressive neoplasm with a unique cytogenetic profile. Although the clinicopathologic and radiologic features of AT/RT have been described previously, to the authors' knowledge the cytomorphologic profile of this tumor has not been studied well. METHODS: Nine samples of AT/RT from 8 patients were analyzed from the pathology files of 2 large institutions in a 10-year period (1993-2002). Material consisted of slides made from scraping and smearing (SS) or squash preparation (SP) of the tissue cores (six slides), fine-needle aspiration (FNA) (two slides), and cerebrospinal fluid (one slide). Smears were stained with Diff-Quik, Papanicolaou, and hematoxylin and eosin stains. RESULTS: There were 4 males and 4 females who ranged in age from 1-16 years (mean age, 7.1 years). Cytomorphologic features consisted of hypercellularity (eight of eight tumors); predominantly large tissue fragments with tumor cells surrounding proliferating capillaries depicting a "papillary-like" appearance (five of eight tumors); large, round, "plasmacytoid" cells and characteristic "rhabdoid" cells (i.e., intermediate-sized cells with granular to fibrillary, brightly eosinophilic cytoplasm with or without globoid "inclusions"; large, eccentrically located, round-to-reniform nuclei with single prominent nucleoli; eight of eight tumors); small, round, primitive "neuronal-appearing" cells with a high nuclear to cytoplasmic ratio (five of eight patients); and bizarre, multinucleated giant cells (two of eight tumors). Also seen were numerous apoptotic bodies, mitoses, and significant necrosis (seven of eight tumors), and prominent dystrophic calcification (four of eight tumors). CONCLUSIONS: AT/RT is extremely rare. Cytologic examination by SS, SP, or FNA offers a useful alternative to frozen section during intraoperative consultation. Cytomorphologic features are unique and lead to an accurate diagnosis in the right clinicoradiologic context. The differential diagnosis includes medulloblastoma (in cerebellar tumors), primitive neuroectodermal tumor (in suprasellar tumors), choroid plexus carcinoma, and malignant glioma.     

Establishment and characterization of a tumorigenic murine vascular endothelial cell line (F-2)

A new murine cell line, designated F-2, was established from an ultraviolet light-induced tumor which developed on the back skin of a BALB/c x C57BL/6 F1-nu/nu nude mouse. More than 1 x 10(5) F-2 cells injected into nude mouse skin produced rapidly developing hemangiomatous lesions. F-2 had a 14-h doubling time under 10% fetal calf serum-containing Dulbecco's modified Eagle's medium culture conditions without any cell growth factor supplementation, and it showed "cobblestone" appearance at confluency. F-2 was able to rapidly differentiate on Matrigel with resultant fine network structure and tubule formation. Ultrastructural observations of the tubule demonstrated that the F-2 cells were connected to each other by intermediate junctions and arranged to form spaces, but that no Weibel-Palade bodies were found in the cytoplasm. F-2 also showed the active uptake of fluorescent-labeled acetylated low-density lipoprotein at 37 degrees C but not at 4 degrees C, and it showed prominent binding to the lectin, Griffonia simplicifolia I agglutinin. The addition of fibroblast growth factor did not facilitate the growth of F-2 cells. These findings strongly suggest that F-2 is a transformed cell line with tumorigenicity and vascular endothelial cell properties, and it may be useful in the study of vascular tumor biology.     

Weibel-Palade bodies as a marker for neovascularization induced by tumor and rheumatoid angiogenesis factors

An analysis was made of ultrastructural changes in capillary endothelial cells in experimentally induced angiogenesis and in a human pathological situation known to involve increased angiogenesis. Chick chorioalloantoic membrane (CAM) showing a positive angiogenic response to low-molecular weight angiogenesis factor isolated from rat Walker sarcoma or from human rheumatoid joint was compared with untreated CAM. Serotonin-treated CAM provided an additional control in that serotonin has the capacity to stimulate endothelial cell growth in vitro but did not induce angiogenesis on the CAM. Human rheumatoid joints were studied using normal healthy human joints as controls. The number of Weibel-Palade (W-P) bodies per unit of cytoplasmic area were higher in tumor angiogenesis factor-treated CAMs (not significant) and rheumatoid angiogenesis factor-treated CAMs (P less than 0.008) than in untreated controls. These differences were more pronounced if W-P body volumetric density was analyzed (P in both cases less than 0.008). Serotonin-treated control CAMs did not show higher numbers of W-P body or greater WPV than untreated controls. Numbers of W-P body and W-P body volumetric density were higher (P less than 0.008) in rheumatoid joints than normal joints. Median values for W-P body number were 16-fold higher and, for W-P body volumetric density, they were up to 30-fold higher in rheumatoid joints.     

Neoplastic angioendotheliosis. Immunohistochemical and electron microscopic findings in three cases

Three cases of neoplastic angioendotheliosis (NAE) presenting with central nervous system (CNS) disease but no skin lesions are described. The histogenesis of the neoplastic cells is discussed. Microscopic examination showed accumulation of neoplastic cells in the vascular system throughout the body and their extravascular proliferation in several organs. Electron microscopic and immunohistochemical studies revealed the presence of Weibel-Palade bodies and factor VIII-related antigen in intravascular and extravascular neoplastic cells in two of the three cases. In the first case the neoplastic cells did not have any T-cell markers. However, in one case no specific markers were found in the neoplastic cells by electron microscopic, enzyme histochemical, or immunohistochemical examination. These findings, although supporting the endothelial origin of the neoplastic cells, indicate the need for further consideration of whether NAE is actually a single disease entity or several different diseases.     

Response of brain specific microenvironment to P-glycoprotein inhibitor: an important factor determining therapeutic effect of P-glycoprotein inhibitor on brain metastatic tumors

P-glycoprotein (P-gp), a factor responsible for the multidrug resistance of tumors, is specifically expressed in brain microenvironment. To test its roles in brain metastatic tumor chemoresistance, we implanted the paclitaxel-sensitive melanoma cell line, K1735, into the skin or brain of mice and examined its paclitaxel resistances. When implanted into the skin, paclitaxel inhibited tumor growth, however, it had no inhibitory effect on cells implanted into the brain. The paclitaxel resistance of the brain K1735 tumors was eliminated by combined treatment with a P-gp inhibitor, HM30181A, and paclitaxel. Previously we found that there is a defined therapeutic window for combined treatment of brain tumors with HM30181A and paclitaxel. To determine whether it is due to responses of the brain microenvironment we measured changes in P-gp expression and function of brain endothelial cells in response to HM30181A treatment in vitro and in vivo. They were significantly increased by high-dose HM30181A treatment and it was related with the therapeutic effect loss of high-dose HM30181A treatment. Therefore, P-gp in the brain microenvironment has crucial roles in the brain metastatic tumor chemoresistance and brain microenvironment responses to P-gp inhibitor treatment should be considered in the development of brain endothelial cell-targeted chemotherapy using P-gp inhibitor.     

Immunocytochemical localization of factor VIII-related antigen in tumors of the human central nervous system

Using both immunohisto- and immunocytochemical techniques with periodate-lysine-paraformaldehyde (PLP) fixation, we have studied the distribution of Factor VIII-related antigen (FVIIIR:Ag) in 12 cases of tumors of the human central nervous system (CNS) and one sample of non-tumor brain tissue. FVIIIR:Ag was found both extracellularly and intracellularly. It was localized in the vascular lumen, between endothelial cells, and in the endothelial cell basement membrane. In the endothelial cell cytoplasm, FVIIIR:Ag was found in the endoplasmic reticulum, perinuclear space, and in intracytoplasmic vacuoles and vesicles. Characteristic of malignant tumors (six out of seven) was a strongly-positive dilated endoplasmic reticulum. This may reflect increased FVIIIR:Ag synthesis in the endothelial cells of malignant tumors. Only one of five benign tumors showed such staining. Six of 12 tumors and the non-tumor brain showed perinuclear FVIIIR:Ag. Both ad- and abluminal vesicles in the tumor endothelial cells contained FVIIIR:Ag suggesting that endocytosis, transcellular transport, and/or endocytosis, as well as FVIIIR:Ag synthesis occurs. The non-tumor brain showed normal capillary structure and very little FVIIIR:Ag immunoreactivity. The relationship of these FVIIIR:Ag abnormalities to the hypercoagulable state seen in some malignant brain tumor patients remains to be clarified.     

Embryonic Vasculogenesis in Nodular Melanomas and Tumour Differentiation

The relationship of vasculogenic mimicry to pigment in nodular vertical growth phase [VGP] cutaneous melanomas is assessed in this study. 10 nodules each from 27 tumors, 15 pigmented and 12 amelanotic were sampled in proportion to the pigment level. Serial frozen and paraffin sections subjected to HE, Reticulin, PAS to assess the vascular pattern; Dopa Oxidase and Immunopositivity for HMB45, LN5 [laminin 5] & integrin[α₅β₁], and EM [electron microscopy] to identify Weibel-Palade bodies within endothelial cells. The vascular pattern, pigment and the immunopositivity was mapped to assess the percentage VM [vasculogenic sinusoids] vs INC [incorporated microvasculature]. In pigmented melanomas, INC from pre-existing stromal vessels is predominant. Amelanotic melanomas show embryonic vasculogenic mimicry, a self-propagating system of spaces within the sheets of tumors cells. Both INC and VM co-exist in tumors with both amelanotic and melanotic nodules. In areas with VM, loci of LN5 and α₅β₁ integrin positive cells appear within the proliferating columns, positivity in these cells suggesting a switch to a more aggressive form. Irregular spaces appear lined by tumor cells, with initial hemopoeitic activity, coalesce and interlink into tubular networks. Spaces lined by tumor cells extend into an intricate network which then connects with the angiogenetic system. The tumor cells lining the vasculogenic spaces are positive for LN5, α₅β₁ integrin. Statistically, INC is significantly higher in pigmented melanomas, whereas amelanotic melanomas show significantly higher VM. Pigmentation is correlated positively with INC and negatively with VM. INC and VM are negatively correlated with each other.     

Penetration of intra-arterially administered vincristine in experimental brain tumor

Vincristine is an integral part of the "PCV" regimen that is commonly administered to treat primary brain tumors. The efficacy of vincristine as a single agent in these tumors has been poorly studied. This study was designed to determine whether vincristine enters normal rat brain or an intracranially or subcutaneously implanted glioma and to assess the presence of the efflux pump P-glycoprotein (P-gp) on tumor and vascular endothelial cells. The 9L rat gliosarcoma was implanted intracranially and subcutaneously in three Fischer 344 rats. On day 7, [3H]vincristine (50 microCi, 4.8 microg) was injected into the carotid artery, and the animals were euthanized 10 or 20 min later. Quantitative autoradiography revealed that vincristine levels in the liver were 6- to 11-fold greater than in the i.c. tumor, and 15- to 37-fold greater than in normal brain, the reverse of the expected pattern with intraarterial delivery. Vincristine levels in the s.c. tumor were 2-fold higher than levels in the i.c. tumor. P-gp was detected with JSB1 antibody in vascular endothelium of both normal brain and the i.c. tumor, but not in the tumor cells in either location, or in endothelial cells in the s.c. tumor. These results demonstrate that vincristine has negligible penetration of normal rat brain or i.c. 9L glioma despite intra-arterial delivery and the presence of blood-brain barrier dysfunction as demonstrated by Evan's blue. Furthermore, this study suggests that P-gp-mediated efflux from endothelium may explain these findings. The lack of penetration of vincristine into brain tumor and the paucity of single-agent activity studies suggest that vincristine should not be used in the treatment of primary brain tumors.     

Comparative measurements of hypoxia in human brain tumors using needle electrodes and EF5 binding

Hypoxia is known to be an important prognostic marker in many human cancers. We report the use of two oxygen measurement techniques in human brain tumors and compare these data with semiquantitative histological end points. Oxygenation was measured using the Eppendorf needle electrode and/or EF5 binding in 28 brain tumors. These data were compared with necrosis, mitosis, and endothelial proliferation. In some tumors, absolute EF5 binding was converted to tissue pO(2) based on in vitro calibrations. Eppendorf electrode readings could not be used to identify WHO grade 1/2 versus WHO grade 3/4 tumors, they could not differentiate grade 3 versus grade 4 glial-derived neoplasms, nor did they correlate with necrosis or endothelial proliferation scores. EF5 binding increased as the tumor grade increased and was significantly associated with necrosis and endothelial proliferation. There was no statistically significant correlation between the two hypoxia detection techniques, although both methods indicated similar absolute ranges of tissue pO(2). There was substantial inter- and intratumoral heterogeneity of EF5 binding in WHO grade 4 glial neoplasms. The majority of cells in glial-derived tumor had levels of hypoxia that were mild to moderate (defined herein as 10% to 0.5% pO(2)) rather than severe (defined as approximately 0.1% pO(2)). Immunohistochemical detection of EF5 binding tracks histological parameters in adult brain tumors, with increased binding associated with increasing necrosis and endothelial proliferation. The proportion of moderately to severely hypoxic cells is relatively low, even in the high-grade tumors. Human brain tumors are dominated by oxic to moderately hypoxic cells.     

Photosensitized release of von Willebrand factor from cultured human endothelial cells.

Cultured endothelial cells from the human umbilical vein were incubated with low concentrations (1 microgram/ml) of the photosensitizer Photofrin II. Following a sublethal light exposure, a light dose-dependent release of von Willebrand factor (vWf) into the culture medium was observed. Analysis of the multimeric composition of the released protein indicated that it originated from the intracellular pool of large vWf multimers stored in the Weibel-Palade bodies. This release was detected as early as 1 h postirradiation. Release was inhibited at low temperature and was dependent upon the presence of extracellular calcium. Photosensitization resulted in an influx of calcium whose time course paralleled vWf release from the cells. Since vWf mediates platelet adhesion to the vascular subendothelium, it is possible that its photochemically stimulated release in vivo could contribute to platelet thrombus formation observed in tissue following photodynamic therapy.