Plexogenic angiopathy in pulmonary intralobar sequestrations: Pathogenetic mechanisms

Plexogenic Angiopathy in Pulmonary lntralobar Sequestrations: Pathogenetic Mechanisms

MEENA TANDON,

MD, AND MARTHA L. WARNOCK,

Vascular remodeling to form plexiform (glomoid) lesions is a littleknown manifestation of intralobar pulmonary sequestration. We describe histologic and immunohistochemical features of these lesions in resected specimens from three subjects aged $19, and 28 years. The vascular changes, which included medial and intimal thickening, angioblastic proliferation, plexiform lesions, and dilation lesions, occurred in a setting of hypoxia, chronic inflam-

mation, and high pressure and flow via a systemic arterial supply. We demonstrated strong immunoreactivity of the angioblastic tissue and the plexiform lesions with antibodies to muscle actin, a-smooth muscle actin, and vimentin, and weak to absent reactivity with antibody to desmin. We suggest that in these sequestrations plexiform

lesions develop via angioblastic proliferation at arterial branch points and that dilation lesions develop from subsequent expansion of distal anastomoses. HUMPATHOL 24:263-273. Copyright 0 1993 by W.B. Saunders Company

AND METHODS

Case Selection

and Histologic

ence of intralobar sequestration and the presence of a systemic artery described by the surgeon. Specimens were fixed routinely in 10% buffered formalin. Three to 17 blocks of tissue

were available for each case. Slides stained with hematoxylineosin or the elastic van Gieson stain were reviewedfor evidence of plexogenic angiopathy: plexiform lesions, angiitis, dilation lesions, and concentric inrimal fibrosis. The Giemsa stain was used to identify mast cells, and the alcian blue stain was used to identify intercellular matrix. For case no. 2, 20 serial sections were prepared from each of two blocks. Alternate sections were stained with hematoxylin-eosin or the elastic van Gieson stain.

lmmunohistochemical

Plexogenic angiopathy is an irreversible vascular change that sometimes occurs with pulmonary hypertension. Clinical associations include certain congenital heart diseases, collagen vascular disease, cirrhosis of the liver, certain toxins, schistosomiasis, and foreign body embolization. This angiopathy also occurs in families and idiopathically, usually in young women.’ Not so well known is its occurrence in persons with a systemic arterial supply to the lung, and descriptions of the lesions vary in detail. 2-5 We recently examined resected specimens from two individuals with intralobar sequestration. Both showed plexogenic angiopathy. We describe morphologic and immunohistochemical changes of the lesions, review the findings of an additional case retrieved from our files, and describe possible mechanisms involved in the pathogenesis of plexogenic angiopathy. MATERIALS

MD

Methods

We searched the computer files of the Surgical Pathology Department of the University of California San Francisco Medical Center for cases of pulmonary sequestration acquired from 1983 to 1991. One case was found in addition to the two index cases. For each case we reviewed the patient’s chart for clinical details and the pathology report to verify the presFrom the Department of Pathology, Universityof California San Francisco, San Francisco, CA. Accepted for publicationJune 7, 1992. Key words: intralobar sequestration, lung, plexiform lesions, angiitis, angiogenesis, vascular remodeling. Address correspondence and reprint requests to Martha L. Warneck, MD, Department of Pathology, University of California, San

Francisco, CA 94143-0506. Copyright 0 1993 by W.B. Saunders Company 0046-8177/93/2403-0007$5,00/O

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Methods

To provide more information concerning origin and type of cells involved in the remodeling, we used the following antibodies. We identified antigenic determinants for endothelium with rabbit anti-human von Willebrand factor (Dakopatts Corp, Carpinteria, CA) and with the lectin Ulex europaeus I (E-Y Laboratories, San Mateo, CA). We identified filamentous proteins of connective tissue cells with antibodies to muscle actin (cyand 7 isotypes, clone HHF35; Enzo Diagnostics, Inc, New York, NY), Lu-smooth muscle ((Y-SM) actin (clone lA4; Sigma Chemical Co, St Louis, MO), desmin (clone D33; Dakopatts Corp), and vimentin (clone 9; Accurate Chemical & Scientific Corp. Westbury, NY). Immunoperoxidase and lectin peroxidase stains were performed on 5 pm-thick sections that were cut from formalin-fixed tissue routinely embedded in paraffin. Sections were protease-digested (0.25% trypsin and 0.25% protease [ 1: 1 v/v], Sigma Chemical Co) before staining for all antibodies except desmin and the actins. Sections were incubated overnight at 4’C with primary antibody, except for (YSM actin. Sections for (Y-SM actin were treated with freshly prepared 0.02% sodium borohydride for 2 minutes to block free aldehyde groups6 and then incubated at 37°C for 1 hour with primary antibody at a dilution of 1:500. Staining was accomplished using the avidin-biotin-peroxidase complex (ABC) method (Vectastain ABC, Vector Laboratories, Burlingame, CA).7 Slides were counterstained with hematoxylin. Control methods included appropriate positive and negative controls and the staining reactivity of known positive and negative tissues. Most stains were performed twice on different days on sections, including both normal lung and sequestration if available. Staining was graded 0 to 3+ using airway smooth muscle reactivity as an internal standard.

Morphometric

Methods

To describe the arterial remodeling and the size of vessels involved in the plexiform lesions, we measured the percentage wall thicknesses and arterial diameters, respectively, in the malformation and compared them with those of arteries in the normal lung adjacent to the sequestration. Measurements were made on elastic van Gieson-stained sections of vessels cut in cross-section or near cross-section. For each suitable

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vessel we measured the shortest diameter and the medial thicknesses on both sides at that diameter. The medial thickness included the internal and external elastic laminas. The medial thicknesses of both sides of the vessel were added, and the sum was divided by the diameter and multiplied by 100 to give the percentage medial thickness. Similarly, the thicknesses of the combined media and intima were measured on each side, added together, divided by the external diameter, and multiplied by 100 to give the percentage medial and intimal thicknesses. All suitable muscular arteries less than 1,000 pm in diameter were measured. For case no. 2 the presence of sufficient normal lung permitted classification of vessel size by parenchymal location: bronchiolar, respiratory bronchiolar, or acinar. Based on the calculation of mean arterial diameter (mean f SD) in each location, we grouped vessels into the following categories: ,260, 260 to 120, and <120 pm, respectively, which were used to classify vessels in the sequestrations.

Case Reports CaseNo. 1. A 3-year-old white boy was admitted with pneumonia. A computed tomography (CT) scan revealed a left lower lobe consolidation supplied by a vessel from the descending thoracic aorta. An angiogram confirmed the anomalous arterial supply; the venous drainage was into the azygous vein. The patient underwent resection of an intralobar sequestration weighing 27 g. Parenchyma was firm and mottled gray and yellow-tan with mucous plugging of small airways. No bronchus was identified. Case No. 2. A 28-year-old white man was admitted with a l-month history of recurrent pleuropulmonary infections in the left lower lobe. Chest radiographs and a CT scan showed ill-defined hyperlucency of the lower lobe. Aortography showed an arterial supply from the descending thoracic aorta; venous drainage was into the left atrium. A left lower lobectomy was performed. The 247-g specimen had a cyst (3 cm in diameter) superiorly and a region in continuity inferiorly (6 X 10 cm) composed of several large cysts (3 to 5 cm in diameter) and multiple small cysts. Laterally, a rim of surrounding parenchyma appeared normal. By dissection, air spaces in the sequestration communicated with the resected lobar bronchus. Case No. 3. A 19-year-old Asian woman with end-stage renal disease was admitted for renal transplant. A preoperative chest radiograph showed a 3-cm, rounded, soft tissue mass in the right lower lobe of the lung. The surrounding lung appeared hyperlucent. There was a history of pneumonia in the same area 5 years previously. A CT scan showed an area of abnormally lucent lung in the posterior, medial right lung base adjacent to the rounded mass seen on the chest radiograph. Angiography confirmed the presence of an intralobar sequestration supplied by an artery from the upper abdominal aorta; venous drainage was to the left atrium. The patient underwent a segmental resection of the sequestration. The resected specimen (66 g) contained no bronchus. Sections showed a subpleural wedge of normal parenchyma that blended into cystic parenchyma (Fig 1). A firm, rounded area, 3 cm in diameter, was composed of cysts containing clot. RESULTS Histologic Changes Histologic sections in all except case no. 1 showed rims of normal lung separated by either fibrous septa or ill-defined alveolar boundaries from the sequestration, which had cystic airways and spaces. Cartilage was absent from the sequestrations. All cases showed patchy

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FIGURE 1. Section of the resected lung tissue from case no. 3 showing a wedge of normal lung parenchyma at the upper left that blends into cystic parenchyma. Note the thick-walled artery to the right of the large central cyst that is filled with blood clot. (Magnification Xl .)

chronic interstitial inflammation, fibrosis, and alveolar hemorrhage. The thick-walled supplying systemic artery had continuous (not fragmented, as in large pulmonary arteries) elastic laminas, as in systemic arteries, and showed atherosclerosis. Although not specifically traced in our specimens, the systemic artery appeared to be continuous with the arteries of the sequestration, which had a normal architectural relationship to the conducting airways. We refer to these latter vessels as pulmonary arteries. Similarly, although the parent bronchial artery was not identified, vessels with the location and structure of normal bronchial arteries were apparent in the airway wall. Perfusion studies are needed to confirm these vascular relationships. The pulmonary arteries within the sequestration were tortuous and showed medial and patchy intimal thickening as well as plexiform lesions. The mean percentage medial thickness of vessels in the normal lung (Table 1) is given as a single value, as the means did not vary according to size. For the sequestrations, the means for percentage medial thickness and percentage medial + intimal thickness of vessels by size category are also shown in Table 1. The mean percentage medial wall thickness (range, 3 1% to 46%) in the sequestrations was greatly increased over that in the accompanying normal lung (12% and 14%; P < .OOl; Student’s t test). The large standard deviations for measurements in the sequestrations indicate variable damage to the vessels. There was no trend toward increased medial thickness as vessel diameters declined. In contrast, the mean percentage medial + intimal thickness increased as vessels became smaller in cases no. 1 and 3, but not in case no. 2. These observations suggest that intimal proliferation continues, especially in the smallest arteries after medial thickening has occurred and that medial atrophy does not occur secondary to intimal thickening. Thickened cellular intimas had faint strands of elastin and variable amounts of collagen, and medias had increased elastin deposits appearing as irregular strands (elastic van Gieson stain). Internal and external elastic laminas were irregularly thickened and occasionally fragmented (Fig 2). Many vesseis showed increased

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TABLE 1.

Arterial

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Wall Thicknesses

(Tandon & Warnock)

in Normal Lung and in Sequestrations Vessel Diameter*

Case No. (No. of Vessels Measured) Medial wall thickness 2, Normal lung (n = 14) 3, Normal lung (n = 67) 1, Sequestration 2, Sequestration 3, Sequestration Medial and intimal wall thickness 1 2 3

All *

>260 pm

120-260 pm

<120 pm

14f 6 12+ 8 46 * 15t 39 f 17t 38 + 13t

None 43 f 15 (n = 15) 39 + 10 (n = 14)

45 f 10 (n = 3) 31 + 8 (n = 9) 38 + 20 (n = 5)

46f18(n=S) 40 f 22 (n = 19) 34+15(n=7)

61 f 22 38+ 15 61 + 27

82k 18 49 * 22 87 + 15

492 54+

None 19 12

Abbreviation: n, number of vessels measured. * Mean percentage f standard deviation. t Significantly different from vessels in either normal lung: P < 0.001; Student’s t test.

alcian blue staining of matrix components, sometimes in the intima and cell layer immediately beneath it, and at other times more diffusely through the wall. Just outside the external elastic lamina of some arteries, a narrow zone of adventitia staining faintly with the van Gieson stain contained spindled cells and increased elastic fibers. Sometimes, emerging in the same region was a localized, outer longitudinal muscle bundle external to the elastic lamina (Fig 2). In some cases this bundle had no elastic layer delimiting it from either the media or the adventitia. Adventitial fibrosis was moderate to marked in all cases. In addition to the conventional arteries that followed airways, supernumerary arteries were identified. These vessels, which in the lobular region are approximately four times as numerous as the conventional axial arteries, traverse the parent artery wall as unobtrusive slits at right angles and give rise to arteries with diameters on average one quarter that of the parent artery.‘,’ The supernumerary vessels led directly to the alveolar capillary bed (Fig 3). These vessels appeared to be important in the vascular remodeling associated with the

plexiform lesion (see below). Further details of the histology in individual cases follow. In case no. 1 no normal lung was identified. Much of the sequestration was filled with a chronic inflammatory infiltrate, and dilated airways were filled with purulent exudate. In areas without chronic inflammation, 11 vessels in two slides showed plexiform (glomoid) lesions with multiple lumens and absence of elastic lam-

FIGURE 2. Representative muscular artery showing medial and intimal thickening with increased medial elastic tissue (case no. 2). There is some destruction of elastic laminas at the upper right. Note the outer longitudinal muscle bundle (left). (Elastic van Gieson stain; magniftcation x250.)

I lectin-peroxidase

FIOURE 3.

Supernumerary arten/ stained with Ulex europaeus to show its origin from a muscular or-ten/ at right angles. Note the narrow diameter and thin wall of the supernumerary that is directed toward the alveolar septal capillary bed (arrow). The related airway is at the lower right (case no. I). (Magnification xl 10.)

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FIGURE 4. Plexiform lesion from a 3-year-old child (case no. 1). (Left) The parent artery is seen at the center. The upward unconnected branch has a plexiform lesion with multiple lumens and thick media. (Elastic van Gieson stain; magnification x80.) (Right) A higher-magniftcation view of the plexiform lesion shown in the left panel. Although a definite outer and a faint inner elastic layer can be seen at the proximal part of the artery (lower arrow), both elastic layers are lost distally (upper arrow). (Elastic van Gieson stain; magnification X300.)

inas (Fig 4). Of 11 vessels measured, 10 (91%) had intimal thickening. Of these, two had occlusive concentric intimal fibrosis. In case no. 2 no remnants of cartilage were found in the sequestration despite the communication of the cysts with the resected bronchus. Intimal thickening occurred in only 55% of vessels compared with 90% found in the other cases. The medial and intimal thickening was accompanied by a few discrete plexiform lesions and a spectrum of angioblastic proliferative lesions consisting of nodular collections of nonspecific spindled cells and minute vessels at localized points along muscular pulmonary arteries varying from 150 to 500 pm in diameter (39 lesions in seven slides). Some of the angioblastic foci occurred at sites of supernumerary arteries that showed intimal proliferation narrowing the lumens and involving the contiguous part of the parent artery. Nodular angioblastic foci of spindled cells and minute vessels (Fig 5) were found in the adjacent adventitia. In other cases it was not always possible to determine whether the angioblastic foci originated from supernumerary arteries or from other sites (eg, conventional branch points or random points in the artery). Other lesions showed replacement of media, intima, and adventitia by more haphazardly arranged cells forming angioblastic foci (Fig 6). In some instances the angioblastic process was very extensive and destructive, essentially obstructing the lumen of the parent artery (Fig 266

7). The angioblastic foci were not surrounded by vascular media or elastic laminas from the parent artery. In serial sections we could show that the parent vessel connected with the vessels in the angioblastic lesions, which then connected with thin-walled channels already present in the adventitia (dilation lesions). These channels, in turn, appeared to connect with the alveolar capillaries and also to have connections with the bronchial arteries (Fig 8). Study of specimens perfused with barium-gelatin will be necessary to confirm these relationships. Among the spindled cells of the angioblastic lesions were scattered inflammatory cells, including lymphocytes, eosinophils, and mast cells, as well as extravasated red blood cells and hemosiderin (Fig 9). The eosinophils were most prominent at the edges of the proliferations. Mast cells, demonstrated by the Giemsa stain, were located more centrally. With decreased cellularity, the eosinophils tended to disappear. The mast cells remained, but were absent in sparsely cellular lesions. Stroma of the cellular lesions was alcian blue positive and had fine strands of elastic tissue and pale pink, van Gieson-positive staining. Bundles of cells with eosinophilic cytoplasm resembling smooth muscle were occasionally present within these nodules. Observation of multiple lesions suggested that there were two different outcomes of the angioblastic foci: development of muscular arteries or compact plexiform

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FIGURE 5. (Top left) A low-power view of muscular arteries and distorted air spaces (case no. 2). (Elastic van Gieson stain.) (Top right) Detail of the top left showing origin of a supernumerary artery narrowed by intimal proliferation that also involves the intima of the adjacent portion of the parent arten/. Adjacent and separate angioblastic nodules (arrows) have no elastic tissue. (Elastic van Gieson stain.) (Bottom) An adjacent section of the same vessel showing continuity of the angioblastic foci shown in the top right panel. Note the circumferential layering of spindled cells around one vascular lumen (arrow). (Hemaloxylin-eosin stain. Magnifications: top left, x50; top right, x 120; bottom, xl 10.)

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FIGURE 6. (Top left) A muscular artery from case no. 2 with elastic-van Gieson stain showing irregular intimal thickening and an angioblastic lesion related to a defect in the media of the parent artery (arrow). The airway is seen at the right. (Magnification X35.) (Top right) The same muscular artery stained for muscle actin. The ill-defined nodules (arrows) of angioblastic tissue are composed of actin-reactive spindled cells and new vessels. (Magnification X68.) (Bottom) The same muscular artery showing reactivity with Ulex europaeus I of the endothelium of the new vessels in the angioblastic lesions as well as of the endothelium of the parent artery. Circumferential layering of cells around lumens is absent. (Magnification X68.)

outer, or both elastic laminas) appeared (Fig 8). The compact plexiform lesions (Fig 10) appeared to develop as other angioblastic foci became less cellular. In case no. 3, as in case no. 1, vessels showed an increased frequency of intimal thickening (90%), in-

lesions. Some angioblastic foci showed circumferential layering of spindled cells around one or more of the lumens (ie, incipient muscular arteries) (Fig 5, bottom). In other cases clusters of muscular arteries (which differed from normal arteries in that they lacked inner, 268

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FIGURE 8. Plexiform lesion (arrow) showing vascular branches in proximity to dilated adventitial vessels and bronchial vessels (right). The plexiform lesion here had no elastic lamina in an adjacent section stained with elastic van Gieson. The medial cells of the plexiform lesion resemble smooth muscle cells with hematoxylin-eosin staining and showed reactivity with actin in an adjacent section (case no. 2). Ulex europaeus I-peroxidase stain; magnification X70.)

reactive with actin. Vimentin-positive cells also included stromal fibroblasts, monocytes/macrophages, lymphocytes, and vascular endothelium. Desmin reactivity was faint and patchy in arterial smooth muscle of normal lung or sequestration (0 to l+), absent or faint in proliferating myointimal cells and angioblastic lesions, and absent in plexiform lesions. Ulex europaeus I and antibody to von Willebrand factor reacted with normal vascular endothelium as well as with the endothelium lining the new channels and with some single cells within the angioblastic lesions (Fig 6, bottom). These stains were helpful in tracing connections from the parent artery to the angioblastic lesions and from there to dilated thin-walled vessels within the adventitia and to capillary walls (Fig 8). The thin-walled, postplexiform vessels were present but less prominent in cases no. 1 and 3 than in case no. 2. In each case both Ulex europaeus I and antivon Willebrand factor reactivity outlined bronchial vessels arranged regularly on either side of the airway smooth muscle layer both within the sequestration and

FIQURE 7. Muscular artery (case no. 2) with luminal occlusion, and medial and adventitial destruction by angioblastic tissue (arrow). (Elastic van Gieson stain; magnification X30.)

chiding concentric fibrosis, compared with case no. 2. Angioblastic lesions were absent and a few plexiform lesions were present, but many vessels varying from 160 to 500 pm in diameter had a necrotizing arteritis, all at the same stage of development (18 lesions in 13 slides). These vessels had intact outlines and intact elastic laminas, but the media showed fibrinoid necrosis and infiltration by red blood cells, acute inflammatory cells, and proteinaceous exudate, all of which also spilled into the adventitia (Fig 11). Numerous eosinophils were present in the exudate surrounding the necrotic arteries. lmmunohistochemical

Results

Airway smooth muscle showed 3+ reactivity with antibody to both actins or desmin and no reactivity with antibody to vimentin. In comparison, arterial smooth muscle reactivity for both actins was 3+ in normal lung and in the sequestrations, but reactivity for vimentin and desmin in the sequestrations was variable (0 to 2+) and even less in normal lung (0 to l+). Reactivity of myointimal cells and angioblastic and plexiform lesions with both actin antibodies was similar (3+) to that of airway and arterial smooth muscle. Vimentin reactivity was variable but slightly stronger (0 to 2+) in the smooth muscle of arteries in the sequestration compared with normal arteries (0 to I+). It also was variable in myointimal cells (1 to 3+) but was strong in angioblastic and plexiform lesions (3+). In adjacent sections it appeared that at least some of the vimentin-positive cells also were

FIGURE 9. An angioblastic focus stained with the Giemsa stain to show the infiltration of eosinophils, mast cells, and lymphocytes (case no. 2). (Magnification X245.)

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FIGURE 10. (Left) Plexiform lesion (arrow). The airway is at the lower left (case no. 3). (Right) A higher-magnification view of the left panel showing minute lumen of parent artery (below) with intimal thickening and intact elastic laminas, except at right adjacent to the plexiform lesion where muscle streams from the parent vessel to the branch. The plexiform lesion has multiple slit-like lumens and no elastic layers, and its cells do not resemble smooth muscle. An adjacent section showed the connection of the two vessels, (Elastic van Gieson stain. Magnifications: left, Xl IO: right, X150.)

in the normal lung. Where remodeled pulmonary arteries in the sequestration were adjacent to airways, the thin-walled vessels were closely associated with the bronchial arteries, which were then enlarged (Fig 8). DISCUSSION We have described arterial abnormalities associated with a systemic arterial supply to the lung in three cases of intralobar pulmonary sequestration. The vascular changes that we found are similar to those described in other situations in which plexogenic angiopathy occurs’ and also mimic changes described in systemic arteries in malignant hypertension. lo Plexiform lesions have been reported in normal lung supplied by a systemic artery3s5 and also have been mentioned in reports of cases of intralobar sequestration. s Medial hypertrophy and necrotizing arteritis have been attributed to increased vascular pressure in extralobar sequestration.4 Nevertheless, the plexiform lesions have not been described in detail, and recent reviews of pulmonary sequestration have not mentioned plexogenic vascular changes.““6 270

We think that because they are not recognized the changes we describe are underreported in intralobar sequestrations, as well as in other cases of systemic arterial supply to the lung. Furthermore, resected sequestrations provide the opportunity to observe many vessels in different stages of remodeling in well-preserved tissue. This natural experiment provides a means of corroborating information gained from experimental models of vascular remodeling following construction of systemic to pulmonary arterial anastomoses (see below). The changes we have described provide a basis for reconstructing the development of the lesions. The morphometric data on medial and intimal changes suggest that medial thickening was followed by intimal thickening, which progressed distally, being more marked in cases no. 1 and 3 than in case no. 2. The appearance of longitudinal muscle along the outer media also was noted. These changes have been well documented in patients with congenital heart disease.’ The adventitial fibrosis that occurred has been described in hypoxic pulmonary hypertension” but not in plexogenic angiopathy.’ Study of the lesions at various stages of

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FIGURE 11. (Left) A low-power view showing necrotizing arteritis in a vessel in distorted lung parenchyma (case no. 3). (Right) A higher-power view of the same vessel shows uniform ftbrinoid necrosis of the media and inflammatory cells in the intima, media, and adventitia. (Hematoxylin-eosin stain. Magnifications: left, X30; right, X 120.)

stain, the compact plexiform lesions were characterized by typical smooth muscle cells, nonspecific spindled cells, or both, and a lack, in part, of one or more elastic layers (Figs 4 and 10). The recognition of isolated new small muscular arteries requires use of elastic tissue stains to demonstrate lack of one or more elastic laminas. It is not clear whether these vessels eventually develop elastic laminas, as they would then resemble normal vessels. Burke et al have beautifully illustrated both the neovascularization (angioblastic) and plexiform lesions of primary pulmonary hypertension and of hypertension secondary to congenital heart disease.” The presence of these varied changes in this case suggests that remodeling is a continuous process, with early proliferative lesions present along with established late lesions. Thus, the descriptive terms (concentric intimal hyperplasia or fibrosis, angioblastic lesion, plexiform or glomoid lesion, muscular artery neovascularization, and dilation [angiomatoid] lesion) all refer to related components of the vascular remodeling of plexogenic angiopathy. Arteritis with fibrinoid necrosis at the same stage of development in all vessels was prominent in mediumsized muscular arteries in case no. 3 (Fig 11). It involved the entire cross-sectional wall of the vessel uniformly and was not related to branch points. Perhaps it resulted

development in case no. 2 suggested the following sequence of events in the development of plexogenic angiopathy. The ill-defined angioblastic lesions, also described by other investigators,‘*~ig seemed to develop at the origin of supernumerary arteries, although an origin at other sites or branch points of conventional, axial arteries could not be excluded. More numerous than the dichotomously branching axial arteries that follow the airways, supernumerary arteries arise at right angles from the parent vessel to supply the acinar alveoli directly.8~gThe junctions of these branches with the parent artery may represent points of least resistance, as the supernumeraries have relatively thin walls. The angioblastic proliferation may serve to buttress this weak point in the arterial wall and to protect the capillary bed from the rising pressure. During the cellular angioblastic phase the newly formed vessels appeared to anastomose with nearby vessels supplying the alveolar walls, as well as with bronchial arteries. Anastomoses possibly occur with more proximal and distal supernumerary arteries. Observation of transitional forms suggested that the initially ill-defined proliferation became sharply circumscribed from the adventitia to form either compact plexiform lesions or new small muscular arteries lacking one or more elastic laminas. With the hematoxylin-eosin 271

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from a more acute insult to the vessel wall in contrast to the chronic, proliferative response represented by the angioblastic lesion. We saw no evidence in our material that the proliferative lesion was initiated by an acute angiitis. Both lesions occurred in vessels of similar size, but the pathogenesis, natural history, and relationship of the arteritis to vascular remodeling remain obscure. The plexiform lesions in our cases closely resemble those produced experimentally in dogs that developed extreme pulmonary hypertension following construction of systemic to pulmonary artery shunts.lg Between 4 and 11 months after construction of the anastomoses, angioblastic proliferations as well as more compact, plexiform lesions occurred. The angioblastic proliferations were presumed on morphologic grounds to be composed of endothelial cells and smooth muscle precursors; we have confirmed that observation with endothelial and smooth muscle markers. In the dogs the vessels of the angioblastic lesions were shown to anastomose with the bronchial arteries by perfusion with a gelatin mass injected via the bronchial artery. Thus, the bronchial artery appeared to be one source of blood for the postplexiform, thin-walled vessels (dilation lesions). Another potential source of blood supply to the dilation lesions is anastomosis with supernumerary arteries proximal to the plexiform lesion. Anastomoses with supernumerary arteries distal to the plexiform lesion might supply the more distal vascular bed. Inflammatory cells have not been considered to be a part of the plexiform lesion, although they have been related to some types of vascular remodeling.” Eosinophils were present in large numbers, especially at the margins of the cellular angioblastic tissue masses. Their role in remodeling the interstitial matrix has been investigated recently and a granule-contained collagenase active against types I and III collagen has been described.?’ The location of the eosinophils at the junction of the cellular lesions and the collagenous tissue suggests a possible role in breaking down the collagenous matrix. The mast cells present centrally in the angioblastic masses may be involved in angiogenesis.22 The spindled cells of the angioblastic lesions reacted with muscle and a-SM actins and vimentin. While a few of these cells appeared with hematoxylin-eosin staining as smooth muscle cells, most did not. The latter cells thus qualify as myofibroblasts.2”~24 In our angioblastic proliferations, the streaming of cells from the parent artery suggests that the cells are derived from smooth muscle cells there. Muscle actin reactivity was similar to that in the parent artery, but vimentin reactivity was clearly greater, diminishing when cells developed the appearance of smooth muscle either in the plexiform lesion or in new muscular arteries emanating from the angioblastic lesion. Thus, this study of the morphologic features of vascular remodeling at different stages suggests that smooth muscle cells may assume a fibroblastlike morphology in response to injury and then become typical smooth muscle cells as the lesion becomes established. This transition from a contractile phase to a phase designated “synthetic” after injury and back to a contractile phase may correspond to changes of smooth 272

muscle cells seen in tissue culture.25 In contrast, the smooth muscle cells of the intima did not revert back to a smooth muscle morphology. Roberts has summarized the proposed pathogenetic mechanisms for the plexiform lesion as (1) the consequence of ajet lesion, (2) thrombosis and recanalization of an aneurysm, or (3) a site of healed necrotizing arteritis2’j Our observations sugg est a fourth hypothesis: angioblastic proliferation and collateral expansion at branch points. We propose that mechanical stress and hypoxia acting at structurally weak branch points cause an angioblastic proliferation that develops into a plexiform lesion with distal anastomoses with bronchial arteries, alveolar capillaries, and, perhaps, proximal and distal supernumerary arteries. As a result of medial hypertrophy and intimal thickening of the parent artery, progressive narrowing of the peripheral arterial bed occurs. We speculate that the thin-walled, postplexiform vessels form dilation lesions in response to increased pressure and flow via anastomoses with bronchial arteries and more proximal supernumerary arteries. In summary, we suspect that plexogenic angiopathy is associated with intralobar sequestration more commonly than has been reported. Mechanical injury produced by increased pressure and flow, hypoxia associated with hypoventilation of the involved lung, and gradual inflammatory destruction of the vascular bed, generally over a period of years, all contribute to the vascular remodeling. Sequestrations are relatively common occurrences that, when resected, provide large amounts of tissue for study. Thus, these lesions are valuable for confirming in humans the morphologic and molecular data obtained from experimentally induced systemic artery to pulmonary artery shunts and for comparing plexogenic angiopathy in this disorder to that arising in other situations. Acknowledgment. The authors thank Drs Courtney Broaddus and Walter Finkbeiner for critical comments.

REFERENCES 1. Wagenvoort CA, Wagenvoort N: Pathology of Pulmonary Hypertension. New York, NY, Wiley, 1977 2. Ostrow PT, Salyer WR, WhiteJJ, et al: Hypertensive pulmonaty vascular disease in intralobar sequestration. Am J Path01 70:33a-34a, 1973 3. Ko T, Gatz MG, Reisz GR: Congenital unilateral absence of a pulmonary artery: A report of two adult cases. Am Rev Respir Dis 141:795-798, 1990 4. Heath D, Watts GT: The significance of vascular changes in an accessory lung presenting as a diaphragmatic cyst. Thorax 12: 142147,1957 5. Holder PD, Langston C: Intralobar pulmonary sequestration. (A nonentity?) Pediatr Pulmonol 2:147-153, 1986 6. Elias JM: Immunohistopathology. A Practical Approach to Diagnosis. Chicago, IL, ASCP Press, 1990, p 50 7. Hsu SM, Raine L, Fanger H: Use of avidin-biotin-peroxidase complex (ABC) in immunoperoxidase techniques: A comparison between ABC and unlabeled antibody (PAP) procedures. J Histochem Cytochem 29:577-580, 1981 8. Elliott FM, Reid L: Some new facts about the pulmonary artery and its branching pattern. Clin Radio1 16: 193-l 98, 1965 9. Hislop A, Reid L: Intra-pulmonary arterial development during fetal life-branching pattern and structure. J Anat 113:35-48, 1972

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