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MORPHOLOGIC FEATURES OF HOMOTRANSPLANTED CANINE MITRAL VALVES
MORPHOLOGIC FEATURES OF H O M O T R A N S P L A N T E D C A N I N E MITRAL VALVES Peter D. Van Vliet, M.D.* Jacob Berghuis, M.D.*** Rochester,
Jack L. Titus, M.D.,
Ph.D.,**
and F. Henry Ellis, Jr.,
M.D.,****
Minn.
A
several studies of the homotransplantation of cardiac valves have . been reported, 1 the morphologic features of the grafted valves at various time intervals after operation have not been described, to our knowledge, in detail. Such a study might help explain the apparent long-term maintenance of some cardiac valve homografts. Therefore, the gross morphologic and histologic features of orthotopically homotransplanted canine mitral valves were studied in animals that had received sufficient antibiotics to prevent bacterial endocarditis but had not received immunosuppressive agents. The surgical tech niques, results, and physiologic data of some of the animals comprising this study have been reported previously. 2 LTHOUGH
MATERIALS AND METHODS
After excision of the recipient dog's mitral valve, a mitral valve from an unrelated donor was obtained and immediately sutured into place in its exact anatomic position.2 The donor valve consisted of the anterior and posterior mitral leaflets in continuity with a thin rim of the annulus fibrosus of the mitral valve and the chordae tendineae of the mitral valve in continuity with the tips of the papillary muscles. Fifteen animals survived more than 10 days and had no clinical or mor phologic evidence of bacterial endocarditis. Of these 15 animals, 11 form the basis for the morphologic study reported herein. The remaining 4, currently alive and in apparent good health, are being traced for evaluation of the results after more than 1 year. Morphologic examination was carried out at the follow ing time intervals after operation: 10 days (one animal), 2 weeks (two animals), 3 weeks (three animals), 3 months (one animal), 4 months (one animal), 5 months (one animal), 8 months (one animal), and 12 months (one animal). Prom the Mayo Clinic and Mayo Foundation. Rochester, Minn. Received for publication July 23, 1964. *Fellow in Pathology. ** Section of Experimental and Anatomic Pathology. ***Fellow in Thoracic Surgery. ****Section of Surgery. 504
Vol. 49, No. 3 March, 1965
HOMOTRANSPLANTED CANINE MITRAL VALVES
505
Complete necropsy was performed on all animals. Blocks for histologic sections were taken of the mitral valve annulus, the valve leaflets, the chordae tendineae, and the papillary muscles. After formalin fixation and paraffin embedding, histologic sections were cut at a thickness of 8 microns and stained with hematoxylin and eosin, Lawson's elastic stain with van Gieson counterstain, the Mallory-Heidenhain stain, the Gram stain, the von Kossa stain, the Prussian blue reaction, and the silver chromate stain.
Fig. 1.—Anterior leaflet of homografted mitral valve, 10 days after transplantation, was cov ered by fibrin (Lawson's elastic van Gieson; X15).
RESULTS
Ten days after operation, 1 animal succumbed of pulmonary edema, which resulted from partial separation of the anterolateral papillary muscle that produced mitral insufficiency. Except for the separation of the anterolateral papillary muscle, the valve graft was firmly attached at all points. Pale yellow fibrin covered all surfaces of the grafted valve leaflets, annulus, chordae tendineae, and papillary muscle tips. This fibrinous coat also extended onto the adjacent endocardial surfaces of the recipient heart. The grafted valve leaflets were moderately thickened but pliable. Histologically, histiocytes were present on the surface of the fibrin covering the graft and the adjacent endocardium (Fig. 1). Lesser numbers of histiocytes also were present in the deeper layers of the fibrinous covering. No bacteria or fungi were observed. The collagen of the grafted valve leaflet appeared to be necrotic in that there was loss of fibrillar detail and fragmentation of the fibrils. The fibroblastic nuclei of the valve stroma were pyknotic. In addition, hemorrhagic edema and a few round cells were present in the grafted valve leaflet. A moderately heavy inflammatory cellular infiltrate was observed at the valve annulus fibrosus where donor tissue joined recipient tissue. This inflammatory cellular reaction consisted mainly of histiocytes and lymphocytes, although a few polymorphonuclear leukocytes were present. Like the collagen of the valve annulus, the collagen of the chordae
506
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J. Thoracic and Cardiovas. Surg.
tendineae appeared to be partially necrotic, but the chordae tendineae were intact. The grafted tips of the papillary muscle were necrotic and covered by fibrin. At the junctional zone between the graft and recipient papillary muscles, an intense inflammatory reaction was observed. This reaction consisted of large numbers of lymphocytes and histiocytes, with a few polymorphonuclear leu kocytes. In this region, phagocytosis of the necrotic graft muscle was taking place in association with fibroblastic proliferation. Calcine deposits were present in scattered foci in the surrounding recipient myocardium. Two dogs that had survived 2 weeks after operation were examined. Both animals had died of pneumonia. In both instances, the grafted valves grossly appeared similar to the grafted valve examined 10 days after transplantation, in that a layer of fibrin covered all surfaces of the graft and extended onto the adjacent recipient atrial and ventricular endocardium, and the grafted valves were thickened. However, the thickened valves were pliable and appeared to be competent. Histologically, the layer of fibrin completely covering the graft had undergone organization by granulation tissue. There was no evidence of infection. At the valve annulus, some phagocytosis of fragmented necrotic collagen of the graft had occurred. The chordae tendineae, which were covered by a layer of organized fibrin, showed necrotic changes of the collagen. Phagocy tosis of the necrotic muscle in the grafted tip of the papillary muscle was present, and replacement of the removed muscle by loose fibrous tissue had oc curred. These changes were similar to those observed after 10 days, but more muscle removal and replacement had occurred. Small vessels in the inflammatory reaction of the recipient myocardium showed vasculitis. Focal regions of calcifi cation of the recipient myocardium were observed in the regions of the operative site. Three weeks after operation, 3 dogs were examined. One animal had died of pulmonary edema due to mitral insufficiency resulting from partial separation of a papillary muscle suture line; 1 animal had mitral insufficiency due to partial dehiscence of the anterior mitral leaflet from the annulus of the valve, and the third animal had died of empyema complicating wound infection. The transplanted valve of the third animal was grossly intact. In the other 2 dogs, except for the regions of separation described, the valves were firmly in place. Although the valve leaflets were approximately twice the thickness of the mitral valve of the normal canine, they were distinctly thinner than in the animals examined at 2 weeks. In no instances was morphologic evidence of infection observed. Histologically (Fig. 2, a-e), the fibrinous layer that completely covered all of the grafted tissues was organized by granulation tissue which contained Pig. 2.—Histologic sections 21 days after transplantation, a, Anterior leaflet of homotransplanted mitral valve shows organization of the fibrin layer that covered the valve (hematoxylin and eosin ; X10). 6, Fibrinous layer that covered the valve leaflet in a is seen partially organized and containing mononuclear inflammatory cells (hematoxylin and eosin; X700). c, Annulus flbrosus of grafted mitral valve shows necrotic collagen (bottom and left) partially replaced by granulation tissue (top and right) (hematoxylin and eosin; X50). d, Grafted papillary muscle of mitral valve was necrotic and partially replaced by young fibrous tissue (hematoxylin and eosin; X90). e, Higher power view of portion of papillary muscle in d shows cellular detail of fibrous tissue that replaced grafted muscle (hematoxylin and eosin; X400). (Pigs. 2e and e from Berghuis, J., Rastelli, G. C, Van Vliet, P. D., Titus, J. L,., Swan, H. J. C, and Ellis, P. H., J r . : Circulation, 39[Suppl. 1 ] : 47, 1964. By permission of the publishers, The American Heart Association, Inc.)
Vol. 49, No. 3
March, 1965
HOMOTBANSPLANTED CANINE MITRAL VALVES
Fig. 2.—For legend see opposite
page.
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ET AL.
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Pig. 3.—Grafted mitral valve leaflet, 3 months after operation, consisted of three layers : on either surface (top and bottom), a layer of fibrous tissue surrounds a central layer of vacuolated, fragmented, edematous collagen of the original donor leaflet (Mallory-Heidenhain stain ; X16).
scattered round cells (Pig. 2, a and b). The grafted valve was less edematous than at the previous time intervals, but the changes in the collagen of the stroma were more marked in that fragmentation and altered staining were more pronounced. The most peripheral layers of the collagen of the annulus flbrosus of the graft were necrotic, and some phagocytosis of the material was observed (Fig. 2, c). A slight amount of mature granulation tissue was present in this region (Fig. 2, c). In the grafted tips of the papillary muscle, phagocytosis, with replacement by young fibrous tissue, continued as at the previous time intervals (Fig. 2, d and e), so that approximately one half of the necrotic grafted muscle had been replaced. The morphologic features of the grafted valves in animals examined 3, 4, and 5 months after operation were similar. One animal was examined from each time interval, and none appeared infected. The transplanted valves appeared normal on gross examination. All structures of the graft were opaque and ap peared to blend into the adjacent endocardium of the recipient heart. Although the valves were somewhat stiffer than normal valves, they were pliable and competent. Histologically, the grafted valve leaflets appeared to consist of three layers (Fig. 3). On either surface of the leaflet was a layer of young fibrous tissue that appeared to be a continuation of the organized fibrinous layer ob served at earlier time intervals. Between these two layers of young fibrous tissue was a central layer composed of vacuolated, fragmented, edematous collagen and elastic fibers. Within this central layer, which appeared to represent the tissues of the original grafted valve, were vascular channels and lipochrome pigment in addition to the degenerating collagen and elastic fibers. This central layer of degenerating donor valve tissue was less prominent in those valves that were examined at the later time intervals. At these time intervals, the chordae tendineae and the papillary muscles of the grafted valve appeared to have been completely replaced by dense fibrous tissue.
Pig. 4.—Eight months after operation, a, Opened homotransplanted mitral valve shows nearly normal appearance, b, Fibrous tissue completely replaced the grafted papillary muscle (Mallory-Heidenhain stain, X15). c, Anterior leaflet homotransplanted valve shows valve com posed of two thin layers of fibrous tissue surrounding thin central layer of degenerating trans planted valvular tissue (Lawson's elastic van Gieson, Xo). (Pigs. 4a and c from Berghuis, J., Rastelli. G. C. Van Vliet, P. D.. Titus, J. L., Swan, H. J. C, and Ellis. F. H., Jr. : Circulation. 39[Suppl. 1] : 47, 1964. By permission of the publishers, The American Heart Association, Inc.)
The morphologic appearance of the grafted valves 8 and 12 months after operation were similar in each dog that was examined. The gross morphologic appearance of the transplanted mitral valve was similar to that observed at the 3- to 5-month time interval, except that the valve leaflets were thinner and approached, in thickness, the appearance of a normal canine mitral valve (Fig. 4, a). Although pliable, the grafted valve appeared somewhat stiffer than a normal valve. The chordae tendineae were of normal diameter and length. Scar tissue occupied the tips of the papillary muscles in the regions in which grafted muscle had originally existed (Fig. 4, b). Histologically, the appearance was similar to that seen in the 3- to 5-month animals (Fig. 4, c). The only significant difference was that the central layer of the valve leaflet was thinner and more vacuolated than at 3 to 5 months. The scattered elastic fibers and fragmented, altered collagen fibers in association with focal deposits of lipochrome pigment observed were considered to represent the remaining central layer. Inflammation was not present at the annulus fibrosus, but dense connective tissue and regions of apparent cartilaginous change were observed in this region. DISCUSSION
Various studies, such as those of Howard and associates3 and Eobicsek and associates,4 have indicated that masses of fibrin frequently accumulate on the
VANVLIET ET AL.
510
J. Thoracic and Cardiovas. Surg.
homografted cardiac valve and result in various complications, such as embolic phenomena. In some instances these fibrinous accumulations have been associated with bacterial infection.3 In the apparently uninfeeted dogs in our study, small amounts of fibrin were deposited on the grafted valves, but emboli were not observed. Bacteria were not found by morphologic examination. Thus, it might be suspected that instances of apparently excessive fibrin deposits, especially when associated with embolic phenomena, are related to infection and not to the grafting procedure per se. Instances of the apparent survival in a functional state for long periods of canine cardiac valve homografts have been reported. 3 - 4 However, in these studies, serial histologic analysis was not reported in detail and, thus, the possi bility that the graft was replaced by recipient tissues during the healing process remains. In the present study the orthotopically homotransplanted canine mitral valve underwent replacement by host tissues in a manner that included the following. The muscular portions of the graft, namely the tips of the papillary muscles, promptly suffered ischemic necrosis and replacement by fibrous tissue derived from adjacent host tissues. The collagen of the chordae tendineae of the graft underwent necrosis and was replaced by dense fibrous tissue, which ap peared to be derived from the organization of a layer of fibrin deposited on the surfaces. The valve leaflet of the graft underwent slow degeneration and replace ment by fibrous tissue derived from organized fibrin layers on its surfaces. At the annulus fibrosus of the grafted valve, the collagen of the graft was replaced, at least in part, by dense connective tissue and by chondroid tissues. Thus, it appeared that the slow rate of replacement of the grafted valve by recipient tissues permitted continued function of the graft, at least for periods of up to 1 year. SUMMARY
The gross morphologic and histologic features of the mitral valves in 11 dogs in which orthotopic homotransplantation of mitral valves were carried out were studied at eight time intervals of 10 days to 1 year after operation. The results indicate that slow replacement of the grafted tissues occurred in such a manner that function of the graft could be maintained. REFERENCES
1. McKenzie, M. B., and Ellis, F. H., Jr.: Current Concepts in Surgical Correction of Acquired Mitral Insufficiency, Circulation 28: 603, 1963. 2. Berghuis, J., Eastelli, G. C, Van Vliet, P. D., Titus, J. L., Swan, H. J. C, and Ellis, F. H., Jr.: Homotransplantation of the Canine Mitral Valve, Circulation 39 (Suppl. 1): 47, 1964. 3. Howard, H. S., Willman, V. L., and Hanlon, C. E.: Mitral Valve Eeplacement, S. Forum 11: 256, 1960. 4. Eobicsek, F., Sanger, P. W., Taylor, F. H., and Eobicsek, L.: Transplantabilitv of Heart Valves, Arch. Surg. 84: i41, 1962.
Jack L. Titus, M.D.,
Ph.D.,**
and F. Henry Ellis, Jr.,
M.D.,****
Minn.
A
several studies of the homotransplantation of cardiac valves have . been reported, 1 the morphologic features of the grafted valves at various time intervals after operation have not been described, to our knowledge, in detail. Such a study might help explain the apparent long-term maintenance of some cardiac valve homografts. Therefore, the gross morphologic and histologic features of orthotopically homotransplanted canine mitral valves were studied in animals that had received sufficient antibiotics to prevent bacterial endocarditis but had not received immunosuppressive agents. The surgical tech niques, results, and physiologic data of some of the animals comprising this study have been reported previously. 2 LTHOUGH
MATERIALS AND METHODS
After excision of the recipient dog's mitral valve, a mitral valve from an unrelated donor was obtained and immediately sutured into place in its exact anatomic position.2 The donor valve consisted of the anterior and posterior mitral leaflets in continuity with a thin rim of the annulus fibrosus of the mitral valve and the chordae tendineae of the mitral valve in continuity with the tips of the papillary muscles. Fifteen animals survived more than 10 days and had no clinical or mor phologic evidence of bacterial endocarditis. Of these 15 animals, 11 form the basis for the morphologic study reported herein. The remaining 4, currently alive and in apparent good health, are being traced for evaluation of the results after more than 1 year. Morphologic examination was carried out at the follow ing time intervals after operation: 10 days (one animal), 2 weeks (two animals), 3 weeks (three animals), 3 months (one animal), 4 months (one animal), 5 months (one animal), 8 months (one animal), and 12 months (one animal). Prom the Mayo Clinic and Mayo Foundation. Rochester, Minn. Received for publication July 23, 1964. *Fellow in Pathology. ** Section of Experimental and Anatomic Pathology. ***Fellow in Thoracic Surgery. ****Section of Surgery. 504
Vol. 49, No. 3 March, 1965
HOMOTRANSPLANTED CANINE MITRAL VALVES
505
Complete necropsy was performed on all animals. Blocks for histologic sections were taken of the mitral valve annulus, the valve leaflets, the chordae tendineae, and the papillary muscles. After formalin fixation and paraffin embedding, histologic sections were cut at a thickness of 8 microns and stained with hematoxylin and eosin, Lawson's elastic stain with van Gieson counterstain, the Mallory-Heidenhain stain, the Gram stain, the von Kossa stain, the Prussian blue reaction, and the silver chromate stain.
Fig. 1.—Anterior leaflet of homografted mitral valve, 10 days after transplantation, was cov ered by fibrin (Lawson's elastic van Gieson; X15).
RESULTS
Ten days after operation, 1 animal succumbed of pulmonary edema, which resulted from partial separation of the anterolateral papillary muscle that produced mitral insufficiency. Except for the separation of the anterolateral papillary muscle, the valve graft was firmly attached at all points. Pale yellow fibrin covered all surfaces of the grafted valve leaflets, annulus, chordae tendineae, and papillary muscle tips. This fibrinous coat also extended onto the adjacent endocardial surfaces of the recipient heart. The grafted valve leaflets were moderately thickened but pliable. Histologically, histiocytes were present on the surface of the fibrin covering the graft and the adjacent endocardium (Fig. 1). Lesser numbers of histiocytes also were present in the deeper layers of the fibrinous covering. No bacteria or fungi were observed. The collagen of the grafted valve leaflet appeared to be necrotic in that there was loss of fibrillar detail and fragmentation of the fibrils. The fibroblastic nuclei of the valve stroma were pyknotic. In addition, hemorrhagic edema and a few round cells were present in the grafted valve leaflet. A moderately heavy inflammatory cellular infiltrate was observed at the valve annulus fibrosus where donor tissue joined recipient tissue. This inflammatory cellular reaction consisted mainly of histiocytes and lymphocytes, although a few polymorphonuclear leukocytes were present. Like the collagen of the valve annulus, the collagen of the chordae
506
V A N V L I E T E T AL.
J. Thoracic and Cardiovas. Surg.
tendineae appeared to be partially necrotic, but the chordae tendineae were intact. The grafted tips of the papillary muscle were necrotic and covered by fibrin. At the junctional zone between the graft and recipient papillary muscles, an intense inflammatory reaction was observed. This reaction consisted of large numbers of lymphocytes and histiocytes, with a few polymorphonuclear leu kocytes. In this region, phagocytosis of the necrotic graft muscle was taking place in association with fibroblastic proliferation. Calcine deposits were present in scattered foci in the surrounding recipient myocardium. Two dogs that had survived 2 weeks after operation were examined. Both animals had died of pneumonia. In both instances, the grafted valves grossly appeared similar to the grafted valve examined 10 days after transplantation, in that a layer of fibrin covered all surfaces of the graft and extended onto the adjacent recipient atrial and ventricular endocardium, and the grafted valves were thickened. However, the thickened valves were pliable and appeared to be competent. Histologically, the layer of fibrin completely covering the graft had undergone organization by granulation tissue. There was no evidence of infection. At the valve annulus, some phagocytosis of fragmented necrotic collagen of the graft had occurred. The chordae tendineae, which were covered by a layer of organized fibrin, showed necrotic changes of the collagen. Phagocy tosis of the necrotic muscle in the grafted tip of the papillary muscle was present, and replacement of the removed muscle by loose fibrous tissue had oc curred. These changes were similar to those observed after 10 days, but more muscle removal and replacement had occurred. Small vessels in the inflammatory reaction of the recipient myocardium showed vasculitis. Focal regions of calcifi cation of the recipient myocardium were observed in the regions of the operative site. Three weeks after operation, 3 dogs were examined. One animal had died of pulmonary edema due to mitral insufficiency resulting from partial separation of a papillary muscle suture line; 1 animal had mitral insufficiency due to partial dehiscence of the anterior mitral leaflet from the annulus of the valve, and the third animal had died of empyema complicating wound infection. The transplanted valve of the third animal was grossly intact. In the other 2 dogs, except for the regions of separation described, the valves were firmly in place. Although the valve leaflets were approximately twice the thickness of the mitral valve of the normal canine, they were distinctly thinner than in the animals examined at 2 weeks. In no instances was morphologic evidence of infection observed. Histologically (Fig. 2, a-e), the fibrinous layer that completely covered all of the grafted tissues was organized by granulation tissue which contained Pig. 2.—Histologic sections 21 days after transplantation, a, Anterior leaflet of homotransplanted mitral valve shows organization of the fibrin layer that covered the valve (hematoxylin and eosin ; X10). 6, Fibrinous layer that covered the valve leaflet in a is seen partially organized and containing mononuclear inflammatory cells (hematoxylin and eosin; X700). c, Annulus flbrosus of grafted mitral valve shows necrotic collagen (bottom and left) partially replaced by granulation tissue (top and right) (hematoxylin and eosin; X50). d, Grafted papillary muscle of mitral valve was necrotic and partially replaced by young fibrous tissue (hematoxylin and eosin; X90). e, Higher power view of portion of papillary muscle in d shows cellular detail of fibrous tissue that replaced grafted muscle (hematoxylin and eosin; X400). (Pigs. 2e and e from Berghuis, J., Rastelli, G. C, Van Vliet, P. D., Titus, J. L,., Swan, H. J. C, and Ellis, P. H., J r . : Circulation, 39[Suppl. 1 ] : 47, 1964. By permission of the publishers, The American Heart Association, Inc.)
Vol. 49, No. 3
March, 1965
HOMOTBANSPLANTED CANINE MITRAL VALVES
Fig. 2.—For legend see opposite
page.
507
508
VANVLIET
ET AL.
J. Thoracic and Cardiovas. Surg.
Pig. 3.—Grafted mitral valve leaflet, 3 months after operation, consisted of three layers : on either surface (top and bottom), a layer of fibrous tissue surrounds a central layer of vacuolated, fragmented, edematous collagen of the original donor leaflet (Mallory-Heidenhain stain ; X16).
scattered round cells (Pig. 2, a and b). The grafted valve was less edematous than at the previous time intervals, but the changes in the collagen of the stroma were more marked in that fragmentation and altered staining were more pronounced. The most peripheral layers of the collagen of the annulus flbrosus of the graft were necrotic, and some phagocytosis of the material was observed (Fig. 2, c). A slight amount of mature granulation tissue was present in this region (Fig. 2, c). In the grafted tips of the papillary muscle, phagocytosis, with replacement by young fibrous tissue, continued as at the previous time intervals (Fig. 2, d and e), so that approximately one half of the necrotic grafted muscle had been replaced. The morphologic features of the grafted valves in animals examined 3, 4, and 5 months after operation were similar. One animal was examined from each time interval, and none appeared infected. The transplanted valves appeared normal on gross examination. All structures of the graft were opaque and ap peared to blend into the adjacent endocardium of the recipient heart. Although the valves were somewhat stiffer than normal valves, they were pliable and competent. Histologically, the grafted valve leaflets appeared to consist of three layers (Fig. 3). On either surface of the leaflet was a layer of young fibrous tissue that appeared to be a continuation of the organized fibrinous layer ob served at earlier time intervals. Between these two layers of young fibrous tissue was a central layer composed of vacuolated, fragmented, edematous collagen and elastic fibers. Within this central layer, which appeared to represent the tissues of the original grafted valve, were vascular channels and lipochrome pigment in addition to the degenerating collagen and elastic fibers. This central layer of degenerating donor valve tissue was less prominent in those valves that were examined at the later time intervals. At these time intervals, the chordae tendineae and the papillary muscles of the grafted valve appeared to have been completely replaced by dense fibrous tissue.
Pig. 4.—Eight months after operation, a, Opened homotransplanted mitral valve shows nearly normal appearance, b, Fibrous tissue completely replaced the grafted papillary muscle (Mallory-Heidenhain stain, X15). c, Anterior leaflet homotransplanted valve shows valve com posed of two thin layers of fibrous tissue surrounding thin central layer of degenerating trans planted valvular tissue (Lawson's elastic van Gieson, Xo). (Pigs. 4a and c from Berghuis, J., Rastelli. G. C. Van Vliet, P. D.. Titus, J. L., Swan, H. J. C, and Ellis. F. H., Jr. : Circulation. 39[Suppl. 1] : 47, 1964. By permission of the publishers, The American Heart Association, Inc.)
The morphologic appearance of the grafted valves 8 and 12 months after operation were similar in each dog that was examined. The gross morphologic appearance of the transplanted mitral valve was similar to that observed at the 3- to 5-month time interval, except that the valve leaflets were thinner and approached, in thickness, the appearance of a normal canine mitral valve (Fig. 4, a). Although pliable, the grafted valve appeared somewhat stiffer than a normal valve. The chordae tendineae were of normal diameter and length. Scar tissue occupied the tips of the papillary muscles in the regions in which grafted muscle had originally existed (Fig. 4, b). Histologically, the appearance was similar to that seen in the 3- to 5-month animals (Fig. 4, c). The only significant difference was that the central layer of the valve leaflet was thinner and more vacuolated than at 3 to 5 months. The scattered elastic fibers and fragmented, altered collagen fibers in association with focal deposits of lipochrome pigment observed were considered to represent the remaining central layer. Inflammation was not present at the annulus fibrosus, but dense connective tissue and regions of apparent cartilaginous change were observed in this region. DISCUSSION
Various studies, such as those of Howard and associates3 and Eobicsek and associates,4 have indicated that masses of fibrin frequently accumulate on the
VANVLIET ET AL.
510
J. Thoracic and Cardiovas. Surg.
homografted cardiac valve and result in various complications, such as embolic phenomena. In some instances these fibrinous accumulations have been associated with bacterial infection.3 In the apparently uninfeeted dogs in our study, small amounts of fibrin were deposited on the grafted valves, but emboli were not observed. Bacteria were not found by morphologic examination. Thus, it might be suspected that instances of apparently excessive fibrin deposits, especially when associated with embolic phenomena, are related to infection and not to the grafting procedure per se. Instances of the apparent survival in a functional state for long periods of canine cardiac valve homografts have been reported. 3 - 4 However, in these studies, serial histologic analysis was not reported in detail and, thus, the possi bility that the graft was replaced by recipient tissues during the healing process remains. In the present study the orthotopically homotransplanted canine mitral valve underwent replacement by host tissues in a manner that included the following. The muscular portions of the graft, namely the tips of the papillary muscles, promptly suffered ischemic necrosis and replacement by fibrous tissue derived from adjacent host tissues. The collagen of the chordae tendineae of the graft underwent necrosis and was replaced by dense fibrous tissue, which ap peared to be derived from the organization of a layer of fibrin deposited on the surfaces. The valve leaflet of the graft underwent slow degeneration and replace ment by fibrous tissue derived from organized fibrin layers on its surfaces. At the annulus fibrosus of the grafted valve, the collagen of the graft was replaced, at least in part, by dense connective tissue and by chondroid tissues. Thus, it appeared that the slow rate of replacement of the grafted valve by recipient tissues permitted continued function of the graft, at least for periods of up to 1 year. SUMMARY
The gross morphologic and histologic features of the mitral valves in 11 dogs in which orthotopic homotransplantation of mitral valves were carried out were studied at eight time intervals of 10 days to 1 year after operation. The results indicate that slow replacement of the grafted tissues occurred in such a manner that function of the graft could be maintained. REFERENCES
1. McKenzie, M. B., and Ellis, F. H., Jr.: Current Concepts in Surgical Correction of Acquired Mitral Insufficiency, Circulation 28: 603, 1963. 2. Berghuis, J., Eastelli, G. C, Van Vliet, P. D., Titus, J. L., Swan, H. J. C, and Ellis, F. H., Jr.: Homotransplantation of the Canine Mitral Valve, Circulation 39 (Suppl. 1): 47, 1964. 3. Howard, H. S., Willman, V. L., and Hanlon, C. E.: Mitral Valve Eeplacement, S. Forum 11: 256, 1960. 4. Eobicsek, F., Sanger, P. W., Taylor, F. H., and Eobicsek, L.: Transplantabilitv of Heart Valves, Arch. Surg. 84: i41, 1962.