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The intima: Historic literature revisited
==========
AnnALS
or AMTOI'IY = = = = = = = = = =
The mtima: Historic literature revisited Annette Draeger and Ceri England
Institute of Anatomy, Department of Cell Biology, University of Bern, BtihlstraBe 26, CH-3000 Bern 9, Switzerland
Key words: Smooth muscle - Endothelium - Blood vessel - Development - Review It is ironic, but nonetheless true, that a structure's physio-
logical significance is very often brought to light only in situations of stress or under pathological conditions. And if there is one supreme example of how a tissue's discrete role in Nature may be obtruded on our notice by the changes thereby incurred, then it is the intima. It is to these modifications that the leading causes of death in America and Europe may be ultimately traced: myocardial infarction and cerebral thrombosis. The architecture of the vascular wall, and the status of the intima therein, has long fascinated scientists (Remak 1850). The intima itself was known to become manifest shortly after birth and to progressively gain in thickness thereafter (von Ebner 1870; Thoma 1888). And an early awareness that cells derived from this layer were possibly involved in atherosclerotic plaque-formation (Gimbert 1865; Langhans 1866) doubtless prompted research into its structure during the latter half of the nineteenth century. Moreover, the recent appearance of several reviews bearing on this enigmatic structure (Owens 1995; Schwartz et al. 1995) serves to show that the subject still has its allurements. Indeed, when one reads the manuscripts written more than a century ago in the light of current knowledge, it soon becomes apparent that many of the questions then addressed remain unsolved and a subject of speculation to this day. Most of the historic studies dealing with intimal structure employed material derived either from the aorta itself or from one of its principal tributaries (von Ebner 1870; Key-Aberg 1881; Thoma 1888). Although the vascular wall was generally recognized as being composed of several layers (Henle 1841; Kblliker 1867; von Ebner 1870; Key-Aberg 1881), there was some disagreement respecting the intima's confines. For Eberth (1871), this Correspondence to: Annette Draeger Ann Anat (1998) 180: 189-192 © Gustav Fischer Verlag
"intermediate" layer was clearly demarcated by the endothelium on the one side and by the internal elastic membrane on the other; but the intima was otherwise described as being contained within "certain stripy layers" (Langhans 1866; von Ebner 1870) or referred to as a "fine elastic network" (Ranvier 1888). One has to bear in mind that during the latter half of the nineteenth century, tissue fixation was in its infancy, and the techniques employed, e. g., air-drying (Langhans 1866), boiling or acidtreatment (von Ebner 1870), by no means facilitated a comparison of data. And this is doubtless reflected in the diversification of observations made and in the terminology employed by the investigators at this time. It was not until 1904 that formalin-fixation and freezing were adopted as accepted methods for the preservation of smooth muscle tissue (Grtitzner 1904). Connective tissue and elastic elements were deemed to be the main structural components of the intima (Ranvier 1888), the large observed quantity of the latter (Kblliker 1867) being probably an artefactual manifestation of the air-drying technique employed. But the widespread occurrence of longitudinally-orientated layers of smooth muscle was also reported by many authors (Remak 1850; Kblliker 1867; von Ebner 1870; Bardeleben 1878), and intimal contraction was believed to be instrumental in maintaining luminal patency (Bardeleben 1878). Frequent observations of longitudinal folds within the vessel wall (Henle 1841; Krause 1876) were perhaps not surprising considering that most of the specimens had undergone post-mortem contraction. Sustenance of this not inconsiderable intimal layer (Fig. 1) also posed food for thought, since vasa vasorum were observed to penetrate no deeper than the tunica media. Though admonishing himself for such an "adventurous thought", Virchow (1856) concluded that it was impossible for the inner segments of blood vessels to be sustained other than by the "circulating fluid", in analogy to synovial fluid supplying nutrients to joint cartilage.
A question which naturally sprang to the minds of these early investigators was whether cells comprising the intima were derived from the endothelium delimiting it on its juxtaluminal aspect, or from the tunica media on the other. The "flat" microscopic appearance of cells within layers immediately adjacent to, and teased away from, the endothelium in native specimens, suggested to Remak (1850) that they were of endothelial origin. And their close proximity and structural resemblance to the endothelium had intimated the same to Henle (1841). Surprising as it may seem, experimental evidence in support of this hypothesis has only recently been forthcoming (DeRuiter et al. 1997). Von Ebner's interesting theory (1870) that the intima was derived from blood cells invading the subendothelial space has not, to our knowledge, been followed up. Indeed, most later studies were conducted along the lines that intimal cells were derived from fully differentiated smooth muscle cells within the tunica media, which subsequently underwent changes in response to stimuli such as injury. In the latter half of the
nineteenth century, vascular smooth muscle cells were envisaged as arising by a mechanically induced transformation of connective tissue elements into "contractile" fibrous ones (Virchow 1856). When one considers the relatively crude means at the disposal of these early investigators, it is indeed astonishing that they were nonetheless able to distinguish cells phenotypically distinct from smooth muscle within the intima (Henle 1841; Remak 1850; Kolliker 1867; von Ebner 1870; Eberth 1871; Key-Aberg 1881), these being described or drawn as "star-like" in appearance by Langhans [(1866); Figure 2 (top)], Kolliker (1867), von Ebner (1870) and Key-Aberg [(1881); Fig. 2 (bottom)]. The silver-enhancement technique combined with air-drying employed by the latter can, however, yield "star-like" artefacts (Draeger, unpublished observation). And since an accurate assessment of cell size was not possible at this time, nor any objective indication of magnification furnished with the drawings, it is difficult to ascertain whether the stellate "cells" perceived by the authors
Fig. 1. Part of a longitudinal section through the aorta, as depicted by Key-Aberg. a = four-layered transitional zone containing several longitudinally orientated smooth muscle cells; b =portion of the intima bordering (a); c = innermost layers of the tunica media ("sketched"); d = zone of elastic fibres running in the main obliquely and transversely. Tissue fixed in wood vinegar. [Figure legend adapted from KeyAberg (1881)].
190
(Langhans 1866; Key-Aberg 1881) do actually correspond to those of "stellate outline" observed ultrastructurally in the avian-aorta intima more than a century later by Moss and Benditt (1970). This question of "outline" was indeed a critical one in these early studies, the putative existence of cell boundaries having been much debated (Kolliker 1866; Eberth 1871). Their apparent absence can doubtless be ascribed to poor tissue-preservation quality in airdried specimens, or to inadequate staining, and the system of channels observed to surround individual intimal cells after silver-enhancement [(Key-Aberg 1881); Fig. 3] may likewise have been attributable to shrinkage in association with the drying technique. This historic literature was sadly overlooked by later workers in the field, who believed the intima and tunica
media to be composed of a single pool of smooth muscle
cells. Indeed, it is only recently that the concept of a unique intimal cell-type has gained, or rather regained, foothold - for we cannot lay claim to originality on this score. And so we come back full circle to these pioneering studies. Acbowledcements. The authors wish to thank Prof. W. Jacobson (Dept. of Pediatrics, University of Cambridge) and Dr. W. B. Amos (Medical Research Council, Laboratory of Molecular Biology, Cambridge) for helpful advice, P. Burkhalter (Department for Historical Medicine, University of Bern) for bibliographic assistance and B. Krieger for photographic expertise. This study was in part supported by a grant from the Swiss National Science Foundation (No. 31-44379.95)
Fig. 2. Star-like cells within the intima, as drawn by Langhans (top) and Key-Aberg (bottom). TIssue was fixed either in Milller's fluid (top) or chromic acid [with haematoxylin-staining (bottom)}. [Figure legend adapted from Langhans (1866) and KeyAberg (1881)} .
191
.n9· 10
Fig. 3. Cellular network within the intima (left-hand side of picture), and adjoining endo~heli~ ("depic~ed wi~hout nuclei to avoid confusion"). Fine pellicle of detached aortal tissue fixed in Hollenstein's solution and stamed WIth Carmme. [Figure legend adapted from (Key-Aberg, 1881)].
References Bardeleben K (1878) Ueber den Bau der Arterienwand. In: Sitzungsberichte der Jenaischen Gesellschaft fur Medizin und Naturwissenschaft, Jena pp XXXIV-XLIX DeRuiter MC, Poelmann RE, VanMunsteren JC, Mironov V, Markwald RR, Gittenberger-de Groot AC (1997) Embryonic endothelial cells transdifferentiate into mesenchymal cells expressing smooth muscle actins in vivo and in vitro. Circ Res 80: 444-451 Eberth CJ (1871) Von den Blutgefassen. In: S Stricker (ed) Handbuch der Lehre von den Geweben des Menschen und der Thiere. Wilhelm Engelmann, Leipzig, pp 191-213 Gimbert (1865) Memoire sur la structure et sur la texture des arteres. Journal de l'anatomie et de la physiologie, 2nd annual volume: 616--652 Grtitzner P (1904) Die glatten Muskeln . Ergebnisse der Physiologie 3. II: 12-88 Henle J (1841) Vom Systeme der Blutgefasse. In: Allgemeine Anatomie. Leopold Voss, Leipzig, pp 473-577 Key-Aberg A (1881) Ueber den Bau der Tunica intima der Aortenwand bei dem erwachsenen Menschen. In: Biologische Untersuchungen von Gustav Retzius, FCW Vogel, Leipzig, pp 27-50 Kolliker A (1867) Von den Blutgefassen. In: Handbuch der Gewebelehre des Menschen. Wilhelm Engelmann, Leipzig, pp 581-598
Krause W (1876) Blutgefasse. In: Allgemeine und microscopische Anatomie. Hahn'sche Hofbuchhandlung, Hannover, pp 304-322 Langhans T (1866) Beitrage zur normalen und pathologischen Anatomie der Arterien. Arch path Anat Physiol36: 187-226 Moss NS, Benditt EP (1970) Spontaneous and experimentally induced arterial lesions. I. An ultrastructural survey of the normal chicken aorta. Lab Invest 22: 166-183 Owens GK (1995) Regulation of differentiation of vascular smooth muscle. Physiol Rev 75: 487-517 Ranvier L (1888) Arterien. In: Technisches Lehrbuch der HistoLogie. FCW Vogel, Leipzig, pp 516-598 Remak R (1850) Histologische Bemerkungen uber die Blutgefasswande. Muller's Arch Anat Physioll: 79-101 Schwartz SM, deBlois D, O'Brien ERM (1995) The intima: soil for atherscIerosis and restenosis. Circ Res 77: 445-465 Thoma R (1888) Ueber die Abhangigkeit der Bindegewebsneubildungen in der Arterienintima von den mechanischen Bedingungen des Bluturnlaufs. Arch path Anat Physiol 93: 443- 505 Virchow R (1847) Ueber die acute Entzundung der Arterien. Arch path Anat Physiol 1: 272-284 von Ebner V (1870) Ueber den Bau der Aortenwand, besonders der Muskelhaut derselben. In: A Rollett (ed) Untersuchungen aus dem Institute fUr Histologie und Physiologie in Graz. Wilhelm Engelmann, Leipzig, pp 32-58 Accepted October 30, 1997
192
AnnALS
or AMTOI'IY = = = = = = = = = =
The mtima: Historic literature revisited Annette Draeger and Ceri England
Institute of Anatomy, Department of Cell Biology, University of Bern, BtihlstraBe 26, CH-3000 Bern 9, Switzerland
Key words: Smooth muscle - Endothelium - Blood vessel - Development - Review It is ironic, but nonetheless true, that a structure's physio-
logical significance is very often brought to light only in situations of stress or under pathological conditions. And if there is one supreme example of how a tissue's discrete role in Nature may be obtruded on our notice by the changes thereby incurred, then it is the intima. It is to these modifications that the leading causes of death in America and Europe may be ultimately traced: myocardial infarction and cerebral thrombosis. The architecture of the vascular wall, and the status of the intima therein, has long fascinated scientists (Remak 1850). The intima itself was known to become manifest shortly after birth and to progressively gain in thickness thereafter (von Ebner 1870; Thoma 1888). And an early awareness that cells derived from this layer were possibly involved in atherosclerotic plaque-formation (Gimbert 1865; Langhans 1866) doubtless prompted research into its structure during the latter half of the nineteenth century. Moreover, the recent appearance of several reviews bearing on this enigmatic structure (Owens 1995; Schwartz et al. 1995) serves to show that the subject still has its allurements. Indeed, when one reads the manuscripts written more than a century ago in the light of current knowledge, it soon becomes apparent that many of the questions then addressed remain unsolved and a subject of speculation to this day. Most of the historic studies dealing with intimal structure employed material derived either from the aorta itself or from one of its principal tributaries (von Ebner 1870; Key-Aberg 1881; Thoma 1888). Although the vascular wall was generally recognized as being composed of several layers (Henle 1841; Kblliker 1867; von Ebner 1870; Key-Aberg 1881), there was some disagreement respecting the intima's confines. For Eberth (1871), this Correspondence to: Annette Draeger Ann Anat (1998) 180: 189-192 © Gustav Fischer Verlag
"intermediate" layer was clearly demarcated by the endothelium on the one side and by the internal elastic membrane on the other; but the intima was otherwise described as being contained within "certain stripy layers" (Langhans 1866; von Ebner 1870) or referred to as a "fine elastic network" (Ranvier 1888). One has to bear in mind that during the latter half of the nineteenth century, tissue fixation was in its infancy, and the techniques employed, e. g., air-drying (Langhans 1866), boiling or acidtreatment (von Ebner 1870), by no means facilitated a comparison of data. And this is doubtless reflected in the diversification of observations made and in the terminology employed by the investigators at this time. It was not until 1904 that formalin-fixation and freezing were adopted as accepted methods for the preservation of smooth muscle tissue (Grtitzner 1904). Connective tissue and elastic elements were deemed to be the main structural components of the intima (Ranvier 1888), the large observed quantity of the latter (Kblliker 1867) being probably an artefactual manifestation of the air-drying technique employed. But the widespread occurrence of longitudinally-orientated layers of smooth muscle was also reported by many authors (Remak 1850; Kblliker 1867; von Ebner 1870; Bardeleben 1878), and intimal contraction was believed to be instrumental in maintaining luminal patency (Bardeleben 1878). Frequent observations of longitudinal folds within the vessel wall (Henle 1841; Krause 1876) were perhaps not surprising considering that most of the specimens had undergone post-mortem contraction. Sustenance of this not inconsiderable intimal layer (Fig. 1) also posed food for thought, since vasa vasorum were observed to penetrate no deeper than the tunica media. Though admonishing himself for such an "adventurous thought", Virchow (1856) concluded that it was impossible for the inner segments of blood vessels to be sustained other than by the "circulating fluid", in analogy to synovial fluid supplying nutrients to joint cartilage.
A question which naturally sprang to the minds of these early investigators was whether cells comprising the intima were derived from the endothelium delimiting it on its juxtaluminal aspect, or from the tunica media on the other. The "flat" microscopic appearance of cells within layers immediately adjacent to, and teased away from, the endothelium in native specimens, suggested to Remak (1850) that they were of endothelial origin. And their close proximity and structural resemblance to the endothelium had intimated the same to Henle (1841). Surprising as it may seem, experimental evidence in support of this hypothesis has only recently been forthcoming (DeRuiter et al. 1997). Von Ebner's interesting theory (1870) that the intima was derived from blood cells invading the subendothelial space has not, to our knowledge, been followed up. Indeed, most later studies were conducted along the lines that intimal cells were derived from fully differentiated smooth muscle cells within the tunica media, which subsequently underwent changes in response to stimuli such as injury. In the latter half of the
nineteenth century, vascular smooth muscle cells were envisaged as arising by a mechanically induced transformation of connective tissue elements into "contractile" fibrous ones (Virchow 1856). When one considers the relatively crude means at the disposal of these early investigators, it is indeed astonishing that they were nonetheless able to distinguish cells phenotypically distinct from smooth muscle within the intima (Henle 1841; Remak 1850; Kolliker 1867; von Ebner 1870; Eberth 1871; Key-Aberg 1881), these being described or drawn as "star-like" in appearance by Langhans [(1866); Figure 2 (top)], Kolliker (1867), von Ebner (1870) and Key-Aberg [(1881); Fig. 2 (bottom)]. The silver-enhancement technique combined with air-drying employed by the latter can, however, yield "star-like" artefacts (Draeger, unpublished observation). And since an accurate assessment of cell size was not possible at this time, nor any objective indication of magnification furnished with the drawings, it is difficult to ascertain whether the stellate "cells" perceived by the authors
Fig. 1. Part of a longitudinal section through the aorta, as depicted by Key-Aberg. a = four-layered transitional zone containing several longitudinally orientated smooth muscle cells; b =portion of the intima bordering (a); c = innermost layers of the tunica media ("sketched"); d = zone of elastic fibres running in the main obliquely and transversely. Tissue fixed in wood vinegar. [Figure legend adapted from KeyAberg (1881)].
190
(Langhans 1866; Key-Aberg 1881) do actually correspond to those of "stellate outline" observed ultrastructurally in the avian-aorta intima more than a century later by Moss and Benditt (1970). This question of "outline" was indeed a critical one in these early studies, the putative existence of cell boundaries having been much debated (Kolliker 1866; Eberth 1871). Their apparent absence can doubtless be ascribed to poor tissue-preservation quality in airdried specimens, or to inadequate staining, and the system of channels observed to surround individual intimal cells after silver-enhancement [(Key-Aberg 1881); Fig. 3] may likewise have been attributable to shrinkage in association with the drying technique. This historic literature was sadly overlooked by later workers in the field, who believed the intima and tunica
media to be composed of a single pool of smooth muscle
cells. Indeed, it is only recently that the concept of a unique intimal cell-type has gained, or rather regained, foothold - for we cannot lay claim to originality on this score. And so we come back full circle to these pioneering studies. Acbowledcements. The authors wish to thank Prof. W. Jacobson (Dept. of Pediatrics, University of Cambridge) and Dr. W. B. Amos (Medical Research Council, Laboratory of Molecular Biology, Cambridge) for helpful advice, P. Burkhalter (Department for Historical Medicine, University of Bern) for bibliographic assistance and B. Krieger for photographic expertise. This study was in part supported by a grant from the Swiss National Science Foundation (No. 31-44379.95)
Fig. 2. Star-like cells within the intima, as drawn by Langhans (top) and Key-Aberg (bottom). TIssue was fixed either in Milller's fluid (top) or chromic acid [with haematoxylin-staining (bottom)}. [Figure legend adapted from Langhans (1866) and KeyAberg (1881)} .
191
.n9· 10
Fig. 3. Cellular network within the intima (left-hand side of picture), and adjoining endo~heli~ ("depic~ed wi~hout nuclei to avoid confusion"). Fine pellicle of detached aortal tissue fixed in Hollenstein's solution and stamed WIth Carmme. [Figure legend adapted from (Key-Aberg, 1881)].
References Bardeleben K (1878) Ueber den Bau der Arterienwand. In: Sitzungsberichte der Jenaischen Gesellschaft fur Medizin und Naturwissenschaft, Jena pp XXXIV-XLIX DeRuiter MC, Poelmann RE, VanMunsteren JC, Mironov V, Markwald RR, Gittenberger-de Groot AC (1997) Embryonic endothelial cells transdifferentiate into mesenchymal cells expressing smooth muscle actins in vivo and in vitro. Circ Res 80: 444-451 Eberth CJ (1871) Von den Blutgefassen. In: S Stricker (ed) Handbuch der Lehre von den Geweben des Menschen und der Thiere. Wilhelm Engelmann, Leipzig, pp 191-213 Gimbert (1865) Memoire sur la structure et sur la texture des arteres. Journal de l'anatomie et de la physiologie, 2nd annual volume: 616--652 Grtitzner P (1904) Die glatten Muskeln . Ergebnisse der Physiologie 3. II: 12-88 Henle J (1841) Vom Systeme der Blutgefasse. In: Allgemeine Anatomie. Leopold Voss, Leipzig, pp 473-577 Key-Aberg A (1881) Ueber den Bau der Tunica intima der Aortenwand bei dem erwachsenen Menschen. In: Biologische Untersuchungen von Gustav Retzius, FCW Vogel, Leipzig, pp 27-50 Kolliker A (1867) Von den Blutgefassen. In: Handbuch der Gewebelehre des Menschen. Wilhelm Engelmann, Leipzig, pp 581-598
Krause W (1876) Blutgefasse. In: Allgemeine und microscopische Anatomie. Hahn'sche Hofbuchhandlung, Hannover, pp 304-322 Langhans T (1866) Beitrage zur normalen und pathologischen Anatomie der Arterien. Arch path Anat Physiol36: 187-226 Moss NS, Benditt EP (1970) Spontaneous and experimentally induced arterial lesions. I. An ultrastructural survey of the normal chicken aorta. Lab Invest 22: 166-183 Owens GK (1995) Regulation of differentiation of vascular smooth muscle. Physiol Rev 75: 487-517 Ranvier L (1888) Arterien. In: Technisches Lehrbuch der HistoLogie. FCW Vogel, Leipzig, pp 516-598 Remak R (1850) Histologische Bemerkungen uber die Blutgefasswande. Muller's Arch Anat Physioll: 79-101 Schwartz SM, deBlois D, O'Brien ERM (1995) The intima: soil for atherscIerosis and restenosis. Circ Res 77: 445-465 Thoma R (1888) Ueber die Abhangigkeit der Bindegewebsneubildungen in der Arterienintima von den mechanischen Bedingungen des Bluturnlaufs. Arch path Anat Physiol 93: 443- 505 Virchow R (1847) Ueber die acute Entzundung der Arterien. Arch path Anat Physiol 1: 272-284 von Ebner V (1870) Ueber den Bau der Aortenwand, besonders der Muskelhaut derselben. In: A Rollett (ed) Untersuchungen aus dem Institute fUr Histologie und Physiologie in Graz. Wilhelm Engelmann, Leipzig, pp 32-58 Accepted October 30, 1997
192