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Comparison of aortic lamellar unit structure in birds and mammals
Atherosclerosis,
19 (1974) 47-59
47
~0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
COMPARISON OF AORTIC AND MAMMALS
C. L. BERRY,
JOCELYN
GERMAIN
LAMELLAR
AND
UNIT
PATRICIA
STRUCTURE
IN BIRDS
LOVELL
Department of Pathology, Guy’s Hospital Medical School, London, SEI 9R7 (Great Britain)
(Received February 7th, 1973)
SUMMARY
The structure of the aortic media at different levels in a number of orders of birds has been examined. In general the avian aorta shows a laminar structure, but differs from the mammalian vessel in the occurrence of broad and distinct smooth muscle cell and connective tissue layers in its proximal part. Part of the vessel has a structure which resembles that of mammals; the aorta distal to the origin of the renal arteries is a mixed musculo-elastic structure. Differences between avian and mammalian aortic physiology and their relevance to structural findings are discussed.
Key words: Atherogenesis - Avian aorta - Elasticity - Laminar structure - Physiological properties
INTRODUCTION
We have recently described the chemical and morphological development of the aorta in the rat and in manl-3. Mechanical factors affect the histogenesis of the media of large vessels, and their effects have been discussed in detail elsewhere4. The results of these studies suggest that aortic medial structure is determined in a large part by the stresses acting on it during growth. The media has well defined mechanical characteristics, and consists of a “two phase” system of collagen and elastin. The circumferentially-aligned collagen fibres bear tangential stressing forces, while the elastin network distributes these forces uniformly throughout the walls,s. The components of the media are arranged in a laminar structure composed of smooth muscle cells attached to compact elastic lamellae with interlamellar elastin running between the cells (Fig. 1). This structure develops as the aorta grows, the number of lamellar units being related to the vessel diameter and hence to the tension in the wal17s.
48
C. L. BERRY, J. GERMAIN, P. LOVELL
Fig. 1. Diagrammatic representation of lamellar structure of adult aortic media in mammals. Smooth muscle cells are apparently attached to dense elastic lamellae with finer fibres running between many cells. Collagen is shown as a stippled investment of the elastic network.
Fig. 2. Pheasant aortic root, showing double lamellar structure, with darker smooth muscle bands and pale scleroprotein and GAC-containing zones. Toluidine blue; x 500.
AORTIC LAMELLAR
STRUCTURE IN BIRDS AND MAMMALS
49
The lamellar unit “two phase” structure is found in many mammalian groupssp7, including diving mammalss. In primates, reduction in the number of lamellar units in the aortic media is consistent as the aorta diminishes in diameterr0Jr. The extensive distribution of this type of structure in different species and the way in which it is modified by mechanical factor@ suggests that it has considerable biological advantages as a method of handling and “damping” pulsatile blood flow. With these considerations in mind we have undertaken a study of the morphology of the avian aorta. Since physical factors are considered to be of considerable significance in atherogenesis 12,13the structure of the aortic media, determined as it is by stresses acting during growth and development, may be of importance in experimental design and interpretation of work on avian atherogenesisr4-17. Previous studies of normal aortic structure in birds are few. Argaudr8 studied the eagle, Pfisterig examined a number of species, and more recently Moss and Benditt20 have carried out a detailed study on the chick. Finlayson2r included birds in his extensive report on arterial disease in many animals, and from his comments and illustrations useful information may be obtained about the normal histology of the avian aorta. MATERIALS AND METHODS
The following birds were examined : Anseriformes (duck-Anus platyrhyncha pfatyrhyncha), Galliformes (pheasant-P/z&anus colchicus, chicken-Gallus domesticus), Columbiformes (pigeon-Columba palumbus palumbus), Charadriiformes (woodcockScolapax rusticola rusticola) and Passeriformes (thrush-Turdus ericetorum ericetorum).
Fig. 3. Line drawing of pheasant dorsal aorta. Smooth muscle zones are shown stippled. ing nature of the network is evident. x 120.
The branch-
50
C. L. BERRY, J. GERMAIN, P. LOVELL
Fig. 4. Detail from Fig. 3. The irregular thickness of the smooth muscle lamellae is caused by variations in pitch of the helices of cells in different areas. x 5.50.
Fig. 5. Pheasant aorta. Here only smooth muscle cells and elastic tissue are shown. The two distinct zones are evident. x 800.
AORTIC
LAMELLAR
STRUCTURE
IN BIRDS AND MAMMALS
51
Fig. 6. Electron photomicrograph of duck aortic root. Darkly staining smooth muscle cells are seen in distinct laminae, the elastic tissue is homogeneous, collagen fibres are clearly seen. The pale cells in this zone are the so-called “inter-1amellar” cells. x 1200.
All birds were apparently
young
adults.
Game
birds were shot with a Holland
and
Holland 12 bore sporting gun, using “Impax” number 6 cartridges. The aorta was removed entire within 20 min of death and fixed in buffered formalin. Small (1 mm) segments of the aorta from other birds were fixed in glutaraldehydeandpost-osmicated. In all birds blocks were taken from the root of the aorta, from the innominate arteries, from the dorsal aorta above the intercostal branches and immediately above the coeliac artery, from immediately below the anterior mesenteric artery, below the renal arteries and below the trifurcation at which the sciatic arteries arise. After embedding in epoxy resin 2 ,U sections were cut on an LKB “Pyramitome” or ultramicrotome and stained with 1% toluidine blue in 1% borax. Electron microscopy was carried out on osmium-treated material.
52
C. L. BERRY, J. GERMAIN, P. LOVELL
Fig. 7. Woodcock dorsal aorta. Mammalian type of structure with smooth muscle attached to elastic tissue. Occasional vacuolated cells are seen. Tohtidine blue: x 800.
Fig. 8. Duck aorta between coeliac and anterior mesenteric arteries. Transitional x 100. and single lamina unit structure is shown. Van Giesonelastic;
zone between double
AORTIC LAMELLAR STRUCTURE IN BIRDS AND MAMMALS
53
Fig. 9. Pheasant aorta. Part muscular part elastic structure with well-defined IEL and smooth muscle cells in elastic network. For clarity some smooth muscle cell nuclei are omitted, as is some of the outer elastic tissue.
After examination of a number of 2 ,u sections, routine paraffin-processing was also carried out on formalin-fixed material and 5 ,u sections were examined for glycosaminoglycans (GAC) by the method of Scott and Dorlingz2. RESULTS
In general the basic aortic medial structure was similar in all birds examined. The aortic trunk showed a well-defined laminar structure, but with two distinct types of lamellar unit present. These consisted of smooth muscle cells arranged in compact bands, with marginal elastic laminae separated by clear zones containing collagen and elastin and in addition material shown by special staining of paraffin sections to be mainly carboxylated GAC (Fig. 2). These two lamellar structures formed a complex branching network (Figs. 3 and 4). Fig. 5 shows the structural organisation in detail, here only smooth muscle cells and elastic tissue are shown, cells between the
54
C. L. BERRY, J. GERMAIN, P. LOVELL
Fig. 10. Pheasant infrarenal aorta. Smooth muscle cells are seen in a distinct network of elastic tissue with a tendency to lamellar arrangement. Toluidine blue; x 550.
compact zones are omitted. Fig. 6 is an electron photomicrograph of this region and shows smooth muscle lamellae with elastic tissue, collagen and amorphous material between them. In all birds examined, this type of structure was seen in the aorta as far as the origin of the coeliac artery and in the innominate arteries. Distal to this a more “mammalian” lamellar structure was seen with smooth muscle cells running between well-defined elastic lamellae in helices of Varying pitch (Fig. 7). Occasional muscle cells appeared pale and contained vacuoles. The transitional zone between the two structures was short and was generally found between the coeliac and anterior mesenteric arteries (Fig. 8). This “mammal’‘-like structure is short lived; from the level of the renal arteries the aorta assumes a partly muscular, partly elastic structure which is quite unlike the mammalian artery (Fig. 9). The internal elastic lamina (IEL) is well-defined and complete; this is not found in the proximal aorta. There is a zone of smooth muscle cells enmeshed in elastic tissue, with some tendency to lamellar arrangement (Fig. 10) and with areas of longitudinally and transversely-running fibres (Fig. 11). Outside this are a number of elastic lamellae with interspread smooth muscle cells. Fig. 12 is an electron photomicrograph showing the relationship between smooth muscle cells and intercellular elastin.
AORTIC LAMELLAR
STRUCTURE
IN BIRDS AND MAMMALS
55
Fig. 11. Similar zone with IEL at top right, and outer lamellae at bottom left. An inner longitudinal
and outer circular arrangement of muscles is seen. Toluidine blue; x 800. DISCUSSION
There are some important
anatomical
great vessels. In the birds the systemic systemic
trunk
differences
arteries
between avian and mammalian
resemble
close to the heart two short innominate
those of reptile+. arteries
From the
arise and divide into
carotid arteries and sub-clavian vessels, respectively supplying the head and neck and wing and its muscles. The development of the sub-clavian vessels is singular and does not resemble
that in mammals 23. The continuation
of the aorta
is smaller
than the
parent trunk and gives off vessels in a normal vertebrate plan, paired intercostal and lumbar arteries corresponding to the intervals between vertebrae, ventral vessels running to the viscera, and the sciatic arteries supplying the posterior part of the kidneys as well as the leg+. In addition, important physiological differences exist between the two groups. The mammalian vascular tree is nonuniform in terms of its elastic behaviour, i.e.it becomes increasingly stiff along its length 25. This confers particular advantages, notably in isolating the low impedance of the proximal large arteries from the high impedance termination of the peripheral resistance, and in reducing the oscillatory component of the work required to maintain the cardiac output. Further, the overall distensibility of the system is reduced by stiff peripheral vessels, providing the advantage of rapid adaptation to pressure changes and reducing the tendency to cause large volume changes in veins. The vessels of the goose and turkey behave quite differently2s.
56
C. L. BERRY, J. GERMAIN, P. LOVELL
Fig. 12. Electron photomicrograph of duck infrarenal aorta. The close relationship cells to surrounding elastic tissue is evident. x 2000.
of smooth muscle
and resemble the classical windkessel, i.e. a system of uniform distensibility and increasing cross-sectional area. From the data of Newman, Gosling and Bowden it appears that the chicken aorta behaves in the same way. A change implicit in these findings is that the pulse form shows virtually no change along the aorta and arterial branches, which act as rigid distributing channels in birds. A further important difference between birds and mammals lies in the relatively high blood pressure of the former group14927. These various factors may account for differences in histological appearances between the two groups. Pfisterls pointed out the absence of an internal elastic lamina in the thoracic aorta in birds, and the close similarity in structure between the large branches and the aorta itself. In Moss and Benditt’s20 more recent study the authors described the structure of the aorta in detail from aortic root to “bifurcation”. They stated that the smooth muscle lamellae in this species were generally incomplete, rarely extending around more than a quarter of the circumference of the vessel, and were clothed on each aspect by a dense elastic layer, whilst finer elastic fibres were found in the interstitial lamellae. Finlayson 21 had noted the presence of intermuscular
AORTIC LAMELLAR
STRUCTURE IN BIRDS AND MAMMALS
57
laminae in the proximal aorta and brachiocephalic trunks of birds and illustrations in his paper show these to be present in the kite, stone curlew and whooper swan. Both papers20921 describe a gradual change in pattern from this type of structure to the mammalian form we have illustrated in Fig. 7. In the more distal vessel, Moss and Benditt20 illustrate an “inner muscular media” with an outer elastic zone. These authors describe the cells of the lamellae of connective tissue and GAC as “interlamellar connective tissue cells”, apparently the “interlamellar” cells illustrated by Cooke and Smith28 in the developing pigeon aorta. Cooke and Smith considered these cells to be like fibroblasts. Recent studies on scleroprotein synthesis by smooth muscle cells2s~a0,together with the work of Cliffsi, suggest that the spindle cell of the aortic media may be pluripotential and that distinctions between different types are invalid. However, the work of Moss and Benditt demonstrates that the cells of the connective tissue laminae have many characteristics that differ from normal smooth muscle cells. Our findings are in general agreement with these previous studies. The occurrence of large amounts of carboxylate GAC in the media of the proximal great arteries may affect the rigidity of the vessel wall due to changes in hydration; changes of this kind occur in mammalian vessels in coarctation and after surgery32. This modification of lamellar unit structure may explain the curious physiological behaviour of avian arteries. We differ from other authors in our view of the structure of the distal aorta. Here there is a distinct internal elastic membrane and the smooth muscle cells found are arranged in a well-defined elastic network with a tendency to lamellar formation. Elastic lamellae are seen in the outer zone. We consider that the changes described by Gresham, Howard and Jennings 14 deep to atherosclerotic lesions in the turkey reflect normal structure rather than a repair phenomenon. Intimal cushions are found in birds17 and other animalssa-39 and may be largely composed of smooth muscle cells. We consider that these structures are distinct from the normal thick smooth muscle cell layer of the distal avian aorta. A possible explanation for this curious structure may be found in a further consequence of the unique physiological behaviour of the aorta in birds. The rigidity of the vessels may result in a reduction in amplitude of variations in the pulse wave: one of the advantages of the mammalian system pointed out by Taylor25 is that disturbances will be amplified in a pulse wave travelling along a tube of decreasing distensibility. Perhaps the musculo-elastic vessels of the bird represent a modification of the basic structure which permits changes in vessel radius to modify the effects of the drastic circulatory changes in the posterior part of the body that must occur during flight. Whatever explanation is preferred, it is evident that the aorta of birds differs from that of mammals, and that these differences are important if mechanical factors are considered to be of major importance in atherogenesis. Although a number of authors have commented on differences between avian and mammalian vascular structure, little has been published on the possible reasons for, or effects of, such differences. Many aspects of atherogenesis may be usefully studied in birds, but, in attempt-
C. L. BERRY,
58
J. GERMAIN,
P. LOVELL
ing to apply the results of such experiments to man, fundamental differences in aortic vessel physiology should be remembered. ACKNOWLEDGEMENTS
We should like to acknowledge the help of Mr. George Lloyd Roberts and friends in obtaining the specimens of game birds studied. The Pyramitome was purchased with a grant from the Medical Research Council.
REFERENCES 1 BERRY, C. L., LOOKER,T., AND GERMAIN,J., The growth and development of the rat aorta, Part 1 (Morphological aspects), J. Anut., 113 (1972) 1. 2 LOOKER,T., AND BERRY, C. L., The growth and development of the rat aorta, Part 2 (Changes in nucleic acid and scleroprotein content), J. Anat., 113 (1972) 17. 3 BERRY, C. L., LOOKER,T., AND GERMAIN,J., Nucleic acid and scleroprotein content of the developing human aorta, J. Puthol., 108 (1972) 265. 4 BERRY, C. L., The growth and development of major arteries. In: D. H. M. WOOLLAM AND G. MORRIS (Eds.), Advances in Embryology and Teratology, Paul Elek (Scientific Books), London, 1973. 5 WOLINSKY,H., AND GLAGOV, S., Structural basis for the static mechanical properties of the aortic media, Circ. Res., 14 (1964) 400. 6 WOLINSKY, H., AND GLAGOV, S., A lamellar unit of aortic medial structure and function in mammals, Circ. Res., 20 (1967) 99. 7 WOLINSKY,H., AND GLAGOV, S., Nature of species differences in the medial distribution of aortic vasa vasorum in mammals, Circ. Rex, 20 (1967) 409. 8 BERRY, C. L., The establishment of elastic structure of arterial bifurcations and branches: Its relevance to medial defects of cerebral arteries, Atherosclerosis, 18 (1973) 117. 9 BERRY, C. L., Unpublished observations. 10 AHMED, M. M., Microscopic anatomy and the attenuation of elastic tissue in the Slow Loris
(Nycticebus Coucang Coucang), Folia Primat., 8 (1968) 290. 11 BERRY, C. L., AND GERMAIN,J., Changes in aortic medial elastic structure at different sites in the aorta of three primates, Lab. Animals, In press. 12 CARO, C. G., FITZ-GERALD, J. M., AND SHROTER, R. C., Arterial wall shear and distribution of early atheroma in man, Nature (London), 223 (1969) 1159. 13 M~KHELL, J. R. A., AND SCHWARTZ, C. J., Arterial Disease, F. A. Davis Co., Philadelphia, 1965, p. 50. 14 GRESHAM,G. A., HOWARD, A, N., AND JENNINGS,I. W., Dietary fat and aortic atherosclerosis in the turkey, J. Pathol. Bacterial. 85 (1963) 291. 15 FINLAYSON, R., SYMONS, C., AND T-W-FIENNES, R. N., Atherosclerosis: A comparative study, Brit. Med. J., i (1962) 501. 16 FINLAYSON,R. AND HIRCHINSON,V., Experimental atheroma in Budgerigars, Nature (London),
192 (1961) 369. 17 SANTERRE,R. F., WIGHT, T. N., SMITH,S. C., AND BRANNIGAN,M. S., Spontaneous atherosclerosis in pigeons, Amer. J. Pathol., 67 (1972) 1. 18 ARGAUD, C.R. Ass. Anatomistes, 1904, cited by H.I.C. PFISTER. 19 PFISTER,H. I. C., On the distribution of the elastic tissue in the blood vessels of birds, J. Anat., 61 (1927) 213. 20 Moss, N. S., AND BENDITT,E. P., Spontaneous and experimentally induced arterial lesions, Part 1 (An ultrastructural survey of the normal chicken aorta), Lab. Invest., 22 (1970) 166. 21 FINLAYSON,R., Spontaneous arterial disease in exotic animals, J. Zool., 147 (19651 239. 22 Scorr, J. E., AND DORLING, J., Differential staining of acid glycosaminoglycans (mucopolysaccharides) by Alcian Blue in salt solutions, Histochemie, 5 (1965) 221.
AORTIC LAMELLAR
STRUCTURE IN BIRDS AND MAMMALS
59
23 CARTER, G. S., Structure and Habit in Vertebrate Evolution, Sidgwick and Jackson, London, 1961, p. 334, 24 BRADLEY, 0. C., AND GRAHAME,T., The Structure of the Fowl, Oliver and Boyd, Edinburgh and London, 1960, p. 80. 25 TAYLOR, M. B., Wave travel in arteries and the design of the cardiovascular system. In: P&utile Blood Flow, McGraw-Hill, New York, Toronto, London, 1964, p. 343. 26 NEWMAN, D. L., GOSLING,R. G., AND BOWDEN,N. L. R., Changes in aortic distensibility and area ratio with the development of atherosclerosis, Atherosclerosis, 14 (1971) 231. 27 WEISS, H. S., AND SHEAHAM,M., The influence of maturity and sex on the blood pressure of the turkey, Amer. J. Vet. Rex, 19 (1958) 209. 28 COOKE, P. H., AND SMITH,S. C., Smooth muscle cells. The source of foam cells in atherosclerotic white Carneau pigeons, Exp. Mol. Pathol., 8 (1968) 171. 29 Ross, R., AND KLEBANOFF,S. J., The smooth muscle cell, Part 1 (m viva synthesis of connective tissue proteins), J. Cell Biol., 50 (1971) 159. 30 Ross, R., The smooth muscle cell, Part 2 (Growth of smooth muscle in culture and formation of connective tissue proteins), J. Cell Biol., 51 (1971) 172. 31 CLIFF, W. J., The aortic tunica media in growing rats studied with the electron microscope, Lab. Invest., 17 (1967) 599. 32 BERRY, C. L., AND TAWES, JR., R. L., Mucopolysaccharides of the aortic wall in coarctation and recoarctation, Cardiovasc. Res., 4 (1970) 224. 33 MUSTARD, J. F., MURPHY, E. A., ROSWELL, H. C., AND DOWNE, H. G., Platelets and atherosclerosis, J. Atheroscler. Res., 4 (1964) 1. 34 GUTSTEIN,W. H., SCHNECK,D. J., AND MARKS, J. O., In vitro studies of blood flow disturbance in the region of separation, J. Atheroscler. Res., 8 (1968) 381. 35 CLARKSON, T. B., PRICHARD, R. W., NETSKY, M. G., AND LOFLAND, H. B., Atherosclerosis in pigeons: its spontaneous occurrence and resemblance to human atherosclerosis, Arch. Pathol., 68 (1959) 143. M. M., GILLOT, P., PARMENTIER, R., AND MORTELMANS, J., S&rose et athCrosclCrose 36 VASTESAEGER, du reseau arttriel coronarien chez des mammifkres et oiseaux morts en jardin zoologique, Bull. Sot. Roy. 2001. (Anvers), 14 (1959) 3. 37 VASTESAEGER,M. M., VERCRUYSSE,J., AND DELCOURT,R., Tendance sptcifique $ I’athCrosclCrose et lipides alimentaires chez les animaux sauvages, BUN. Sot. Roy. Zool. (Anvers), 27 (1963) 3. 38 GRUNBERG,W., Spontane Arteriosklerose beim Vogel, Bull. Sot. Roy. Zoo/. (Anvers), 34 (1964) 21. 39 CLARKSON,T. B., MIDDLETON,C. C., PRICHARD,R. W., AND LOFLAND,H. B., Naturally occurring atherosclerosis in birds, Ann. N. Y. Acad. Sci., 127 (1965) 685.
19 (1974) 47-59
47
~0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
COMPARISON OF AORTIC AND MAMMALS
C. L. BERRY,
JOCELYN
GERMAIN
LAMELLAR
AND
UNIT
PATRICIA
STRUCTURE
IN BIRDS
LOVELL
Department of Pathology, Guy’s Hospital Medical School, London, SEI 9R7 (Great Britain)
(Received February 7th, 1973)
SUMMARY
The structure of the aortic media at different levels in a number of orders of birds has been examined. In general the avian aorta shows a laminar structure, but differs from the mammalian vessel in the occurrence of broad and distinct smooth muscle cell and connective tissue layers in its proximal part. Part of the vessel has a structure which resembles that of mammals; the aorta distal to the origin of the renal arteries is a mixed musculo-elastic structure. Differences between avian and mammalian aortic physiology and their relevance to structural findings are discussed.
Key words: Atherogenesis - Avian aorta - Elasticity - Laminar structure - Physiological properties
INTRODUCTION
We have recently described the chemical and morphological development of the aorta in the rat and in manl-3. Mechanical factors affect the histogenesis of the media of large vessels, and their effects have been discussed in detail elsewhere4. The results of these studies suggest that aortic medial structure is determined in a large part by the stresses acting on it during growth. The media has well defined mechanical characteristics, and consists of a “two phase” system of collagen and elastin. The circumferentially-aligned collagen fibres bear tangential stressing forces, while the elastin network distributes these forces uniformly throughout the walls,s. The components of the media are arranged in a laminar structure composed of smooth muscle cells attached to compact elastic lamellae with interlamellar elastin running between the cells (Fig. 1). This structure develops as the aorta grows, the number of lamellar units being related to the vessel diameter and hence to the tension in the wal17s.
48
C. L. BERRY, J. GERMAIN, P. LOVELL
Fig. 1. Diagrammatic representation of lamellar structure of adult aortic media in mammals. Smooth muscle cells are apparently attached to dense elastic lamellae with finer fibres running between many cells. Collagen is shown as a stippled investment of the elastic network.
Fig. 2. Pheasant aortic root, showing double lamellar structure, with darker smooth muscle bands and pale scleroprotein and GAC-containing zones. Toluidine blue; x 500.
AORTIC LAMELLAR
STRUCTURE IN BIRDS AND MAMMALS
49
The lamellar unit “two phase” structure is found in many mammalian groupssp7, including diving mammalss. In primates, reduction in the number of lamellar units in the aortic media is consistent as the aorta diminishes in diameterr0Jr. The extensive distribution of this type of structure in different species and the way in which it is modified by mechanical factor@ suggests that it has considerable biological advantages as a method of handling and “damping” pulsatile blood flow. With these considerations in mind we have undertaken a study of the morphology of the avian aorta. Since physical factors are considered to be of considerable significance in atherogenesis 12,13the structure of the aortic media, determined as it is by stresses acting during growth and development, may be of importance in experimental design and interpretation of work on avian atherogenesisr4-17. Previous studies of normal aortic structure in birds are few. Argaudr8 studied the eagle, Pfisterig examined a number of species, and more recently Moss and Benditt20 have carried out a detailed study on the chick. Finlayson2r included birds in his extensive report on arterial disease in many animals, and from his comments and illustrations useful information may be obtained about the normal histology of the avian aorta. MATERIALS AND METHODS
The following birds were examined : Anseriformes (duck-Anus platyrhyncha pfatyrhyncha), Galliformes (pheasant-P/z&anus colchicus, chicken-Gallus domesticus), Columbiformes (pigeon-Columba palumbus palumbus), Charadriiformes (woodcockScolapax rusticola rusticola) and Passeriformes (thrush-Turdus ericetorum ericetorum).
Fig. 3. Line drawing of pheasant dorsal aorta. Smooth muscle zones are shown stippled. ing nature of the network is evident. x 120.
The branch-
50
C. L. BERRY, J. GERMAIN, P. LOVELL
Fig. 4. Detail from Fig. 3. The irregular thickness of the smooth muscle lamellae is caused by variations in pitch of the helices of cells in different areas. x 5.50.
Fig. 5. Pheasant aorta. Here only smooth muscle cells and elastic tissue are shown. The two distinct zones are evident. x 800.
AORTIC
LAMELLAR
STRUCTURE
IN BIRDS AND MAMMALS
51
Fig. 6. Electron photomicrograph of duck aortic root. Darkly staining smooth muscle cells are seen in distinct laminae, the elastic tissue is homogeneous, collagen fibres are clearly seen. The pale cells in this zone are the so-called “inter-1amellar” cells. x 1200.
All birds were apparently
young
adults.
Game
birds were shot with a Holland
and
Holland 12 bore sporting gun, using “Impax” number 6 cartridges. The aorta was removed entire within 20 min of death and fixed in buffered formalin. Small (1 mm) segments of the aorta from other birds were fixed in glutaraldehydeandpost-osmicated. In all birds blocks were taken from the root of the aorta, from the innominate arteries, from the dorsal aorta above the intercostal branches and immediately above the coeliac artery, from immediately below the anterior mesenteric artery, below the renal arteries and below the trifurcation at which the sciatic arteries arise. After embedding in epoxy resin 2 ,U sections were cut on an LKB “Pyramitome” or ultramicrotome and stained with 1% toluidine blue in 1% borax. Electron microscopy was carried out on osmium-treated material.
52
C. L. BERRY, J. GERMAIN, P. LOVELL
Fig. 7. Woodcock dorsal aorta. Mammalian type of structure with smooth muscle attached to elastic tissue. Occasional vacuolated cells are seen. Tohtidine blue: x 800.
Fig. 8. Duck aorta between coeliac and anterior mesenteric arteries. Transitional x 100. and single lamina unit structure is shown. Van Giesonelastic;
zone between double
AORTIC LAMELLAR STRUCTURE IN BIRDS AND MAMMALS
53
Fig. 9. Pheasant aorta. Part muscular part elastic structure with well-defined IEL and smooth muscle cells in elastic network. For clarity some smooth muscle cell nuclei are omitted, as is some of the outer elastic tissue.
After examination of a number of 2 ,u sections, routine paraffin-processing was also carried out on formalin-fixed material and 5 ,u sections were examined for glycosaminoglycans (GAC) by the method of Scott and Dorlingz2. RESULTS
In general the basic aortic medial structure was similar in all birds examined. The aortic trunk showed a well-defined laminar structure, but with two distinct types of lamellar unit present. These consisted of smooth muscle cells arranged in compact bands, with marginal elastic laminae separated by clear zones containing collagen and elastin and in addition material shown by special staining of paraffin sections to be mainly carboxylated GAC (Fig. 2). These two lamellar structures formed a complex branching network (Figs. 3 and 4). Fig. 5 shows the structural organisation in detail, here only smooth muscle cells and elastic tissue are shown, cells between the
54
C. L. BERRY, J. GERMAIN, P. LOVELL
Fig. 10. Pheasant infrarenal aorta. Smooth muscle cells are seen in a distinct network of elastic tissue with a tendency to lamellar arrangement. Toluidine blue; x 550.
compact zones are omitted. Fig. 6 is an electron photomicrograph of this region and shows smooth muscle lamellae with elastic tissue, collagen and amorphous material between them. In all birds examined, this type of structure was seen in the aorta as far as the origin of the coeliac artery and in the innominate arteries. Distal to this a more “mammalian” lamellar structure was seen with smooth muscle cells running between well-defined elastic lamellae in helices of Varying pitch (Fig. 7). Occasional muscle cells appeared pale and contained vacuoles. The transitional zone between the two structures was short and was generally found between the coeliac and anterior mesenteric arteries (Fig. 8). This “mammal’‘-like structure is short lived; from the level of the renal arteries the aorta assumes a partly muscular, partly elastic structure which is quite unlike the mammalian artery (Fig. 9). The internal elastic lamina (IEL) is well-defined and complete; this is not found in the proximal aorta. There is a zone of smooth muscle cells enmeshed in elastic tissue, with some tendency to lamellar arrangement (Fig. 10) and with areas of longitudinally and transversely-running fibres (Fig. 11). Outside this are a number of elastic lamellae with interspread smooth muscle cells. Fig. 12 is an electron photomicrograph showing the relationship between smooth muscle cells and intercellular elastin.
AORTIC LAMELLAR
STRUCTURE
IN BIRDS AND MAMMALS
55
Fig. 11. Similar zone with IEL at top right, and outer lamellae at bottom left. An inner longitudinal
and outer circular arrangement of muscles is seen. Toluidine blue; x 800. DISCUSSION
There are some important
anatomical
great vessels. In the birds the systemic systemic
trunk
differences
arteries
between avian and mammalian
resemble
close to the heart two short innominate
those of reptile+. arteries
From the
arise and divide into
carotid arteries and sub-clavian vessels, respectively supplying the head and neck and wing and its muscles. The development of the sub-clavian vessels is singular and does not resemble
that in mammals 23. The continuation
of the aorta
is smaller
than the
parent trunk and gives off vessels in a normal vertebrate plan, paired intercostal and lumbar arteries corresponding to the intervals between vertebrae, ventral vessels running to the viscera, and the sciatic arteries supplying the posterior part of the kidneys as well as the leg+. In addition, important physiological differences exist between the two groups. The mammalian vascular tree is nonuniform in terms of its elastic behaviour, i.e.it becomes increasingly stiff along its length 25. This confers particular advantages, notably in isolating the low impedance of the proximal large arteries from the high impedance termination of the peripheral resistance, and in reducing the oscillatory component of the work required to maintain the cardiac output. Further, the overall distensibility of the system is reduced by stiff peripheral vessels, providing the advantage of rapid adaptation to pressure changes and reducing the tendency to cause large volume changes in veins. The vessels of the goose and turkey behave quite differently2s.
56
C. L. BERRY, J. GERMAIN, P. LOVELL
Fig. 12. Electron photomicrograph of duck infrarenal aorta. The close relationship cells to surrounding elastic tissue is evident. x 2000.
of smooth muscle
and resemble the classical windkessel, i.e. a system of uniform distensibility and increasing cross-sectional area. From the data of Newman, Gosling and Bowden it appears that the chicken aorta behaves in the same way. A change implicit in these findings is that the pulse form shows virtually no change along the aorta and arterial branches, which act as rigid distributing channels in birds. A further important difference between birds and mammals lies in the relatively high blood pressure of the former group14927. These various factors may account for differences in histological appearances between the two groups. Pfisterls pointed out the absence of an internal elastic lamina in the thoracic aorta in birds, and the close similarity in structure between the large branches and the aorta itself. In Moss and Benditt’s20 more recent study the authors described the structure of the aorta in detail from aortic root to “bifurcation”. They stated that the smooth muscle lamellae in this species were generally incomplete, rarely extending around more than a quarter of the circumference of the vessel, and were clothed on each aspect by a dense elastic layer, whilst finer elastic fibres were found in the interstitial lamellae. Finlayson 21 had noted the presence of intermuscular
AORTIC LAMELLAR
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laminae in the proximal aorta and brachiocephalic trunks of birds and illustrations in his paper show these to be present in the kite, stone curlew and whooper swan. Both papers20921 describe a gradual change in pattern from this type of structure to the mammalian form we have illustrated in Fig. 7. In the more distal vessel, Moss and Benditt20 illustrate an “inner muscular media” with an outer elastic zone. These authors describe the cells of the lamellae of connective tissue and GAC as “interlamellar connective tissue cells”, apparently the “interlamellar” cells illustrated by Cooke and Smith28 in the developing pigeon aorta. Cooke and Smith considered these cells to be like fibroblasts. Recent studies on scleroprotein synthesis by smooth muscle cells2s~a0,together with the work of Cliffsi, suggest that the spindle cell of the aortic media may be pluripotential and that distinctions between different types are invalid. However, the work of Moss and Benditt demonstrates that the cells of the connective tissue laminae have many characteristics that differ from normal smooth muscle cells. Our findings are in general agreement with these previous studies. The occurrence of large amounts of carboxylate GAC in the media of the proximal great arteries may affect the rigidity of the vessel wall due to changes in hydration; changes of this kind occur in mammalian vessels in coarctation and after surgery32. This modification of lamellar unit structure may explain the curious physiological behaviour of avian arteries. We differ from other authors in our view of the structure of the distal aorta. Here there is a distinct internal elastic membrane and the smooth muscle cells found are arranged in a well-defined elastic network with a tendency to lamellar formation. Elastic lamellae are seen in the outer zone. We consider that the changes described by Gresham, Howard and Jennings 14 deep to atherosclerotic lesions in the turkey reflect normal structure rather than a repair phenomenon. Intimal cushions are found in birds17 and other animalssa-39 and may be largely composed of smooth muscle cells. We consider that these structures are distinct from the normal thick smooth muscle cell layer of the distal avian aorta. A possible explanation for this curious structure may be found in a further consequence of the unique physiological behaviour of the aorta in birds. The rigidity of the vessels may result in a reduction in amplitude of variations in the pulse wave: one of the advantages of the mammalian system pointed out by Taylor25 is that disturbances will be amplified in a pulse wave travelling along a tube of decreasing distensibility. Perhaps the musculo-elastic vessels of the bird represent a modification of the basic structure which permits changes in vessel radius to modify the effects of the drastic circulatory changes in the posterior part of the body that must occur during flight. Whatever explanation is preferred, it is evident that the aorta of birds differs from that of mammals, and that these differences are important if mechanical factors are considered to be of major importance in atherogenesis. Although a number of authors have commented on differences between avian and mammalian vascular structure, little has been published on the possible reasons for, or effects of, such differences. Many aspects of atherogenesis may be usefully studied in birds, but, in attempt-
C. L. BERRY,
58
J. GERMAIN,
P. LOVELL
ing to apply the results of such experiments to man, fundamental differences in aortic vessel physiology should be remembered. ACKNOWLEDGEMENTS
We should like to acknowledge the help of Mr. George Lloyd Roberts and friends in obtaining the specimens of game birds studied. The Pyramitome was purchased with a grant from the Medical Research Council.
REFERENCES 1 BERRY, C. L., LOOKER,T., AND GERMAIN,J., The growth and development of the rat aorta, Part 1 (Morphological aspects), J. Anut., 113 (1972) 1. 2 LOOKER,T., AND BERRY, C. L., The growth and development of the rat aorta, Part 2 (Changes in nucleic acid and scleroprotein content), J. Anat., 113 (1972) 17. 3 BERRY, C. L., LOOKER,T., AND GERMAIN,J., Nucleic acid and scleroprotein content of the developing human aorta, J. Puthol., 108 (1972) 265. 4 BERRY, C. L., The growth and development of major arteries. In: D. H. M. WOOLLAM AND G. MORRIS (Eds.), Advances in Embryology and Teratology, Paul Elek (Scientific Books), London, 1973. 5 WOLINSKY,H., AND GLAGOV, S., Structural basis for the static mechanical properties of the aortic media, Circ. Res., 14 (1964) 400. 6 WOLINSKY, H., AND GLAGOV, S., A lamellar unit of aortic medial structure and function in mammals, Circ. Res., 20 (1967) 99. 7 WOLINSKY,H., AND GLAGOV, S., Nature of species differences in the medial distribution of aortic vasa vasorum in mammals, Circ. Rex, 20 (1967) 409. 8 BERRY, C. L., The establishment of elastic structure of arterial bifurcations and branches: Its relevance to medial defects of cerebral arteries, Atherosclerosis, 18 (1973) 117. 9 BERRY, C. L., Unpublished observations. 10 AHMED, M. M., Microscopic anatomy and the attenuation of elastic tissue in the Slow Loris
(Nycticebus Coucang Coucang), Folia Primat., 8 (1968) 290. 11 BERRY, C. L., AND GERMAIN,J., Changes in aortic medial elastic structure at different sites in the aorta of three primates, Lab. Animals, In press. 12 CARO, C. G., FITZ-GERALD, J. M., AND SHROTER, R. C., Arterial wall shear and distribution of early atheroma in man, Nature (London), 223 (1969) 1159. 13 M~KHELL, J. R. A., AND SCHWARTZ, C. J., Arterial Disease, F. A. Davis Co., Philadelphia, 1965, p. 50. 14 GRESHAM,G. A., HOWARD, A, N., AND JENNINGS,I. W., Dietary fat and aortic atherosclerosis in the turkey, J. Pathol. Bacterial. 85 (1963) 291. 15 FINLAYSON, R., SYMONS, C., AND T-W-FIENNES, R. N., Atherosclerosis: A comparative study, Brit. Med. J., i (1962) 501. 16 FINLAYSON,R. AND HIRCHINSON,V., Experimental atheroma in Budgerigars, Nature (London),
192 (1961) 369. 17 SANTERRE,R. F., WIGHT, T. N., SMITH,S. C., AND BRANNIGAN,M. S., Spontaneous atherosclerosis in pigeons, Amer. J. Pathol., 67 (1972) 1. 18 ARGAUD, C.R. Ass. Anatomistes, 1904, cited by H.I.C. PFISTER. 19 PFISTER,H. I. C., On the distribution of the elastic tissue in the blood vessels of birds, J. Anat., 61 (1927) 213. 20 Moss, N. S., AND BENDITT,E. P., Spontaneous and experimentally induced arterial lesions, Part 1 (An ultrastructural survey of the normal chicken aorta), Lab. Invest., 22 (1970) 166. 21 FINLAYSON,R., Spontaneous arterial disease in exotic animals, J. Zool., 147 (19651 239. 22 Scorr, J. E., AND DORLING, J., Differential staining of acid glycosaminoglycans (mucopolysaccharides) by Alcian Blue in salt solutions, Histochemie, 5 (1965) 221.
AORTIC LAMELLAR
STRUCTURE IN BIRDS AND MAMMALS
59
23 CARTER, G. S., Structure and Habit in Vertebrate Evolution, Sidgwick and Jackson, London, 1961, p. 334, 24 BRADLEY, 0. C., AND GRAHAME,T., The Structure of the Fowl, Oliver and Boyd, Edinburgh and London, 1960, p. 80. 25 TAYLOR, M. B., Wave travel in arteries and the design of the cardiovascular system. In: P&utile Blood Flow, McGraw-Hill, New York, Toronto, London, 1964, p. 343. 26 NEWMAN, D. L., GOSLING,R. G., AND BOWDEN,N. L. R., Changes in aortic distensibility and area ratio with the development of atherosclerosis, Atherosclerosis, 14 (1971) 231. 27 WEISS, H. S., AND SHEAHAM,M., The influence of maturity and sex on the blood pressure of the turkey, Amer. J. Vet. Rex, 19 (1958) 209. 28 COOKE, P. H., AND SMITH,S. C., Smooth muscle cells. The source of foam cells in atherosclerotic white Carneau pigeons, Exp. Mol. Pathol., 8 (1968) 171. 29 Ross, R., AND KLEBANOFF,S. J., The smooth muscle cell, Part 1 (m viva synthesis of connective tissue proteins), J. Cell Biol., 50 (1971) 159. 30 Ross, R., The smooth muscle cell, Part 2 (Growth of smooth muscle in culture and formation of connective tissue proteins), J. Cell Biol., 51 (1971) 172. 31 CLIFF, W. J., The aortic tunica media in growing rats studied with the electron microscope, Lab. Invest., 17 (1967) 599. 32 BERRY, C. L., AND TAWES, JR., R. L., Mucopolysaccharides of the aortic wall in coarctation and recoarctation, Cardiovasc. Res., 4 (1970) 224. 33 MUSTARD, J. F., MURPHY, E. A., ROSWELL, H. C., AND DOWNE, H. G., Platelets and atherosclerosis, J. Atheroscler. Res., 4 (1964) 1. 34 GUTSTEIN,W. H., SCHNECK,D. J., AND MARKS, J. O., In vitro studies of blood flow disturbance in the region of separation, J. Atheroscler. Res., 8 (1968) 381. 35 CLARKSON, T. B., PRICHARD, R. W., NETSKY, M. G., AND LOFLAND, H. B., Atherosclerosis in pigeons: its spontaneous occurrence and resemblance to human atherosclerosis, Arch. Pathol., 68 (1959) 143. M. M., GILLOT, P., PARMENTIER, R., AND MORTELMANS, J., S&rose et athCrosclCrose 36 VASTESAEGER, du reseau arttriel coronarien chez des mammifkres et oiseaux morts en jardin zoologique, Bull. Sot. Roy. 2001. (Anvers), 14 (1959) 3. 37 VASTESAEGER,M. M., VERCRUYSSE,J., AND DELCOURT,R., Tendance sptcifique $ I’athCrosclCrose et lipides alimentaires chez les animaux sauvages, BUN. Sot. Roy. Zool. (Anvers), 27 (1963) 3. 38 GRUNBERG,W., Spontane Arteriosklerose beim Vogel, Bull. Sot. Roy. Zoo/. (Anvers), 34 (1964) 21. 39 CLARKSON,T. B., MIDDLETON,C. C., PRICHARD,R. W., AND LOFLAND,H. B., Naturally occurring atherosclerosis in birds, Ann. N. Y. Acad. Sci., 127 (1965) 685.