AUTHOR’S ABSTRACT O F T H I S PAPER ISSUED B Y T H E BIBLIOGRAPHIC SERVICE, J A N U A R Y 14 ON THE VARIATIONS O F WALL THICKNESS I N EMBRYONIC ARTERIES J O H N LEWIS BREMER Harvard Medical School, Boston, Massachusetts SIX FIGURES The relative thickness of arterial walls in embryos has not, to my knowledge, been the subject of special study, yet on looking at any number of embryos of different ages and of different species with this in view one cannot but be impressed by the great variety of appearances. One expects the larger, main vessels to be more heavily coated than their smaller branches, the arteries near the heart, on which the full force of the pulse beat comes, to have thicker walls than those more distally placed. It is known that in young embryos the arteriez I r e relatively enormous and the walls a simple endothelium. Large caliber and thin walls seem to go together, and to indicate that with the slow stream and low pressure suggested by the large caliber mesenchymal differentiation to strengthen the walls is not necessary. Mesenchymal differentiation appears at widely different periods in different types of embryos and in different arteries, and is usually accompanied by a relative or actual narrowing of the vessels, as the circulation becomes more definite. But in addition to this there may be, even in certain arteries of individual embryos, great variation in the thickness of the wall in different regions. The thinner o r thicker areas in the wall of an artery may be very local and readily understandable, if the probable cause for the presence of a coat is kept in mind. The walls develop in response to pressure within the vessel, and, as a 1 THE ANATOMICAL BIGCORD, VOL. 27, NO. J A h U S R Y , 1924 1 2 J O H N LEWIS BREMER corollary to this, the greater the pressure the stronger the walls. In the embryo, before the mesenchyma is much differentiated, the term stronger can probably be interchanged with thicker, though it is by no means certain that this woulc! hold true later. One instance of inequality in the arterial wall is shown in figure 1. This is of a carotid branch cut lengthwise at a rather sharp bend, in the region of a probably swift current, and the wall on the outer curvature, where Fig. 1 Branches of the carotid artery. The blood flows in the direction of the arrow into the three branches, all of which, but especially that to the left, show a thickening of the wall on the outer curvature, as the stream is deflected. A group of corpuscles lies in the lumen near the thin wall. Chick, 14 d. 5 h., €1. E. C. No. 1968, sect. 741, transverse. Fig. 2 Ductus arteriosus entering the dorsal aorta. The walls are thin, where they are supported by the vagus and recurrent laryngeal nerves, thicker where not so supported. Man, 18.8 mm., H. E. C. no. 2246, sect. 582, transverse. Fig. 4 Ductus arteriosus of turtle entering the dorsal aorta. The walls a r e thin far beyond the area of contact with the vagus and recurrent laryngeal nerves, which are shown in black on either side. The dorsal aorta shows thick walls. Chrysemys marginata, 32 mm., H. E. C. no. 1127, sect. 947, transverse. Fig. 6 Arteries of chick. The large thin-walled aorta is shown giving off its branch, the vitelline artery, smaller but with thicker walls. Another branch from a higher level, the coeliac artery, is shown below, of much smaller caliber but with thicker walls. Chick, 14 d. 5 h., H. E. C. no. 1968, sect. 2184, transverse. WALL T H I C ~ N E S S IN THE EMBRYONIC ARTERIES 3 the pressure of the blood would be the greatest, is much thicker than that on the inner curvature. Similar conditions can be met with frequently, where arteries curve; but in noting them one must be careful to eliminate the possibility that the supposed thickening is in fact only due to oblique sectioning of the wall. A familiar instance in sagittal sections is the inequality in the ventral and dorsal walls of the aorta; the dorsal wall, on the outer curvature, receives its coat earlier and is usually somewhat thicker than the ventral. Another type of local inequality in wall thickness is that in which s m e outside structure sufficiently supports the vessel wall, rendering further efforts to withstand the pressure from within unnecessary, at least temporarily. I n these instances the mesenchyma does not differentiate locally, as is shown in figure 2. Here it is a nerve, the vagus and its branch the recurrent laryngeal, against which the vessel, the ductus arteriosus, is resting and which makes the strength of the. wall in that particular region. I have also noticed vessels where veins, other arteries, even the angularly set branch of the same vessel served to relieve the pressure and allow a thin place in the wall. Nature seems to add the coat only in order to withstand the internal pressure, and to make use of any other available means to attain the desired result. The effect is purely local, as the mesenchymal layers are interrupted only f o r the nerve or other support, which thus becomes imbedded in a wall otherwise of even thickness. A rarer and more extensive change of caliber and of wall thickness in continuous vessels is shown in the older stages of the turtle, Chrysemys marginata. At 29 mm. the whole arterial system of these embryos is formed of thick-walled, massive vessels, except for the two ductus arteriosi, right and left, which are greatly expanded, thin-walled sacs, in striking contrast t o the neighboring arteries, as is shown in figures 3 and 4. The whole extent of these dorsal portions of the pulmonary arches, from the points of origin of the pulmonary arteries to the dorsal aorta, is of the same character, the thin wall merging at each end into the adjacent thick ones by 4 JOHN LEWIS BREMEB rapidly tapering zones, wedge-shaped in longitudinal sectiori. From the heart the blood must pass through the thick-walled proximal portions of tho pulmonary arches either to the. pulmonary arteries, also thick-walled but of much smaller caliber, o r into these expanded sacs on its way to the dorsal aortae. There must be an abrupt lessening of pressure and rate of flow during this passage. Fig. 3 Reconstruction from sagittal sections of the right aortic and pulmonary arches of a turtle embryo, as seen from the right side. The vessels a r e f o r the most p a r t represented as bisected longitudinally to show the relative thickness of their walls. The dorsal aorta lies at the left of the figure, the heart would be to the right. The pulmonary arch gives off the pulmonary artery a n d continues as a thin-walled, curved, bulging ductus arteriosus, which joins the aorta. Chrysemys marginata, 31.7 mm., I€. E. C. no. 1101. This condition is first noticeable in Chrysemys embryos of about 11mm., where a distinct relation with the vagus nerve can be observed, for the large vagus indents deeply the lateral side of the vessel. From then on the ductus grows in caliber and length, but the wall remains thin far beyond the limit of the nerve. Even earlier than this the pulmonary and aortic arches differ from each other, the pulmonary being much WALL THICKNESS I N THE EMBRYONIC ARTERIES 5 the larger throughout its extent. Ultimately the dilated ductus comes in contact with the overlying fourth arch, around which it bulges both mesially and laterally, as shown in the figure, and which may thus lend support to the thin walls ; but the sides and floor of the ductus are not so supported, and yet remain thin. The interpolation of these flaccid, sac-like segments along the course of the pulmonary stream may remind one of the similar thin-walled enlargements seen frequently in the early embryonic conus arteriosus, and called the ‘aortic sac’ by Congdon (’23); and the purpose may be the same in both cases, namely, to reduce the full force of the pulse beat and insure a more regular flow of blood along the distal vessels. Yet in the turtle of this age there is no such thin-walled segment on the conus nor on any of the other arteries leaving the heart, which seem able of themselves to regulate the blood stream, as do adult arteries ; and it would seem unnecessary t o have the pulmonary arch alone provided with such a safety valve. Another thought might occur, that the thin walls in this particular place represent merely an attempt at economy of effort on nature’s part, in refusing to provide muscular walls of any great thickness for a vessel so soon to become obliterated. If this were the case, one would expect to find somewhat similar appearances in other members of the reptile group. I n the snake, Eutaenia radix, however, where only one ductus persists till hatching, the channel of the arch is of the same caliber throughout, and the walls of the same thickness as those of neighboring arteries. (It is curious that in this form, whereas the left lung and left pulmonary artery are absent, it is nevertheless the left ductus arteriosus that is used, the right disappearing some time before hatching. Thus, as there is no left pulmonary artery, the whole arch becomes ductus arteriosus, and is obliterated in toto.) The other reptiles examined show also no marked difference in thickness in different parts of the pulmonary arch. The peculiarity is certainly not a reptilian characteristic. 6 J O H N LEWIS BREMER The fate of these enlarged sacs in the turtle may point toward the explanation of their presence. Shortly after hatching1 changes take place leading toward the final closure of the vessel. At one stage the walls are folded irregularly inward, until the passage is almost occluded ; within three or four days the closure is complete. The procedure differs from that in other vertebrates, however, in that it is the distal end of the tube which clo~esfirst, and in that the ductus becomes longer and straighter than before. The closure is initiated by the contraction of the muscles of the narrow tapering zone, near the aorta. The result is a relatively capacious caecum or blind branch appended to the pulmonary artery, in its course to the lung. The sac thus formed is broader than the aortic arch, and capable of holding a considerable quantity of blood. If one blows into a canula whose point is inserted in the pulmonary trunk of a turtle a few days old the flaccid ductus can be seen to expand and become tense. No air leaks into the aorta. When the pressure is released, the ductus is immediately much reduced in size, as though its walls contained elastic elements, and finally resumes its collapsed condition. The force of the pulse beat is not, however, sufficient to dilate the vessel to a noticeable extent, as observation with the hand lens while the heart is still beating shows no variation in caliber in the ductus synchronizing with the pulse. At fifteen or twenty days after hatching the ductus has become shrunken and obliterated for fully half its length, and the proximal, remaining half has been reduced in caliber by a rearrangement of the tissue of its walls, which are now nearly as thick as in the ventral portion of the arch. The obliteration of the lumen proceeds from near the dorsal aorta toward the pulmonary artery, and soon is completed for the whole ductus. The two vessels, right and left, progress together at every step. The presence of an expansile sac of considerable capacity on each of the pulmonary arteries in the young turtle, a sac, 'I wish t o express my thanks t o Dr. Pauline Kimball and Dr. Frank A. Stromsten, who have kindly supplied me with these older stages. WALL THICKNESS IW THE EMBRYONIC ARTERIES 7 which gradually diminishes in size and finally disappears entirely, leads one to speculate as to the probable usefulness of these structures and the necessity for them in these special reptiles.2 They could serve either as reservoirs for blood o r as reducers of the pulse beat, or in both of these capacities. Tbere should be some peculiarity of the turtle, among vertebrates, which might call for these special functions. The lungs are, of course, indicated as the site of this peculiarity, as the organs to which the arteries in question pass. The turtle lung has long been recognized as one in which the phylogenetic step has just been made from the primitive saccular form to the higher lobular type with branching bronchioles leading to independent air sacs. The branching is only slight, however, as was shown by Miller ('04), since the air sacs number only seven in each lung. The branching of the pulmonary artery is also slight, and the resulting arterioles break up almost immediately into thin-walled capillaries. This rapid change from thick-walled artery to thinwalled capillary might readily result in excessive pressure within the smaller vessels, especially in those of the lung which must already withstand the shock of rapid expansion with breathing. To lessen this danger the distal end of the pulmonary artery proper is found t o be, in turtles fifteen to twenty days old, of large caliber and with thin walls, as opposed to the smaller caliber and thicker walls found at its union with the pulmonary arch, so that the pressure diminishes as the vessel is traversed by the blood. This is not the case in turtles just before hatching, in which the artery is of a uniform small caliber throughout. It occurs to me that, as the young normally enter a hibernating period almost immediately on hatching, the small artery may be sufficient in that state to supply the necessary blood t o the lungs; but that when the turtles are prevented from hibernating and 'A. Brenner noted a slight rounded swelling a t the junction of the pulmonary artery and the obliterated ductus botali in Test,udo graeca, and figures the pulmonary arch as of greater caliber than the branch t o the lung. Arch. f. Anat. u. Entwick., 1883. 8 J O H N LEWIS BREMER forced to lead an active life, as in the case of those examined and described here, the artery becomes enlarged distally as a means of protection. The expansile sac of the ductus arteriosus may act as an additional safety valve. With more mature adjustment the lung arteries become better able to regulate the blood stream. I n the adult turtle the entire ductus has been reduced t b the usual fibrous cord, and the adult pulmonary artery, from heart to lung, is of even large caliber. The crocodile is classed with the turtle by Wiedersheim (in his Lehrbuch) in respect to the phylogenetic position of the lung. I have not the necessary older stages of crocodile embryos to learn of the later condition of the ductus arteriosus in this form, whether o r not it shows the characteristics described for the turtle as possibly corresponding to the lung structure. In the young alligator embryos available, however, the pulmonary arches are much larger than the others, and it will be remembered that this was characteristic also of the younger turtle embryos. The coincidence is suggestive. The whole stream from the right ventricle does not pass t o the lungs in the turtle, as the left fourth arch, as is well known, also arises from this chamber of the heart t o join the dorsal aorta, after giving off branches to the stomach, liver, and intestine. Also from the left pulmonary arch a small branch passes mesially and ventrally to a parathyroid gland and to the wall of the trachea. The lack of a complete interventricular septum probably allows a mixture of venous and arterial blood, so that these various organs are not so poorly served as one would at first imagine. The whole force of the pulse beat is not, however, directed toward the lungs. In the chick the ductus arteriosus also exhibits thin walls, in contrast to the more massive pulmonary artery. Here again, just before hatching, the two pulmonary arches are complete, though only the right fourth arch persists, the left having long before degenerated and disappeared. From each pulmonary arch branches a pulmonary artery, of much W A L L THICKNESS I N THE EMBRYONIC ARTERIES 9 smaller caliber but with a continuation of relatively thick walls. From this point to the dorsal aorta on each side extends the ductus arteriosus, slightly larger than the proximal segment of the arch, but with thinner walls. The striking effect made in the turtle b:r the sudden changes in caliber and thickness of wall is lost, however, in the chick, because the ductus is straight and tubular, instead of saccular, and because it joins a thin-walled dorsal aorta. I n fact, examination of all the main arteries of an older chick embryo shows that the thinness of wall of the ductus is only one example of numerous similar differences in wall thickness throughout the arterial system. As can be seen in figure 5, of a chick of nineteen days’ incubation, the aortic arch is massive, but is continued into a dorsal aorta of much larger caliber but with much thinner walls. The t,ransition is abrupt. I n the pulmonary arches the transition, as has been mentioned, occurs at the proximal end of the ductus. The carotid, or brachiocephalic arteries, though of small caliber, are provided near the heart with coats equaling in thickness those of the arch of the aorta, the diameter of which is several times as great. If the vessels are traced further, beyond the limits of reconstruction, the descending aorta remains thin-walled and of large caliber, but its first branch, the coeliac artery, is again small and provided with a coat actually thicker than that of the parent stem, proportionately much thicker, as is shown in figure 6. The vitelline and allantoic arteries are also thick-walled. I n the head region the external carotid artery is a thin-walled, flaccid continuation of the massive brachiocephalic trunk, and as such extends throughout the long neck into the head. There, near its terminal branches to the face and the eye, it abruptly assumes massive walls and a small caliber, which are continued along the branches. The transition here is on the artery itself, not, as in the coeliac artery, definitely on the branch. It will be seen that in both directions, cranially and caudally, the blood from the heart passes first through relatively narrow, thick-walled arteries, then through long 10 J O H N LEWIS BRGIMER stretches with larger caliber and thinner walls, then again through smaller thick-walled vessels. Not in every case, however, for the brachial branch of the brachiocephalic artery shows no such expanded area and no thinning of the walls beyond what is usual with lessening size after repeated branching; and, on the other hand, the smaller branches of the dorsal aorta to the back and to the mesoiiephros show no return to the thick-walled condition. Several suggestions can be made in explanation of these facts, but only, I think, to be discarded. Congdon mentions Streeter 's view that the early proliferation of endothelium entailed in vessels of great size may serve as a reserve for the future rapid differentiation of the vascular system. While this suggestion may explain the relatively enormous vessels of young embryos, in which connection it was put forth by Streeter, it would not so readily apply t o the established arteries of chicks or turtles about to hatch, nor t o the ductus arteriosus, which is soon to become obliterated. On the other hand, the supposition that muscular and elastic tissue had not been supplied to the ductus arteriosus of these two forms because nature had foreknowledge of their future obliteration and declined to expend unnecessary energy on tissue building, is denied by the numerous instances of purely embryonic vessels which are provided with thick coats, and would fail to explain the thin walls found in many permanent arteries, The possibility that vessels of different embryological origin may retain, in certain forms, characteristic rates of wall development, though already united and used as a continuous channel, might be considered. Thus the aortic arches might conceivably develop walls thicker than those of the dorsal aorta. This might explain the sudden change in character of the aorta shown in figure 5, as the area of rapid reduction of wall thickness may very probably mark the junction of fourth arch and dorsal aorta, as is seen by studying younger chick embryos in which the dorsal aorta, between the third and fourth arches, is still present. The massive WALL THICKNESS I N T E E EMBRYONIC ARTERIES 11 portion may represent arch, the thin portion aorta. Carrying this idea to the pulmonary arch, one might consider that the thin-walled ductus arteriosus represents that portion of the arch which has developed as an outgrowth of the dorsal aorta, and therefore inherited its characteristics, though in Fig. 5 Reconstruction from tzransverse sections of the right aortic and pulmonary arches of a chick embryo of nineteen days’ incubation, as seen from the left or medial side. The heart, therefore, would lie t o the left of the drawing. The left pulmonary arch and ductus arteriosus, though present in the embryo, are not shown in the reconstruction. The two brachioeephalic arteries, pointing upward toward the head, have small caliber and thick walls. The dorsal aorta shows a sudden increase in caliber and reduction in wall thickness, as does also the ductus arteriosus. 12 J O H N LEWIS BREMER that case one would expect this influence to be shown on the fourth arch as well, and also on the branches of the dorsal aorta, which have been shown, on the other hand, to be in general thicker than the parent vessel. And if one traces the vessels cranially no such differentiation into thick arch and thin dorsal aorta appears. No part of the internal carotid artery, which should represent the third arch and cranial dorsal aorta, shows the expected differentiation. On the other hand, the sudden thickening of the walls of the external carotid artery in the head, already referred to, falls into no scheme based on the aortic arches. It would be difficult indeed in the chick t o correlate all the facts with the known embryological development of the vessels. The arterial walls of the older chicks are much thicker as a whole near the heart, and appropriate stains reveal a large amount of elastic tissue in the arch of the aorta and in the proximal parts of the carotid arteries and pulmonary arches, in contrast t o the absolute lack of it in the heart proper and to the small amount found in the more distal parts of these vessels. This looks like, and probably is, a special arrangement connected with the heart beat, but proximity to the heart and the necessity to withstand and lessen the force of the pulse cannot be given in explanation of the thickness of wall of distal branches, like the coeliac artery. The possibility that the thick-walled segments of small caliber are merely the results of greater contraction in these areas merely carries the question one step further, f o r it suggests differences in structure or arrangement of the muscles to account for the differences in contraction, without offering any explanation of the varying types of wall. While, therefore, plausible explanations can be given for the short, localized variations in the thickness of arterial walls, and even f o r the peculiarities of the ductus arteriosus in the turtle, I fail to find any reasoaable explanation f o r the abrupt changes in caliber and character of the arteries of the older chick embryos. That the implied variations in rate and pressure of the blood stream must have some WALL T H I C K N E S S I N T H E EMBRYONIC ARTERIES 13 advantageous relation, past, present, or future, to the development of the embryo, I do not doubt. The passage OP blood from the large thin-walled aorta or carotid arteries, where most of the force of the pulse beat must be lost and the rate of flow must be slow, into narrower channels with much more muscular walls would seem to imply a peristahic action or autonomous pulsation in these distal arteries, capable of increasing the rate until it conforms with the type of vessel. Such independent arterial pulsation has been recognized in invertebrates, notably in the squid, but is usually denied f o r vertebrates. SUMMARY The walls of embryonic arteries, of various vertebrate types and different ages, occasionally show marked variation in the thickness of the muscular coat, The cause f o r the particular thinning o r thickening may be easily understood in some cases, as when the outer curvature of a bend is especially strengthened t o withstand the pressure of the stream or when the vessel is supported by some other structure, as for instance a nerve, and remains locally thin-walled. Photographs of these two conditions are shown. More extensive thinning is seen in the ductus arteriosus of the older turtle embryos, which is reduced to a flaccid expanded sac. The changes in this sac after hatching are followed, and its probable service in protecting the pulmonary capillaries from excessive blood pressure is suggested. In the older chick embryos the whole dorsal aorta is thin-walled and of large caliber, though continuous with massive aortic arches and with many thick-walled branches. No cause but that of a supposed physiological advantage can be given f o r this curious variation in thickness of arterial walls. BIBLIOGRAPHY CONGDON,E. D. 1923 Transformation of the aortic-arch system during the development of the human embryo. Contrib. t o Emb., no. 68. MILLER, W. S . 1904 Development of the lupg in Chryseniys picta. Am. Jour. Anat., 1701. 3, no. 1, pp. 15-16.