Talk:Paper - The relation between the size of the artery and the capillary bed in the embryo (1937)

From Embryology


H. H. WOOLLARD anv J. A. HARPMAN Department of Anatomy and Embryology, University College, London

THE work of Mall), Evans), Woollard(7), Hughes(3,4), and others has established that there is a stage in the embryo after the formation of the capillary bed when definitive arterial stems arise by enlargement and fusion in part of the capillary bed, and coincident atrophy in other parts. The emergence of these definitive stems is gradual and intermediate stages can be discovered where the fusion of the capillaries is not yet complete. Observations of close stages of injected preparations make it certain that this mode of development is an accurate account of what happens.

Clearly it is to this stage of the erection of definitive stems that Thoma’s laws apply, the earlier phase of capillary elaboration, it is generally agreed, being an angioblastic response. Thoma’s laws have often been stated, but perhaps they may be repeated here in the form quoted by Mall (5). The first law is that increase in the size of the lumen of the vessel, or, what is the same thing, the increase of the surface of the vessel wall, depends upon the rate of the blood current. The second law states that the growth in thickness of the vessel wall is dependent upon the tension. Further, the tension of the wall is dependent on the diameter of the lumen of the vessels and upon the blood pressure. The third law, which is the most contentious one, states that an increase of blood pressure in an area leads to the new formation of capillaries. .

The background of these laws has been discussed several times, most recently by Hughes (4,5), and it seems clear that endothelium can grow by its own intrinsic properties. It is not the blood flow through them that induces them to grow; this merely keeps up the regular pattern. Hughes and others have, however, pointed out that this conclusion is not necessarily in conflict with Thoma’s third law, since it is quite possible that in a particular field one of the factors inducing new capillary formation might be an increase in the blood pressure of the particular region concerned. This third law has been invoked by Clark e¢ al(1), for instance, to explain the growth of the new capillaries which they observed in their experiments on the rabbit’s ear.

It is believed that the facts drawn from pathology and assembled by Thoma himself have given an assured basis to the second of these laws. The most spectacular example of the second law is found in the arterial-like changes The Capillary Bed in the Embryo 19

which a vein undergoes when it is subjected to the forces operating upon an arterial wall by introducing it into an arterial stem.

Thoma’s work on the area vasculosa of the chick has long been regarded as the principal support of the first of these laws, but it is true that for the most part in the embryological field this law has been based on its interpretative value rather than supported by quantitative investigation. Hughes in his latest work (which we have had the privilege of seeing in manuscript) has attempted to make good this defect by a very extensive quantitative investigation on the blood vessels of the developing chick. He has taken as his source of observations the marginal velocity of the corpuscles wherever measurable in arteries and veins. Owing to‘the very great difficulties encountered in plotting vessel size against such velocities as can be seen and measured, there is a considerable degree of inconstancy in the results obtained.


It occurred to us that in fixed injected specimens it might not only be possible to make more accurate measurements, but also that it would be much easier to measure the diameter of the entering and leaving blood vessels and the area of the capillary bed which they supplied. This implies of course the assumption that the amount of blood delivered to the capillary area would be proportional to its area and would also be proportional to the cross-sectional area of the vessel of supply.

There were available to us a number of pig embryos which had been injected while still living with indian ink. These we had used previously for the study of the development of the blood vessels of the forelimb. The injected specimens had been fixed in formalin and cleared through oil of wintergreen. Out of the material we selected a number in which the limbs had been detached and in which the filling of the vessels appeared to be complete.

Under the microscope, by means of camera lucida tracing, it was possible to measure the diameter of the entering vessels and the area covered by the injected capillaries. Areas were measured either by using squared paper or by means of a planimeter. The diameter of the entering blood vessel was obtained in the same way.

The graph shown as Fig. 1 is the outcome of these measurements upon arteries and capillary beds. It will be seen that at the stage when the arteries are becoming differentiated fromthe capillary bed the graph shows a close approximation to a straight line. These measurements cannot claim any great precision, for the pressure (blowing by the mouth) at which the injections were made might differ somewhat, the resistance to distension perhaps would vary in the different embryos, and the state of contraction or dilatation of the capillary bed might not be the same. These considerations can be used either to suggest that the agreement between vessel size and capillary bed is greater than appears, or that the relationship is more apparent than real. It is also to be

2—2 20 H. H. Woollard and J. A. Harpman

observed that the graph shows a change from the linear relation at the beginning and towards the end of the instances plotted. The earlier curve occurs presumably because then our examples fall within the period of the angioblastic response which is forming the capillary bed, and the second later curve because we now reach toward the more stable relation that arises as the rate of growth declines. The curve hereafter will approach more and more to the horizontal as growth becomes slower and slower.

Area of capillary bed in sq. mm.

10 5 e e 0 0 0-05 1-0 0-15 Cross sectional area of artery in sq. mm.

Fig. 1

The illustrations which accompanied our earlier paper (7) could be used also for the construction of a graph, since in each the capillary bed and the size of the artery had been accurately drawn. Since, however, they had not been all drawn at the same degree of magnification it was necessary, in order to make a graph, to plot the ratio of the size of the artery to the area of the capillary bed against the length of the embryo. This gave us a straight line, but the cases illustrated are too few to justify publishing the result.

In these computations we have measured the area of the capillary bed rather than its volume. Obviously it would have been more satisfactory to have used this latter measurement. In practice the difficulties of getting an accurate measure of cubic capacity would have been so great that many new The Capillary Bed in the Embryo 21

errors would have been introduced. The measurement of the area was really of more value.

_The measurements obtained for the veins were unsatisfactory. In our preparations the diameters of the veins change frequently and irregularly along their course. Further, sometimes a vein which may have attained a relatively considerable volume along its course might relapse into a capillary network again in the vicinity of the body wall. However, when selecting older specimens in which these difficulties did not occur, it can be said that there is some correspondence between vein size and the area of the capillary bed.

After taking into account the various sources of error in these measurements, and after the admission that they measure only indirectly the rate of blood flow into a capillary bed, we believe that they help us to justify the validity of Thoma’s law in regard to the development of blood vessels in the embryo. The matter seems to us of some importance. Frequently the development of blood vessels, for example those of the lower extremity, are described as exhibiting a series of changes wherein the pattern of large definitive stems changes in a manner for which there seems to be neither functional nor morphological reason. If one accepts Thoma’s laws, or even only the first one of them, one must feel some scepticism about these apparently inexplicable changes.

The evidence behind these laws of Thoma is now considerable. There is the great work of Mall(5) on the structural unit of liver, wherein he showed by numerous measurements how Thoma’s laws hold throughout development, and serve to ensure the functional value of the adult arrangements. Upon the development of veins the laws of Thoma seem much more doubtful. Even in the earlier stages of development the correspondence, measured as described above, between size of vein and capillary bed is not close and in the adult the dimensions of the venous wall, the size of the vein, and the pressure within it have only the most general relationship to one another.


1. Using the injected blood vessels of pig embryos at different ages the size of the artery and the area of the capillary bed supplied in the limb have been measured.

2. Plotting these against each other an approximate straight line has been obtained within certain limits.

3. Within these limits it has therefore been concluded that the size of the artery depends on the amount of blood passing unit cross-sectional area in unit time, or the rate of the flow of blood which is Thoma’s first law.

4, The same observations applied to veins at the same stage of development exhibit only very slight agreement. 22 H. H. Woollard and J. A. Harpman


(1) Cuarx, E. R., Hrrscuter, W. J., Rex, R. O., Kirsy-Smitu, H. T. & Smiru, J. H. (1931). “Observations on the ingrowth of new blood vessels into standardized chambers in the rabbit’s ear, and the subsequent changes in the newly grown vessels over a period of months.” Anat. Rec. vol. L, p. 129.

(2) Evans, H. M. (1912). “‘The development of the vascular system.” Manual of Human Embryology, vol. 11, p. 570. Philadelphia and London: Keibel and Mall.

(3) Huauss, A. F. W. (1935). “Studies on the area vasculosa of the embryo chick. I.” J. Anat., Lond., vol. Lxx, p. 76.

(4) (1937). ‘‘Studies on the area vasculosa of the embryo chick. II.” J. Anat., Lond.,

(in the Press). These papers contain all the relevant literature on this subject.

(5) Mata, F. P. (1906). ‘A study of the structural unit of the liver.” Amer. J. Anat. vol. v,

p. 227.

(6) Sastn, F. (1922). “Direct growth of veins by sprouting.” Contr. Embryol. Carneg. Instn. No. 65, vols. xmI-xtv, p. 1. (7) Woottarp, H. H. (1922). ‘The development of the principal arterial stems in the forelimb of the pig.” Contr. Embryol. Carneg. Instn. No. 70, vol. xiv, p. 139.