CHANGES IN VASCULAR PATTERNS 



1259 



a capillars exceeds a certain level, a sprout is sent out. 

 If on the contrary there is a diminution below a 

 critical point, there results a decrease in the caliber. 



Hughes (82) in 1937 for the first time supplied 

 measurements of the actual rate of blood flow and 

 was able to substantiate Thoma's first principle, when 

 he found a relationship between the diameter of an 

 artery and the rate of blood flow in the area vasculosa 

 of the chick. He had found that the first differentia- 

 tion of the vitelline artery was mainly effected by the 

 disappearance of connections with smaller vessels to 

 either side (81). As the vessel increased in diameter 

 the endothelial cells, and their nuclei, became 

 elongated at right angles to the axis of the vessel. 

 Mitosis then occurred. By counting and measurement 

 Hughes (82) was later able to establish that increase 

 in cells is proportionate to increase in area. He ad- 

 mitted however that some of these cells might repre- 

 sent incorporated mesenchymal elements, rather 

 than the products of division of earlier endothelial 

 cells, especially in veins. 



The development of the transparent chamber 

 method made it possible to explore under direct 

 vision to what extent angiogencsis in the adult would 

 recapitulate ontogeny. Ziegler (196) first introduced 

 glass chambers for studying the genesis of tubercles, 

 but these were buried in muscle or subcutaneous 

 tissue. Sandison (152), at the suggestion of E. R. 

 Clark, made transillumination possible by bringing 

 the chamber to the surface. This enabled an intimate 

 and continuous observation of vessels in process of 

 formation in adult animals. Sprout formation, 

 fusion of adjacent sprouts, the development of lumina 

 in newly formed capillaries, atrophy of some capil- 

 laries, and the differentiation of others into arterial 

 and venous vessels, were all observed in transparent 

 chambers in the ears of rabbits by Sandison (152) 

 and the Clarks (33, 35), and in similar chambers in 

 dogs (122). For a further discussion of the transparent 

 chamber technique see Chapter 27. 



Williams (190), in autografts of subcutaneous tissue 

 to previously vascularized rabbit ear chambers, 

 found that new vessels can be formed by lateral saccu- 

 lation or "herniation" of the endothelial wall as well 

 as by canalization of sprouts. Such adjacent branches 

 can fuse to give rise to new connecting channels. He 

 recorded also the development of arteriovenous 

 anastomoses. In previous work with omental auto- 

 grafts he observed the formation of connections 

 between host and graft vessels, in some instances 

 within 48 hours (191). It was astonishing to note the 



reconstitution of some of the original vessels within 

 separate fragments of the grafts, and that this could 

 take place before blood flow began. He considered 

 that the principal stimulus to endothelial prolifera- 

 tion was probably hypoxia of a certain degree and 

 duration (190). Under conditions where hypoxia 

 was unquestionably present and maintained for more 

 than 3 days, vascular proliferation was extreme. When 

 free blood flow was established rapidly, however, 

 plexus formation was inhibited. In similar chambers 

 within dorsal skin flaps of mice Merwin & Algire (1 20) 

 found that the original vascular channels in grafts of 

 thyroid tissue survived and were the active structures 

 in establishing connections with host vessels. Grafts 

 of tumor tissue, however, were quickly provided 

 with a rich vascular network derived from the host. 

 Solid sprouts of the host vessels can grow against the 

 "grain" of connective tissue, while ordinary vessels of 

 the graft are oriented parallel to the cells and fibers 

 of the connective tissue. 



The Clarks (32) found in rabbit ear chambers that 

 arteries can be developed without acquiring a nerve 

 supply, and that these do not contract spontaneously 

 but react passively. They can, however, respond to 

 chemical stimuli. 



Types of Collaterals 



Collateral vessels may be preformed, or newly 

 formed, but it may be difficult to distinguish the two. 

 Weyrauch & De Garis (189), after ligature of suffi- 

 cient original arteries and veins in the mesentery, 

 considered some vessels newly formed on the basis of 

 vastly increased numbers and unusual position. It 

 may be questioned, however, whether such vessels 

 were not developed from smaller arterial or venous 

 channels. Under some circumstances, as for example 

 in vascular channels traversing a former serous cavity, 

 there can be no doubt of their new formation. 



In discussing pre-existing collaterals a convenient 

 terminology has been devised by Longland (108) 

 according to their ultimate function. The plexus of 

 connected arteries that may span an obstructed por- 

 tion of a major artery he has called the "midzone," 

 the main artery proximal to the midzone, the "stem," 

 and the distal, he has termed the "re-entrant" (fig. 

 6). His measurements showed that in the extremities of 

 the rabbit, at least during the first 16 weeks, the ves- 

 sels in the midzones grow relatively more than the 

 stem or re-entrant vessels (fig. 7). 



Learmonth (96) has described two types of col- 



