692 WOUND HEALING 8 



in turn align into fiber strands by lateral association (Porter and Hawn, 1949). The 

 type of fiber formed depends upon the pH of the medium (Hawn and Porter, 

 1947): in acid media the fibers are of a larger diameter. 



This fibrin net serves as a framework into which the capillary buds and adjacent 

 fibrocytes migrate. A similar phenomenon occurs in tissue culture where syneresis 

 of the plasma clot residts in alignment of fibrin filaments according to tension 

 forces existing within the clot (Weiss, 1929), and these filaments in turn orient 

 the outwandering cells (Weiss, 1933). According to Weiss (1949a) the sequence 

 is essentially as follows: tension produces an alignment of the matrix molecules 

 into chains which in turn build up submicroscopic fibers and subsequently larger 



Fig. 14. Photomicrograph of a 12-day old rat burn. The wound is sectioned parallel to the 



skin surface and at the level of the lower portion of the dermis. The wound area is to the 



right of the figure and the fibrocytes of the adjacent normal skin are oriented in a radial 



fashion. (Hematoxylin and Eosin, X24). 



units. The cytoplasmic filaments of cells already present and those invading the 

 area assume a shape according to surrounding tension forces and extend along 

 the surfaces of these fibers. Normally a single explant of fibroblasts grows in a 

 radial manner, but when two such explants are placed close to one another an 

 attraction field is formed. Katzberg (1951), using polarized light, observed a 

 birefringent pattern between two such explants and believed that syneresis of 

 the plasma clot had developed tension forces which subsequently oriented the 

 fibrin. He noted that fibroblasts of an explant, outside the attraction field, grew 

 less rapidly than those within it. The cells in the field stopped growing when the 

 gap was bridged ; this also occurs in vivo, for example, between cut ends of healing 

 tendons (Buck, 1953). In liquid media cells migrate only after a fibrous exudate 

 has been produced, and their orientation is govered by the fibrin strands (Weiss, 



