It has been demonstrated that activated factor XIII may catalyze the formation of covalent cross-links between fibrin and collagen. This is shown by the disappearance of the gamma-gamma dimer band in PAA-SDS gel electrophoresis when fibrinogen is clotted in presence of collagen, factor XIII and Ca ions, and by the binding of labeled fibrinogen. This reaction may explain the outstanding physiological importance of factor XIII.
The end of the collagen fibril is an important and unique morphological site. Growth of fibrils in vitro and in vivo appears to occur by addition of ordered subassemblies of collagen aggregates at the fibril end. In tissues the fibril end appears closely related to the cell surface in both mesenchymal cells and epithelia. In the latter cell type there is an apparent relationship between the basement membrane and fibril end. Platelets do not aggregate upon exposure to basement membranes or collagens derived therefrom. We suggest that the end of the fibril which is embedded in the outer face of the basement membrane may be involved in interaction with the platelet.
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