Observations on the Formation of Connective Tissue Fibers.

Some information on the origin and development of collagenous fibers has been obtained from the study of cultured tissues- and wound healing. In such material fine fibrils can be observed to develop between the cells. These subsequently increase in number and finally coalesce to form larger fibers. It is generally thought that the fibrils are organized from an intercellular protein that is produced by or at least influenced by the surrounding cells, but the precise mechanism is not understood. One approach to the examination of this problem is by way of direct observations on the early sub-microscopic sequences involved in fiber formation. To that end we have utilized the combined methods of electron microscopy and tissue culture. The material studied has included fibrous arrays formed in vitro in association with ex-plants of chick embryo skin and foregut, rabbit thymus and rat pericardium. In preparation for microscopy the cultures containing these arrays were washed in a balanced salt solution, fixed briefly over vapors of OsO4 and thereafter mounted on electron microscope screens by the same technics that are used for cells. Only areas of the culture in which the plasma clot had been completely lysed were suitable for microscopy. In such areas the presence or absence of connective tissue fibers could be determined with the light microscope. Generally the cell population was sparse. Some units appeared as macrophages and fibrocytes and were spread out quite thinly on the coverglass whereas others of unknown nature remained rounded up and were dispersed over the surface of the fiber mat. As was more of less expected, it was found that these fibrous mats contain a great many more fibers than are apparent with the light microscope. In fact, the predominant type, hereinafter referred to as unit fibers, has a diameter usually less than 500 Å (Fig. 1 to 5).