In living cultures of various kinds of adult frog tissues, which have been made according to the hanging drop method, there occurs, in many cases, a transformation of the plasma clot by which it becomes entirely changed from a typical fibrin net both in appearance and structure.
The changes in the fibrin net generally begin when the culture is from two to three days old. During these changes it appears that the elements of the fibrin net fuse or consolidate, and as a result a great number of fine wavy fibrils are formed which unite to form wavy bundles of fibers, and these freely intertwine and anastomose as they ramify through the area of the plasma clot. The transformation of the fibrin net occurs first in the region of the clot which lies next to the imbedded tissue, gradually extends to the distal regions of the clot, and in time—as a rule in about two weeks—the entire plasma clot becomes changed from the fibrin net into a structure which to all appearances is identical with regular connective tissue. Photographs of both living and preserved cultures have been made to show the course of the transformation of the plasma clot and the development of the fibers.
Experiments have been made which show that the fibers which are formed are not outgrowths of the imbedded tissue. Also they are not formed by an intracellular action, but arise directly by a transformation of the fibrin elements of the plasma clot.
Experiments have been made which indicate that the transformation of the fibrin net will not occur unless it has come under the influence of living tissues or of living isolated cells. However, mechanical means, such as exerting tension on the clot with needles, may hasten the formation of the fibers. Also, in some cultures, movements of living isolated cells appear to aid in the formation of the fibers. The living tissues alone, however, are able to cause the fibers to form without the aid of any apparent mechanical factor. This is shown by cultures of various tissues in which no cell movement occurs and in which the plasma clot is undisturbed and yet a prolific formation of fibers may take place.
Experiments have been made in order to determine the true nature of the transformed plasma clot and to see if the new fibrous tissue were still fibrin in character. The results that have so far been obtained from these tests have not been entirely conclusive and leave the question unsettled.
The transformation of the fibrin net results in a shrinkage of the clot. It also becomes very tough and resistant to injury and, therefore, entirely different from the fragile and easily destroyed fibrin net when in its original condition. It is believed that such a reaction must play an important part in wound healing. A study of the relation between connective tissue fibers formed in wound healing and in embryonic development to the fibers formed in the plasma clot is being made, and the results will appear in a later paper.