238 EMBRYOLOGY 



dermal layer, the dermis; deeper within the limb is a layer of connective 

 tissue; and within this is muscle and finally bone (Fig. 160). After the 

 amputation, the surface of the wound heals by a migration of the epidermis 

 over the cut surface. And then one sees a dedifferentiation of structures take 

 place in the course of which cells break off from the muscle and from the 

 bone. These cells lose their typical structure and are said to dedifferentiate. 

 Cells from the connective tissue and from the dermis join the dedifferentiated 

 muscle and bone cells. These cells, together with some cells from the epi- 

 dermis, finally form a large, compact mass at the site of amputation. At the 

 cut end of the limb a large mass of new cells accumulates. These have arisen 

 by migration of the cells from the dedifferentiating structures. These cells 

 form a dense, white mass of tissue called the blastema, which is composed of 

 undifferentiated cells. 



When this blastema reaches a certain size the dedifferentiation of the 

 bone and muscle stops, and the blastema begins to differentiate and replace 

 the part of the limb which was removed. Specifically, if the forearm is cut off 

 the blastema differentiates directly into the bone and cartilage and skin of 

 the forearm. This experiment gives no indication of any so-called reserve of 

 cells. The cells of the blastema, as far as one can tell, come from the tissues 

 of the limb itself. 



Nature of dedifferentiation 



There is still a very difficult problem which is not solved by these observa- 

 tions. Is there any true dedifferentiation of muscle and bone cells? Do these 

 cells return to an embryonic condition? And if they do, can they then dif- 

 ferentiate into any type of cell? More specifically, when a muscle cell de- 

 differentiates can it then differentiate into a bone cell in the new tissue ? Such 

 a change of cell type from muscle to bone would be a rather drastic change 

 and would indicate that the process of development is reversible. There is 

 no conclusive evidence for this kind of true dedifferentiation. Most of the 

 evidence is probably against such a radical dedifferentiation, because the 

 growth of various adult tissues in tissue culture has shown that different cell 

 types retain their individuality for years. They do not, in general, change 

 from one type of cell to another. However, because of the dynamic state of 

 the constituents of the body, we must not close our minds to the possibility 

 of true dedifferentiation. If cellular constituents are in a chemical equilibrium 



