DIFFERENTIAL GROWTH IJT, 



work in which it occurs. Yet growth itself plays a decisive role in creat- 

 ing those physical conditions which, in turn, will orient its further 

 course. A few examples may illustrate this principle of reverberation. 



In the preceding section reference was made to the effect of tension 

 on the form of growth in epithelial tissues. Even more spectacular is its 

 effect on the architecture of tissues of mesenchymal origin. Connective 

 tissue, tendon, fascia, cartilage, bone, blood vessels, muscles, all respond 

 to mechanical tensions by orientation of the component cells and inter- 

 cellular fiber systems along the lines of stress. Experimental analysis 

 of this response in tissue culture showed that the observed cell orienta- 

 tion is a secondary product following a primary orientation of the col- 

 loidal matrix in which the cells lie embedded (44). The sequence is as 

 follows : ( I ) Tension produces alignment of aggregates of the aniso- 

 diametric molecules of the matrix into chains. (2) Such chains join to 

 build up submicroscopic fibrils, which eventually may merge into mi- 

 croscopic fibers. (3) Cytoplasmic filaments (filopodia) from cells in- 

 habiting the matrix extend along the interfaces between these solid 

 micellar threads and the interstitial liquid. (4) The rest of the cell body 

 follows the advancing filopodia. In this manner the cell becomes greatly 

 elongated and is made to trace the ultrastructural pattern of its sur- 

 roundings. Nerve processes advance in the same fashion, except that 

 their cell body remains anchored (45). 



In the original tissue culture experiments the matrix consisted of 

 blood plasma, with fibrin constituting the orientable ultrastructural 

 reticulum. Later experiments (53) revealed that even in so-called cul- 

 tures in liquid medium, cell orientation is brought about by a similar 

 reticulum, namely, coagulable fibrous material exuding from the cells 

 themselves, spreading along the surfaces of the liquid and constituting 

 a trellis ("ground mat") on which the cells then advance. Tensional 

 fibrin organization in blood clots has likewise been disclosed as the 

 mechanism orienting cell growth in wound healing and nerve regenera- 

 tion in the body (57). In embryonic tissues, other fiber proteins can be 

 expected to operate in like manner. All pertinent observations and ex- 

 periments made thus far confirm the conclusion that tension, by way of 

 its organizing effect on the colloidal cellular milieu, can determine {a) 

 orientation of cell movement, (h) cell elongation, (r) cell shape, {d) 

 orientation of mitotic spindles, hence, direction of division, (r) rate of 

 movement, (/) rate of multiplication, {g) deposition of fibrous cell 

 products. Figure i illustrates diagrammatically how different degrees 

 of stretch applied to a fibrous colloid translate themselves into the mor- 



