364 



Special Vertebrate Organogenesis 



rial moves on peripherad, thus widening the 

 formerly stunted distal portion (/). The rate 

 of this movement was estimated to be of the 

 order of a few millimeters per day, which 

 corresponds closely to the optimal rate of 

 free advance in regenerating fibers (see p. 

 356). 



These results have led to the conclusions 

 that ( 1 ) axoplasm is synthesized solely in the 



terminal swellings of blocked regenerating 

 nerve fibers (see Cajal, '28; Nageotte, '22). 

 Although we know nothing about the nature 

 of the axonal pumping mechanism,* it is 

 reasonable to assume that it is the same for 

 first outgrowth (see p. 350) and regenera- 

 tion. While the fiber tip advances, the ma- 

 terial is used for elongation; after the fiber 

 has ceased to elongate, the continuing sup- 



Fig. 135. Damming of axoplasm in constricted nerve fibers. A-E, consecutive stages of unimpeded regener- 

 ation; F-H, consecutive stages of regeneration with "bottleneck"; /, following H after release of constriction. 

 (From Weiss and Hiscoe, '48.) 



central cell body near the nuclevis; (2) axo- 

 plasm is conveyed peripherad in a steady 

 movement accommodated to the width of 

 the tube which serves as channel; (3) axo- 

 plasm is subject to continuous catabolic deg- 

 radation all along the fiber. Accordingly, 

 any local reduction of the width of the chan- 

 nel throttles downward flow, hence reduces 

 the rate of replacement of the "downstream" 

 portion, while excess material accumulates on 

 the "upstream" side. Thus is visualized 

 directly what used to be postulated by earlier 

 students of nerve growth as "vis a tergo" 

 (Held, '09) or "formative turgor" (Cajal, 

 '28). 



Damming of axoplasm can now be taken as 

 a direct sign of obstructed axonal transport. 

 As such it is seen, for instance, in the 



ply adds to its width until a steady state is 

 reached between rate of supply and rate of 

 catabolic consumption. Since there is evi- 

 dence (Weiss and Hiscoe, '48) that this 

 centrifugal supply stream continues through- 

 out the life of the mature neuron, nerve re- 

 generation turns out to be but a special 

 manifestation of a perpetual growth process. 

 This explains why nerves can regenerate 

 repeatedly in succession with undiminished 

 vigor (Dimcan and Jarvis, '43). 



* If the rhythmic pulsations demonstrated for 

 central glia cells (Pomerat, '51) were also a prop- 

 erty of peripheral Schwann cells and if these were 

 coordinated in the manner of heart muscle contrac- 

 tions or ciliary beats, this might offer a mechanism 

 for the massaging of axoplasm downward within its 

 sheath. 



