File:Analysis of development (1955) (17549733973).jpg

Title: Analysis of development
Identifier: analysisofdevelo00will (find matches)
Year: 1955 (1950s)
Authors: Willier, Benjamin H. (Benjamin Harrison), b. 1890
Subjects: Embryology; Embryology
Publisher: Philadelphia, Saunders
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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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-
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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.

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