34§ 



NA TURE 



[Sei'Tember 9, 192: 



cally unaltered throughout the individual's existence, 

 growing no further. If severed, say, by a wound, 

 they die for their whole length between the point 

 of severance and the muscle or skin they go to. 

 Then at once the cut ends of the nerve-fibres start re- 

 growing from the point of severance, although for years 

 they have given no sign of growth. The fibre, so to say, 

 tries to grow out to reach to its old far-distant muscle. 

 There are difficulties in its way. A multitude of non- 

 nervous repair cells growing in the wound spin scar 

 tissue across the new fibre's path. Between these alien 

 cells the new nerve-fibre threads a tortuous way, avoid- 

 ing and never joining any of them. This obstruction it 

 may take many days to traverse. Then it reaches a 

 region where the sheath-cells of the old dead nerve- 

 fibres lie altered beyond ordinary recognition. But the 

 growing fibre recognises them. It joins them and, tunnel- 

 ling through endless chains of them, arrives finally, 

 after weeks or months, at the wasted muscle-fibres which 

 seem to have been its goal, for it connects with them at 

 once. It pierces their covering membranes and re- 

 forms with their substance junctions of characteristic 

 pattern resembling the original that had died weeks or 

 months before. Then its growth ceases, abruptly, as it 

 began, and the wasted muscle recovers and the lost 

 function is restored. 



Can we trace the causes of this beneficent yet so 

 unaccountable reaction ? How is it that severance can 

 start the nerve re-growing. How does the nerve-fibre 

 find its lost muscle microscopically miles away ? What 

 is the mechanism that drives and guides it ? Is it a 

 chemotaxis like that of the antherozooid in the botanical 

 experiment drawn towards the focus of the dissolved 

 malic acid ? If so, there must be a marvellously 

 arranged play of intricate sequences of chemically 

 attractive and repellent substances dissolved suitably 

 point to point along the tissue. It has recently been 

 stated that the nerve-fibre growing from a nerve-cell 

 in a nutrient field of graded electrical potential grows 

 strictly by the axis of the gradient. Some argue for 

 the existence of such potential gradients in the growing 

 organism. Certainly nerve regeneration seems a return 

 to the original phase of growth, and pieces of adult 

 tissue removed from the body to artificial nutrient 

 media in the laboratory take on vigorous growth. 

 Prof. Champy describes how epithelium that in the 

 body is not growing, when thus removed starts growing. 

 If freed from allfibrous tissue, its cells not onlygerminate, 

 but, as they do so, lose their adult specialisation. In 

 nerve regeneration the nerve-sheath cells, and to some 

 extent the muscle-cells which have lost their nerve-fibre, 

 lose likewise their specialised form, and regain it only 

 after touch with the nerve-cell has been re-established. 

 So similarly epithelium and its connective tissue 

 cultivated outside the body together both grow and 

 both retain their specialisation. The evidence seems 

 to show that the mutual touch between the several cells 

 of the body is decisive of much in their individual 

 shaping and destiny. The severance of a nerve-fibre 

 is an instance of the dislocation of such a touch. It 

 recalls well-known experiments on the segmenting egg. 

 Destruction of one of the two halves produced by the 

 first segmentation of the egg results in a whole embryo 

 from the remaining half-egg ; but if the two blastomeres, 

 though ligated, be left side by side, each then produces 



NO. 2758, VOL. I ioj 



a half-embryo. Each half-egg can yield a whole embryo, 

 but is restrained by the presence of the twin cell to 

 yielding but a half embryo. The nerve severance seems 

 to break a mutual connexion which restrained cell 

 growth and maintained cell differentiation. 



It may be said that the nerve-sheath cells degrade 

 because the absence of transmission of nerve impulses 

 leaves their fibre functionless. But they do not degrade 

 in the central nerve-piece, although impulses no longer 

 pass along the afferent fi I ires. This mechanism of recon- 

 struction seems strangely detached from anv direct 

 performance of function. The sprouting nerve-fibres 

 of a motor nerve with impulses for muscular contraction 

 can by misadventure take their way to denervated skin 

 instead of muscle. They find the skin-cells the nerve- 

 fibres of which have been lost, and on these thev bud 

 out twigs, as true sensory fibres would do. Then, 

 seemingly satisfied by so doing, they desist from further 

 growth. The sense-cells, too, after this misunion, regain 

 their normal features. But this joining of motor nerve- 

 fibre with sense-cell is functionless, and must be so 

 because the directions of functional conduction of the 

 two are incompatible. Similarly a regenerating skin- 

 nerve led down to muscle makes its union with muscle 

 instead of skin, though the union is a functional misfit, 

 and cannot subserve function. Marvellous though 

 nerve regeneration be its mechanism seems blind. Its 

 vehemence is just as great after amputation, when the 

 parts lost can of course never be re-reached. Its blind- 

 ness is sadly evident in the suffering caused by the 

 useless nerve-sprouts entangled in the scar of a healing 

 or healed limb-stump. 



There is a great difference, however, between the 

 growth of such regeneration and the growth impulse in 

 pieces of tissue isolated from the body and grown in media 

 outside. With pure cultures, in the latter case, Prof. 

 Champy says the growth recalls in several features that 

 of malignant tumours, for example, multiplication of 

 cells unaccompanied by formation of a specialised adult 

 tissue. A piece of kidney cultivated outside the body 

 de-differentiates, to use his term, into a growing mass 

 unorganised for renal function. But with connective- 

 tissue cells added even breast-cancer epithelium will in 

 cultivation grow in glandular form. New ground is 

 being broken in the experimental control of tissue 

 growth. The report of the Imperial Cancer Research 

 Fund mentions that in cultivation outside the body 

 malignant cells present a difficulty that normal cells do 

 not. To the malignant cells the nutrient soil has to be 

 renewed more frequently, because they seem rapidlv to 

 make the soil in which they grow poisonous to them- 

 selves, though not to normal cells. The following of all 

 clues of difference between the mechanism of malignant 

 growth and of normal is fraught with importance which 

 may be practical as well as theoretical. 



The regenerating nerve rebuilds to a plan that spells 

 for future function, but throughout all its steps prior to 

 the time when it actually reaches the muscle or skin, 

 no actual performance of nerve-function can take place. 

 What is constructed is functionally useless until the 

 whole is complete. So similarly with much of the con- 

 struction of the embryo in the womb for purposes of a 

 different life after emergence from the womb : of the 

 lung for air-breathing after birth ; of the reflex con- 

 traction in the fcetal child of the eyelids to protect the 



