Chapter 7 



REGENERATION AND GROWTH 



A. E. Needham 



I. INTRODUCTION 



By regeneration is understood the restoration of the normal structure of a living 

 organism following the loss of part or parts of it. For a uniform terminology 

 "regenesis" would be a better term, implying a type of morphogenesis which 

 restores the structure initially established in ontogenesis. It is one manifestation 

 of "morphostatic" phenomena (Weisz, 1954), the morphogenetic counterpart of 

 the physiological servo-mechanisms of "homoeostasis" (Cannon, 1932). Regenera- 

 tion usually involves new growth, often as rapid as that of embryos and of malig- 

 nant neoplasms (Christensen and Streicher, 1948). Like any other special case, 

 it has certain advantages for the study of the general problem of growth. 



One advantage is the short time-scale of regeneration. Others are the large size of many 

 regenerates, and the great liberty to select the quantity and quality of the material to be 

 regenerated, and the occasion. Further, adult tissues are generally much hardier than those 

 of embryos. On the other hand regeneration is possibly more complicated than normal 

 ontogenesis because of its dependence on the mature tissues to which the regenerate is 

 attached, and because of the integrating control by nervous, hormonal and vascular 

 systems (pp. 631-647). 



In the most common type of regeneration, "epimorphosis", virtually the whole 

 of the part lost is restored by the growth of a bud or "blastema" of undifferenti- 

 ated cells, which accumulates below the closed surface of amputation. By contrast, 

 some organs, such as the vertebrate liver, after partial hepatectomy, regenerate 

 by vigorous growth throughout the remainder, without restoration of the precise 

 initial form of the organ. This "compensatory hypertrophy" sometimes occurs in 

 an homologous organ topographically remote from the one lost, for example in 

 the partner of a kidney, in a Vertebrate. Losses in sponges, hydroid coelenterates 

 and other colonial animals are often "made good" by compensatory growth else- 

 where, though it has not been shown generally that this is a specific response to 

 the accident or to its sequelae, as it is in the vertebrate liver and kidney. 



In some Protozoa (Summers, 1941), worms (Berrill, 1952) and other animals, asexual 

 reproduction by fission or fragmentation is followed by epimorphic regeneration of each 

 daughter ("architomy"). Other animals, often quite closely related, regenerate the essen- 

 tial parts of the daughters before fission ("paratomy" or "neotomy"). Here regeneration is 



