Section VIII 



ENERGY EXCHANGE AND ENZYME DEVELOPMENT 

 DURING EMBRYOGENESIS* 



E. J. BOELL 



INTRODUCTION: ENERGY REQUIREMENTS 

 OF THE EMBRYO 



Energy is required by the developing or- 

 ganism to sustain four major groups of proc- 

 esses: maintenance, growth, differentiation, 

 and specific functional activities. Mainte- 

 nance processes continue throughout life, 

 but, to a certain extent, the others occur in 

 sequence during development. Normally, 

 they are not completely independent of each 

 other, and, although they can to some ex- 

 tent be separated conceptually,t they are not 

 easily dissociable even by experimental 

 means (Needham, '42). 



Maintenance metabolism may be defined 

 as the sum total of energy-yielding or en- 

 ergy-requiring processes concerned with the 

 preservation of the living system as an or- 

 ganized entity in its environment. It is at 

 the bottom of the steady state equilibria 

 through which, at least in part, integrity 

 of the organism is preserved; it operates in 

 processes of so-called "active transport," and 

 it is essential for replacement of parts of 

 the living machinery that have been sub- 

 ject to metabolic wear and tear through 

 what Pollister ('54) has termed "mainte- 

 nance protein synthesis." For the embryo, as 

 for all living beings, maintenance metabo- 

 lism represents the basic cost of living. Dur- 

 ing development, maintenance metabolism 



* This paper was completed during the tenure of 

 a Fulbright Award for research at the Carlsberg 

 Laboratories in Copenhagen. The author wishes to 

 express his gratitude to Dr. Heinz Holter, Chair- 

 man of the Division of Cytochemistry, and to Dr. 

 Saren Lovtrup for their critical reading of parts of 

 the manuscript. 



f One of the major limitations in conceptual 

 separation of the fundamental processes in ontogeny 

 is that the embryo's response to a given set of ex- 

 perimental conditions is usually the same no matter 

 whether the investigator is thinking more about 

 maintenance than growth and differentiation at the 

 tune, or vice versa. 



would be expected to increase, not because 

 maintenance becomes progressively more dif- 

 ficult as the embryo increases in organiza- 

 tional complexity, but simply because there 

 is more embryo to be maintained. 



Growth can best be defined in terms of 

 protein synthesis. J It is obvious that such 



X Although the concept of embryonic growth has 

 been under consideration for a long time, there is 

 still no measure of its magnitude that is completely 

 adequate or free from objection. If growth is defined 

 as increase in mass, it is apparent that the process 

 may be independent of synthesis of new materials 

 and may simply involve increase in size by imbibi- 

 tion or deposition of inorganic materials. To meas- 

 ure growth in terms of increase in solids, that is, as 

 dry weight, is satisfactory in the case of embryos 

 whose cells do not contain appreciable quantities of 

 the raw materials for development, but it is com- 

 pletely inapplicable to highly lecithal eggs. In the 

 eggs of amphibians, for example, dry weight de- 

 creases during development as yoLk is consumed. 

 The use of protein nitrogen as a measure of growth 

 has the same limited applicability. Increase in num- 

 ber of cells or of nuclei may be used to assess growth 

 in certain instances, but in a number of embryos 

 cell number increases while the size of the organ- 

 ism remains essentially unchanged. The use of 

 DNA, suggested by Berenblum, Chain, and Heatley 

 ('39) and recently emphasized by Davidson and 

 Leslie ('50) may be an adequate measure of growth 

 within certain limits. It is clear the DNA content 

 of the individual cells of a given species is constant 

 (Boivin, Vendrely, and Vendrely, '48; Vendrely 

 and Vendrely, '49; Mirsky and Ris, '49). Accord- 

 ingly, increase in DNA should presumably indicate 

 relative increase in cell number. But it is also 

 equally clear that the karyoplasmic ratio is not con- 

 stant — at least, not in early development. There- 

 fore, DNA content, while in some cases giving an 

 indication of the relative changes in nmnbers of 

 nuclei, is not an effective measure of cytoplasmic 

 mass. Furthermore, it may be noted that the use of 

 DNA as a nuclear measure may be questioned. 

 Hoff-J0rgensen and Zeuthen ('52) have reported 

 that the frog egg contains a store of DNA enough 

 for several thousands of new nuclei, and Lindahl 

 ('53) has demonstrated recently that micromeres 



520 



