64 



Cellular Structure and Activity 



resolve the detailed fine structure of the 

 cell membrane, to say nothing of transient 

 molecular constituents within the membrane, 

 which appear or disappear as development 

 proceeds. And the indeterminacies of fixa- 

 tion are always with us! 



There is of course no substitute for the 

 slow, painstaking structural and chemical 

 analysis of embryonic cells by ever improv- 

 ing techniques. However, pending the ac- 

 cumulation of such data, useful new data 

 and concepts may result from consideration 

 of partial systems along lines briefly ovit- 

 lined below. 



Let us regard embryogenesis as a process 

 of "crystallization" in which the constituent 

 parts are themselves highly diverse, changing 

 in composition and position as development 

 proceeds. Induction of a particular structure 

 corresponds to a crystallization of a pattern 

 within the larger complex of the spatially 

 and temporally integrated complex of pat- 

 terns. How would such a view lead to 

 fruitful experimentation? 



Recalling the results on the in vitro crys- 

 tallization of fibrous macromolecular pat- 

 terns described above, we discern certain 

 similarities to induction at a very much 

 lower level of organization. From an acid 

 solution of collagen may be formed not only 

 typical collagen fibrils but also fibrils hav- 

 ing one-third the normal period, fibrils with 

 no axial period, or long-spacing fibrils, de- 

 pending on the conditions. The specificity 

 and morphogenetic potentiality resides in 

 the fibrous, dissolved collagen; various sub- 

 stances when added to the system may 

 "evoke" one or another of the patterns prev- 

 iously described (Gross, Highberger and 

 Schmitt, '52; Schmitt et al., '53). What forms 

 depends upon the concentration of collagen 

 and of non-collagenous organic material, 

 upon the ionic strength, and upon pH and 

 other factors which can be controlled. The 

 reconstitution of paramyosin affords similar 

 challenging opportunities for studying the 

 way in which the protein molecviles can in- 

 teract despite the fact that almost nothing is 

 known abovit the composition of paramyosin. 



May it not be possible similarly to study 

 the ability of embryonic partial systems, pos- 

 sibly of in vitro suspensions of individual 

 cells or groups of cells, to form specific tissue- 

 like aggregates under conditions in which the 

 chemical environment of the cells is subject 

 to reasonably strict control? Such experi- 

 ments may involve development of special 

 techniques and preparation of material on a 

 rather heroic scale in order to make possible 



all types of combinations and to permit a 

 statistically significant evaluation of the re- 

 sults. Under such controlled conditions (as- 

 suming that the viability of the cells is not 

 damaged too much) it may be possible to 

 introduce materials — or even cells — suspected 

 of having inductive or regulatory signifi- 

 cance. To obtain even a single type of cellular 

 system which is amenable to such manipula- 

 tion may require considerable effort. How- 

 ever, once obtained, it may yield information 

 about factors involved in the formation of 

 cell patterns obtainable in no obvious way 

 from a study of the whole embryo or of parts 

 transplanted into embryos, because under 

 these conditions there are too many variables 

 to permit drawing any simple conclusions 

 directly. 



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