60 



Cellular Structure and Activity 



An equally striking reconstitution of a new 

 fibrous repeating pattern has recently been 

 accomplished by Hodge ('52). The fibrous 

 protein, paramyosin, of the adductor muscles 

 of the clam has an axial repeat pattern of 

 145 A with a main period five times this 

 value, or 725 A (Hall, Jakus and Schmitt, 

 '45). The paramyosin fibrils dissolve in di- 

 lute acetic acid. Reconstitution of the native- 

 type structure has not yet been accomplished. 

 However, upon increase in ionic strength, a 

 new fibrous form was obtained by Hodge 

 which has a repeating period of about 1400 A. 

 This pattern, never observed in nature, has 

 intraperiod band structure strikingly similar 

 to that of skeletal striated muscle; the coun- 

 terparts of the A, I, Z, M, H and A^ bands are 

 all represented. However, the axial repeating 

 period is only one-twentieth to one-thirtieth 

 that of striated muscle. Whether this similar- 

 ity is merely coincidental remains to be 

 determined. 



It is possible that systems which form such 

 highly ordered structural patterns in vitro 

 are, in fact, multicomponent systems in 

 which small amounts of non-fibrous material 

 are required to integrate the protein chains 

 in particular structural arrays. At any rate, 

 the systems are chemically complex and little 

 is as yet known about the conditions re- 

 quired to produce the patterns. Two new 

 patterns have already been obtained (colla- 

 gen and paramyosin) and it may be expected 

 that more will be found when reconstitu- 

 tion studies are made of other fibrous 

 proteins. 



The resvilts described above suggest a 

 point of view regarding the mechanism by 

 which fibrous and possibly other types of 

 structviral patterns are formed in cells. This 

 may be illustrated in the case of muscle. In 

 the premorphological stages of differentia- 

 tion the proteins and other components are 

 synthesized by the cell but have not yet 

 been fashioned into the form characteristic of 

 the differentiated cells. The presence of 

 myosin and actin in predifferentiation stages 

 of muscle has been demonstrated by Her- 

 mann and Nicholas ('48). When the physical 

 chemical environment of the cell is favorable, 

 the components of the fibrous pattern may 

 "crystallize" out spontaneously in a manner 

 similar qualitatively to that observed in in 

 vitro experiments. The pattern may form in 

 several stages. Thus in muscle the first stage 

 of morphological differentiation is the for- 

 mation of unstriated fibers which show posi- 

 tive birefringence. It is not known whether, 

 at this stage, the fibers have an axial perio- 



dicity of 400 A, as do fully differentiated 

 fibers, or whether the characteristic ratios 

 and geometric relations of myosin and actin 

 have already been achieved. Formation of 

 the banded structure (Z, A, I, M and other 

 bands) follows in a sequence and manner 

 which is not yet fully understood. The finally 

 differentiated structure shows a high degree 

 of regularity of axial repeating pattern. As 

 many as four orders of diffraction, repre- 

 senting the sarcomere length included be- 

 tween Z bands, have been observed with 

 visible light (Buchthal and Knappeis, '40). 

 How can such linear repeating patterns, hav- 

 ing periods as large as 3 to 15|U, and in a 

 few cases very much larger, be produced? It 

 seems probable that the pattern-forming po- 

 tentialities reside in the fibrous system itself 

 and that the process is spontaneous as in the 

 in vitro cases. Any other explanation would 

 require that some kind of equivalent regular 

 structural discontinuities which direct the 

 process preexist in the cell; we would then 

 be required to explain the origin of this 

 intrinsic precursor pattern. 



The longest axial period thus far observed 

 by reconstituting fully dispersed fibrous 

 proteins is about 0.3ju, (3000 A) — the so- 

 called collagen long-spacing. It seems reason- 

 able to expect that when the system contains 

 many more constituents (several fibrous pro- 

 teins in muscle and an undetermined num- 

 ber of other participating compounds), the 

 emerging pattern may have much larger 

 dimensions and be more complex. It is in- 

 teresting to note in passing that the pattern 

 of axial structure in striated muscle, as 

 manifested by band characteristics, is the 

 same whether the repeating sarcomere period 

 is 2 ^ or 15 ju, [in the proventriculus muscles 

 of certain marine annelids (cf. Schmidt, '36), 

 the period may be as much as 100 m]. It 

 seems improbable that the myosin and actin 

 molecules in the different species vary so 

 markedly in properties. It is more likely that 

 the differences depend on other pattern- 

 modulating substances and circumstances. 



It is difficult enough to attempt to inter- 

 pret the phenomena which occur when 

 fibrous proteins as well known chemically as 

 collagen are reconstituted in vitro. Experi- 

 ments with more complicated systems such 

 as skeletal muscle would be very empirical 

 indeed. However, it will be many years be- 

 fore the chemistry of muscle constituents is 

 fully known. Meanwhile, stimulating new 

 discoveries and ideas may come from em- 

 pirical attempts at reconstitution of muscle 

 structure which may throw light not only on 



