150 The Nature of Biological Diversity 



systems may sequester glucose and transport it to the triad, or I-band 

 level, for glycolytic breakdown. Fawcett and Revel have made the 

 safe suggestion that the system "takes part in the synthesis of energy- 

 rich compounds." But what role? Actually there is fairly good evi- 

 dence from cytochemical studies, in which isolated fractions of the 

 smooth ER were examined (Muscatello et al., 1961), that the mem- 

 branes of the reticulum contain a relaxing factor (the Marsh factor) 

 which inhibits the ATP-ase activity of the myofibril. And there is 

 further evidence (Ebashi, 1961; Revel, 1962) that the triadic element 

 can concentrate Ca++, itself an inhibitor of the relaxing factor. Thus 

 one sees evolving a hypothesis that would involve the sarcoplasmic 

 representative of the smooth reticulum in muscle relaxation. 



This idea finds some support from comparative morphology of the 

 SR in muscles showing different rates of contraction and relaxation. 

 It is observed that muscles which contract at extraordinarily high 

 rates of frequency, for example, the toadfish swim bladder (Fawcett 

 and Revel, 1961), the cricothyroid muscle of the bat (Revel, 1962), 

 the synchronous flight muscle of the dragonfly (Smith, 1961), and 

 the extrinsic eye muscles of Fundiilus (Reger, 1961), all show ex- 

 traordinarily rich developments of the sarcoplasmic reticulum. Rela- 

 tively slow muscles, on the other hand, like cardiac muscle, especially 

 in the turtle (Fawcett and Selby, 1958), show a minimal expression 

 of the SR. It appears that the SR achieves its maximal development 

 in muscles where relaxation has to be achieved in a few milliseconds. 



The association of the endoplasmic reticulum with intracellular 

 fiber systems is not confined to the myofibril. Faure-Fremiet et al., 

 (1962) have recently drawn attention to its structural involvement 

 with the contractile bundles of filaments in a Vorticella type proto- 

 zoan. Though the extrapolation is tenuously supported, one can rea- 

 sonably wonder if many of the motions of cells, especially those which 

 mark the return of the cell to the relaxed or preferred form, may not 

 depend upon the preferred pattern of the ER and the relaxing factors 

 associated with its component elements. 



In this article, repeated mention has been made of patterns in the 

 ER. These may consist, as already indicated, partly of ribosome-asso- 

 ciated elements and partly of particle-free elements. In speaking of 

 patterns, one refers to the relative amounts of these two forms, and 

 their distribution with respect to one another and to the cell. Thus, 

 although the cells of two salt-secreting organs may look alike in terms 

 of smooth ER, they will differ enough in details of ER structural de- 

 sign or pattern to make a separation of electron microscope images 

 perfectly possible if not easy. 



