136 KEITH R. PORTER 



This smooth form, hke the particle-encrusted portions of the ER 

 possesses a characteristic morphology. The unit of structure is tubular 

 (50 to 100 A in diameter), and these are arranged in varying degrees of 

 compaction to form complex tri-dimensional lattices. Such configurations 

 occur in a wide variety of cell types. In some instances their development 

 appears to be related to maintenance of form, e.g., in the sustentacular 

 cells of the frog olfactory epithelium [24]. In cells of the pigment epithelium 

 of the frog retina a development of the smooth ER occupies a large part 

 of the cytoplasm and shows no affinity for the few ribosomes present [25]. 

 It is the predominant form in the sarcoplasm of striated muscle cells and 

 shows a pattern differentiation related to the sarcomeres of the myofibrils 

 [26]. What if any role it performs in common in these widely different cells 

 is a large question. 



In at least one class of cells, that engaged in the synthesis of lipid 

 materials for export, the smooth surfaced form of the ER occurs con- 

 sistently. It has, e.g., been described as the dominant form in the more 

 mature cells of the sebaceous glands (Meibomian gland of the rat) [27]. 

 The secretory (lipid) droplets form initially in these cells in close relation 

 with the tubular elements of the ER and appear later to be segregated for 

 secretion within the vacuoles of the Golgi complex. A similar development 

 of the agranular ER has been reported in cells producing steroid hormones, 

 for example the interstitial cells of the opossum testis [28]. Here the ER 

 is entirely devoid of dense particles and the cells are intensely acidophilic. 

 These observations have special interest and significance because Lynn 

 and Brown [29] in a biochemical study on similar material found evidence 

 that enzymes involved in the production of testosterone from progesterone 

 are associated with microsomes and that, though inactivated by lipases, 

 they are unaffected by ribonuclease at concentrations that normally inhibit 

 protein synthesis in liver and pancreatic homogenates. The inference is 

 that the enzymes active in steroid synthesis are associated with the RNA- 

 free microsomal membranes. This is further supported by observations on 

 fetal zone cells of the human adrenal in which there is a richly developed 

 smooth ER [29a]. 



These three instances suggest a correlation between the agranular ER 

 and contain phases of lipid metabolism, especially where a product is 

 synthesized for export from the cell. The exact site of synthesis with 

 respect to the ER has not in these cases been clearly defined. Thus one 

 cannot say that it is sequestered within the cavities of the ER and trans- 

 ported thence to the Golgi, though involvement of the Golgi component 

 would suggest that the sequence encountered in protein synthesis may be 

 followed here as well. 



In this regard the morphological phenomena observed in intestinal 

 epithelial cells of the rat during fat absorption can be interpreted to have 



