LAMELLAR SYSTEMS 139 



sembling the structures observed after enzymatic digestion. Based 

 on these collateral data, the distinct granular structure of the dense 

 and intermediate layers in the myelin sheath as revealed by the 

 new low-temperature preparation techniques merits special atten- 

 tion, and will be considered in terms of our present concepts of the 

 molecular organization of myelin. 



It is assumed that the fundamental radial unit of the sheath is 

 formed by two lipoprotein layers consisting of bimolecular leaflets 

 of 67 A with interposed protein and water layers, each contributing 

 approximately 25 A (Schmitt, 1959). The two bimolecular leaflets 

 of mixed lipids are distinguished by a "difference factor" (Finean, 

 1958), which can be accounted for by assuming that in the process 

 of myelin formation the asvmmetric Schwann-cell membrane's wrap- 

 ping itself around the axon (Geren and Schmitt, 1955; Robertson, 

 1959) will produce a symmetry difference in successive layers. Ac- 

 cording to Robertson (1959), each repeating unit would therefore 

 essentially comprise two Schwann-cell membranes in contact along 

 their outside hydrophilic surfaces to form the intraperiod line. Since 

 rapid freezing and the described low-temperature preparation tech- 

 niques directly affect the aqueous interfaces and other hydrated 

 structures, the characteristic changes can be related in part to the 

 regular distribution of the water layers in the myelin sheath. The 

 intensification of the intermediate line observed after freezing and 

 thawing (Fernandez-Moran and Finean, 1957; Finean, 1958) would 

 therefore probably result from ice formation and associated freezing 

 effects along the highly hydrated intraperiod line. By analogy, the 

 swelling of the myelin layers found after glycerol treatment would 

 be produced by infiltration and binding of the glycerol within the 

 same hydrophilic planes. In order to account for the permeability 

 properties of the membrane, the radially oriented lipid molecules 

 with a cross-section of 4.7 A are considered to be sandwiched be- 

 tween monolayers of protein with a fenestrated type of structure 

 ( Engstrom and Finean, 1958; Fernandez-Moran, 1959b ) . Consider- 

 ing the postulated sieve mechanism and the fluid crystalline state of 

 the membranes, the water constituent would again have to play an 

 important role in the organization within the plane of the lipopro- 

 tein layers. The characteristic dissociation of the layers into rod- 

 shaped granules produced by freezing, enzymatic digestion, and 

 glycerol treatment can be understood in terms of an underlying 



