The particulate material of aquaeous homogenate of forebrain was separated by zonal centrifugation in a continuous 0.4‐1.2 m‐sucrose gradient after sedimentation at 100,000 g to eliminate the soluble material. Based on the absorbance at 280 nm, four major peaks were obtained from adult normal mice corresponding to 1.1 m (A), 0.68 m (B), 0.35 m (C), and 0.12 m (D) sucrose. The two smaller and lighter peaks, C and D, were not present when purified myelin was separated by the same procedure. B consisted of pure compact myelin; A was made of vesicles, sometimes with a double membrane. Throughout development, the myelin peak shifted from 0.58 m in young animals to 0.70 m in very old ones. Moreover, the myelin peak B drastically increased during development, as compared with peak A.‐In the Trembler, the profile was close to normal, with a slightly higher yield of myelin which also peaked at a higher density. In the quaking, there were only two shoulders in the myelin density range at 0.68 m and 0.75 m; in the Jimpy only a faint shoulder was seen, at approximatively 0.67 m. In the shiverer, B was absent and only an A peak was present, at approximatively 0.85‐0.90 m, which contained non‐compact lamellar membranes and myelin figures with an abnormal major dense line. In the mid (an allele of shiverer) the density profile resembled the one obtained in the shiverer (one peak in the 0.88 m region). When considering myelin basic protein (MBP) content in the normal developing animal, it was minimum in fraction A, mainly found in the B myelin peak, but also present in the light fraction (C + D). In young animals this latter peak was prominent, in contrast to the adult. In the Trembler mutant, the profile was close to normal; in the quaking, MBP was mainly found in peak A (0.85 m), and in the Jimpy MBP was very low and nearly constant throughout the gradient (with faint quantities in light C + D fraction). In mid, the content was very low, with a peak in the 0.88 m region, in contrast with its shiverer allele, where MBP is hardly detected. More meaningful myelin studies can be carried out by using zonal centrifugation in continuous sucrose gradient, to determine the density of mutant myelin and the degree of myelin maturation in animals.

Wistar rats were fed for three generations with a semisynthetic diet containing either 1.5% sunflower oil (940 mg% of C18:2n-6, 6 mg% of C18:3n-3) or 1.9% soya oil (940 mg% of C18:2n-6, 130 mg% of C18:3n-3). At 60 days of age, the male offspring of the third generation were killed. The fatty acyl composition of isolated capillaries and choroid plexus was determined. The major changes noted in the fatty acid profile of isolated capillaries were a reduction (threefold) in the level of docosahexaenoic acid and, consequently, a fourfold increase in docosapentaenoic acid in sunflower oil-fed animals. The total percentage of polyunsaturated fatty acids was close to that in the soya oil-fed rats, but the ratio of n-3/n-6 fatty acids was reduced by threefold. In the choroid plexus, the C22:6n-3 content was also reduced, but by 2.6-fold, whereas the C22:5n-6 content was increased by 2.3-fold and the ratio of n-3/n-6 fatty acids was reduced by 2.4-fold. When the diet of sunflower oil-fed rats was replaced with a diet containing soya oil at 60 days of age, the recovery in content of n-6 and n-3 fatty acids started immediately after diet substitution; it progressed slowly to reach normal values after 2 months for C22:6n-5 and 2.5 months for C22:6n-3. The recovery in altered fatty acids of choroid plexus was also immediate and very fast. Recovery in content of C22:5n-6 and C22:6n-3 was complete by 46 days after diet substitution.

5′Nucleotidase and Na+,K+‐ATPase are very probably myelin‐associated enzymes, although not specific for this membrane. Thus, it is important to determine their activity in dysmyelinating mutants in either CNS (quaking, jimpy, shiverer, and mld) or PNS (Trembler). CNS: The activity of 5′nucleotidase was lower in mouse than in rat (10.5 and 28.0 nmol/min/mg protein in brain, respectively). In mouse myelin, the activity was 30 nmol/min/mg protein (and 72 in rat myelin). In mutants, the brain activity was very close to normal. In contrast, ATPase, the activity of which was higher in myelin as compared with forebrain homogenate, presented a reduced activity in various 21‐day‐old and adult mutants, except Trembler. It was normal in 8‐day‐old quaking and in cerebella from mutants. PNS: ATPase was lower than in brain and reduced in most mutants, this being expected for Trembler and quaking but not for shiverer and mld. 5′‐Nucleotidase activity was higher compared with that in brain homogenate (relatively stable between 10‐day postnatal and adult). It was affected in the mutants; in Trembler it was nearly normal in young animals but increased during development. Thus in Trembler, two different myelin‐related enzymes and a myelin‐specific enzyme (CNPase) presented different developmental patterns: ATPase was always reduced, 5′‐nucleotidase was normal, and CNPase was slightly below normal in young (68% of the control value); CNPase activity declined during development but 5′‐nucleotidase increased (42% and 190% of the control in 60‐day‐old animals). It is necessary to consider these results in parallel with alterations in the PNS because of Schwann cell abnormalities. Thus, determination of these two enzymes will provide a useful tool to study myelination and myelin assembly under both normal and pathological conditions.