Effect of acclimation temperature on the axon and fiber diameter spectra and thickness of myelin of fibers of the optic nerve of goldfish.

The optic nerves of common goldfish acclimated to 5 and 25 degrees C were fixed with glutaraldehyde in either phosphate buffer or PIPES with EGTA, post-fixed with osmium tetroxide, and examined by electron microscopy. The axon diameter spectra, from axons measured in electron micrographs and those measured on the electron microscope screen, differ noticeably with acclimation temperature. At the lower temperature, there is a definite shift toward the occurrence of larger fibers compared with the spectrum of the 25 degrees C fish. Although the number of fibers assessed is small compared with the total number in the goldfish nerve, these results confirm our previous study. These findings could be attributed to an increase in the number of new fibers during the acclimation to the higher temperature. We discuss this possibility and on the available evidence find it unlikely. Other changes in the axon and fiber are also seen with acclimation temperature. The axon to fiber diameter ratio, made directly from the electron micrographs, shows that axons from the nerves of the higher acclimation temperature fish possess consistently thicker myelin sheaths than are found for axons in nerves of the lower temperature fish. This finding is also in agreement with results obtained by us from measurements independent of each other.

Effect of acclimation and fixation temperatures on the number of lamellae and periodicity of myelin in fibres of the optic nerve of goldfish.

Previous findings from our laboratory have shown that the optic nerves of goldfish acclimated to different temperatures differ considerably in their glycerophospholipid composition. This paper describes changes in the morphology of the nerve with different acclimation and fixation temperatures. Optic nerves of 5 and 25 degrees C acclimated fish were excised and fixed at the temperature of acclimation, or at the reverse temperature, and the morphology observed by electron microscopy. Under all temperature conditions considered there is a statistically significant linear relationship between the radius of the axon and the number of myelin lamellae. However, the temperature of acclimation and fixation both influence the regression coefficients for this relationship, the higher the acclimation temperature the lower the coefficient and the higher the fixation temperature the higher the coefficient. The periodicity of the myelin also alters with these temperatures, being greater in the 25 degrees C fish than in the 5 degrees C ones. Myelin sheath thickness is also significantly greater in the 25 degrees C fish. These results are discussed in relation to observed changes in glycerophospholipid composition and conduction velocities.

Effect of dietary lipid on the acyl group composition of glycerophospholipids of brain endothelial cells in the developing rat.

The effects of dietary lipid on the acyl group composition of glycerophospholipids of isolated brain endothelial cell fractions have been determined in the developing rat. Rats were fed high-protein diets containing differing dietary linoleic/linolenic acid ratios but having a similar caloric percentage, or were fed a fat-free diet. With a diet supplemented with corn oil (linoleic/linolenic ratio, 47:1), the proportion of acyl groups of the linolenic acid series (n-3) declines in all glycerophospholipid fractions compared with the controls (linoleic/linolenic ratio, 8.5:1), but the proportion of linoleic acid series (n-6) tends to rise. Consequently, the n-6/n-3 ratio of these glycerophospholipids is markedly higher with corn oil than the control diet. N-9 groups (oleic acid series) are consistently lower in proportion. With fat-free diet, the proportion of n-9 groups is higher in these glycerophospholipids than in the controls, but there is no change in the n-6/n-3 ratio. Comparing the changes produced in the adult and the developing cell fraction, the developing cell fraction is more responsive to dietary influence than that of the adult.

Glycerophospholipids and acyl group changes in rat brain capillary endothelial cell fraction and the liver during development.

The glycerophospholipid (GPL) content and the acyl group compositions of the ethanolamine and choline glycerophospholipids (EGP, CGP) of the isolated brain capillary cell fractions of the developing rat have been determined. The proportion of EGP/GPL increases during development, whilst CGP/GPL declines: consequently EGP/CGP increases with age. The acyl group compositions of the plasmenylethanolamine (P-GPE) and the diacyl glycerophosphocholine (D-GPC) also change during development, the proportions of saturated fatty acids (SFAs) declining and n-6 groups rising. The rates of accumulation of individual acyl groups of EGP and CGP are highest between the ages of 15 and 20 days; particularly evident is the increase in the medium chain SFAs: 16:0 and 18:0, the monoene 18:1n-9 (oleic acid) and the major-occurring polyunsaturated fatty acid, arachidonic acid, in these GPLs. These changes are compared with those of the liver where the proportion of EGP/CGP remains constant and the acyl group compositions are relatively unchanged during development. Consequently, the rates of accumulation of individual acyl groups of EGP and CGP in liver remain constant or rise slightly during this age period. These results are compared with existing accounts of the desaturation capabilities of brain and liver with age.

Distribution of neurofilaments in myelinated axons of the optic nerve of goldfish (Carassius auratus L.).

Neurofilaments were counted in myelinated axons of the optic nerve of goldfish which were acclimated to 5 degrees and 25 degrees C. The number of neurofilaments increases markedly with increasing axonal size; axons of less than 0.1 micrometer 2 in area contain between 25 and 60 neurofilaments, while in the larger axons of area greater than 1.0 micrometer 2 there are approximately 190. The densities of the neurofilaments in the small axons are noticeably higher than in the larger ones (507 and 160, respectively). A variety of fixation procedures i.e. osmium tetroxide (OsO4) in phosphate buffer, glutaraldehyde (4%) in phosphate buffer or in ethyleneglycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) and piperazine-N-N'-bis-(2-ethanesulphonic acid) (PIPES) and post-fixed with OsO4 had no effect on the numbers of neurofilaments relative to the size of axon. The anaesthetic MS-222 (tricaine methanesulphonate) likewise had no effect on the numbers of neurofilaments. It is proposed that temperature acclimation alters the axon diameter concomitant with an alteration in the number of neurofilaments to fit the new diameter of the axons.

Changes in the content and acyl group composition of glycerophospholipids of brain endothelial cells of the developing rat.

The glycerophospholipid (GPL) content and acyl group compositions of isolated brain endothelial fractions have been determined in the developing rat. During development there is a marked change in the proportions of ethanolamine glycerophospholipids (EGP) to choline glycerophospholipids (CGP), the former rising while CGP falls with age. The acyl group compositions of plasmenylethanolamine (P-GPE) and 1,2-diacyl-sn-glycero-3-phosphocholine (D-GPC) after significantly during development; both show a decline in saturated fatty acids (SFAs) and a rise in the n-6/SFA ratio, in contrast to a constancy in composition of 1,2-diacyl-sn-glycero-3-phosphoethanolamine (D-GPE). The degree of change in the acyl group composition in a particular GPL fraction is related to the rate of its accumulation and to the proportional increase in concentration, fractions accumulating most rapidly or increasing markedly in concentration showing the greatest acyl group compositional change. The possible significance of the high proportion of SFAs in P-GPE and D-GPC fractions in the developing brain endothelial fraction is discussed in relation to the altering blood-brain barrier capacities observed with age.

Influence of diet on the acyl composition of phospholipids in endothelial cells and mitochondria of rat brain.

The effect of diet on phospholipid acyl groups of rat brain endothelial cells and mitochondria and of liver was determined. Rats were fed high-protein diets with a 4:1 linoleate/linolenate ratio but with 4.4%, 1.9%, or 0.8% of the caloric content provided by these essential fatty acids (cal % EFA) or were fed a fat-free diet. In capillary endothelial cells the greatest change occurred in the plasmalogen ethanolamine fraction, there being a significant reduction in the n-3 series of acyl groups and increase in the n-9 series as the cal % EFA was reduced. Other phospholipid fractions changed little. More pronounced changes occurred in brain mitochondria and liver phospholipids. The small changes in capillary endothelia with cal % EFA are in contrast to the great changes produced by a change in the linoleate/linolenate ratio. As the ratio is reduced, there is a progressive increase in the n-3 series in all phospholipid fractions.