The morphology of fractions of rat forebrain synaptosomes separated on continuous sucrose density gradients

Abstract

Density-gradient centrif uging did not achieve a dramatic separation of synaptosomes of different sizes or morphological type. The main population of synaptosomes (i.e. those generating profiles less than O-3/i2 in area) may consist of 2 sub-populations, 1 containing mitochondria and 1 not. The latter tend to remain in the less dense region of the gradient. [Serial sections confirm that synaptosomes without contained mitochondria are present (C. D. Voorhorst, personal communication).] The denser fractions may be ''richer in mitochondrial inclusions as was thought by Weinstein et al., but the statistical analysis failed to demonstrate that a higher proportion of the total synaptosome volume is occupied by mitochondria in the denser fractions. The lightest fractions contained considerable numbers of empty membranous sacs similar in dimensions to synaptosomes but of unknown provenance. The mean profile areas of synaptosomes migrating to the middle regions of the gradient were about 12% greater than those of either lighter or heavier fractions. This wasn''t attributable to variations between samples (though more samples would be needed to establish this with cer- tainty), and suggests the interplay of more than one factor affecting profile area (or synaptosome volume). Possibly the mitochondrion-free synaptosomes of the lightest fractions are smaller than those containg mitochondria, and thus depress the mean value for the total population. The smaller size of the densest synaptosomes may be due to shrinkage brought about by the high osmotic pressure of the densest regions of the gradient; the more readily dehydrated members of the original population may well travel further down the gradient. Further characterization of the fractions will require both better sampling techniques and the use of histochemical methods.