Electron microscopy of the parameres formed by the centromeric heterochromatin of human chromosome 9 at pachytene.

The structure and arrangement of the parameres, which are small bodies representing part of the heterochromatin of human chromosome 9 at pachytene, were studied using transmission and scanning electron microscopy. Parameres appear to be denser than other parts of the chromosomes but have a similar fibrous substructure. The most common arrangement is clusters on the axis of the bivalent, consisting of varying numbers of parameres of variable size. The parameres are joined to each other and to the rest of the chromosome by interconnecting fibres. No evidence was obtained for the organisation of parameres into paired lateral loops, as proposed by previous workers using light microscopy. The combination of osmium impregnation of pachytene chromosomes with a backscattered electron detector in the scanning electron microscope produced very clear images of the pattern of chromomeres. This procedure may prove valuable for pachytene mapping of chromosomes because of the greatly improved resolution compared with light microscopy.