Electron Micrographic Observations of Centromeres from Unsectioned Mammalian Chromosomes Isolated by Micrurgy

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Abstract
Cells of mouse, Chinese hamster and human cell lines were cultivated in vitro and were used during the period of maximum growth rate to obtain many mitoses without colchicine which is believed to alter centromere and spindle structure. Cells in prometaphase or me taphase were selected and incised by microneedle to bring out lengths of chromosomal spindle fibers to which chromosomes were attached at regular intervals. The light microscopic observation that centromeres are attached to one another by chromosomal spindle fibers was substantiated by electron microscopy and suggested this as a mechanism which may provide the geometric order within the nucleus that numerous workers have come to recognize. Electron micrographs of centromeres show them to contain a central pale matrix having amorphous substructure, formed in the shape of a heart and surrounded by fibers 100 Å thick. This central body is firmly attached to the remainder of the chromosome by two or four chromonema fibers about 500 Å thick, which extend from one arm, through the region of the centromere, to the other arm in each chromatid. The central body is attached to four spindle fiber bundles, two for each chromatid. At each of the four sites of spindle fiber attachment there is a region which stains with mitochondrial stains. These regions have been repeatedly demonstrated by light microscopy, but have not been identified with certainty in electron micrographs. These several structural features and relationships have been combined in a model which serves well as an illustration for numerous studies by others.

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