Enhancement of Secondary Constrictions and the Heterochromatic X in Human Cells

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Abstract
Methods are presented which permit identification of the human X, Y, and specific autosomal sites through a differential action upon presumably heterochromatic portions of metaphase chromosomes. Two modifications of the commonly used air-drying technique for spreading chromosomes were studied using human diploid “fibroblastic” strains. Comparative studies were made with mixtures of male and female cells to control sources of variation. A strong enhancement of secondary constrictions in autosomes No. 1, 9, 16, 4–5, and in the Y was obtained with both procedural variants. In female cells a modified fixative was shown to cause one X chromosome to appear negatively heteropycnotic, permitting its identification in a high proportion of metaphases. The appearance of this X and the affected part of the Y was similar to induced secondary constrictions (No. 1, 9, and 16), being diffuse, pale and puff-like. Evidence that the affected metacentric in female cells was indeed an X was provided by a parallel demonstration in the easily recognizable X chromosomes of cultured Syrian hamster cells. The heterochromatic nature of the allocyclic X and the morphologically similar nature of all these described features imply a common response of heterochromatin in general. Techniques presented provide a convenient means for identification of the X chromosome.

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