Ultrastructure of the fragile X chromosome: New observations on the fragile site

Using a nonair-drying modification of a method for longitudinal sectioning of metaphase spreads on glass slides [Wen et al., 1997], we have studied 14 preidentified X chromosomes (10 from fragile X specimens and 4 controls) w ith transmission electron microscopy (TEM), Four of 10 X chromosomes from f ragile X specimens exhibited lighter chromatin density in the area of and d istal to the fragile site, most pronounced under dark-field TEM, A clear li ne of separation at the fragile site locus was also observed by TEM in an X chromosome with no visible fragile site after Q-banding, We hypothesize th at these areas of lighter density, including lines of separation, precede t he appearance of the fragile site that is commonly observed using light mic roscopy. (C) 1999 Wiley-Liss, Inc.