Transmission electron microscopy has been used to study the chromosomes found in the early diplotene oocytes of Xenopus laevis. Chromosomes from 40 micron early diplotene oocytes were found to possess a normal lampbrush chromosome morphology. The contour length of the loops found on these chromosomes was measured, the mean value determined and compared with that for lampbrush chromosomes found on 300 micron mid-diplotene (Dumont stage II) oocytes. The mean contour length of the loops from 40 micron oocytes (12.33 +/− 6.5 micron) was longer than that for the 300 micron oocytes (7.897 +/− 5.22 micron). Analysis of variance showed these two values to be significantly different (P less than 0.01). The mean loop density (number of loops per 25 micron chromosome axis) was also determined for the two size classes of oocytes, being 9.33 +/− 2.05 per 25 micron of chromosome axis for the 40 micron size class and 11.2 +/− 2.435 per 25 micron of chromosome axis for the 300 micron size class. Analysis of variance showed these values to be significantly different (P less than 0.01). The results of the present study demonstrate that the lampbrush chromosomes found in 40 micron early diplotene oocytes possess a greater loop contour length and a lower density of loops than those found in 300 micron, mid-diplotene oocytes. Possible reasons for these observations are discussed.

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