Transformation of the heterochromatin structure during epithelial cell differentiation in Microtus agrestis

The interphase structure of the constitutive heterochromatin of the field vole, Microtus agrestis, was studied in the epithelium of the intestinal and respiratorial tracts, and was compared with the nuclear size and the proliferation rate of these tissues. For determining the nuclear structure and the mitotic and DNA labeling rate, Feulgen and HE-stained paraffin sections were used; measurements of the DNA content and of the nuclear and chromocenter areas were carried out on Feulgen-stained smear preparations of jejunal epithelial cells, nerve cells and glial cells with the integrating microdensitometer of Barr & Stroud and the image analyzer Microvideomat of Zeiss, respectively. In all epithelial tissues, a good correlation was found between the proliferating activity and structure of heterochromatin: the parts of the epithelia with a high amount of compact chromocenters in their nuclei also showed a high cell proliferation rate, whereas in differentiated, nondividing epithelial cells most of the nuclei were free of chromocenters. Between nuclei with and without chromocenters of the same tissues, size differences of about 75% were observed. The areas of chromocenters of diploid G1 nuclei of glial, nerve and jejunal epithelial cells were equal (3-4.8 micrometer2) although the total areas of the nuclei differed to an extent of up to 1,000%. The sum of the areas of two separate chromocenters was about 10% larger than the area of a fused chromocenter.