DYNAMICS OF POLYPLOIDIZATION AND QUANTITATIVE STUDIES OF AG-NORS IN THE INTERPHASE NUCLEOLI OF CAMBIAL TROPHOBLAST CELLS IN THE DEVELOPING RAT PLACENTA

Parameters of silver impregnation of nucleoli of rat cambial trophobla st cells were evaluated at the 12th, 13th, and 14th prenatal day, i.e. in the period of their differentiation. The number of Ag-positive gra nules in the nucleoli has been found to vary from 10 to 120 at all the developmental stages studied, the modal number of the granules decrea sing from 60-80 at the 12th day to 40-60 at the 14th day. At the trans ition to each next ploidy class (2c, 4c, 8c, 16c), the number of the A g-positive granules and the area they occupy in the nucleoli increased but this increase was lesser than two-fold at all the stages studied. The area of the nucleoli, on the contrary, increased strictly paralle l to the ploidy degree. The mean number and the total area of the arge ntophilic granules as well. as the area of the nucleoli calculated for each ploidy class did not change in the course of the placenta develo pment. This indicates an approximately equal level of activity of the nucleolar organizer regions at the development stages studied. The per centage of the cells with the different number of their nucleoli was r elatively stable in the placenta development period studied and in the course of cell polyploidization. In all cases, 80-90% of the nuclei c ontained as a rule from 1 to 3 nucleoli. Association of the nucleolus- forming chromosomes is discussed to be a probable prerequisite for tra nsfer of the trophoblast cells from polyploidizing mitoses to the endo reduplication cycle that results in polyteny.