DNA content and ploidy level of bovine placentomal trophoblast giant cells

Cytophotometric measurement of the DNA content of Feulgen-stained nuclei in touch preparations of bovine placentomes (n =5) revealed that 8C nuclei occurred in all, 16C nuclei in two, and 32C nuclei in one specimen. The determination of ploidy level by in situ hybridization with a Y-chromosome specific DNA probe showed that the majority of the fetal nuclei in touch preparations of placentomes from male fetuses (n =5) are tetraploid. Generally two tetraploid nuclei lie close together. These findings indicate that polyploidization is a normal feature in the development of the mostly binucleate trophoblast giant cells (TGCs). A new model for the development of these cells is proposed: a primary acytokinetic mitosis leads to a binucleate cell with two diploid nuclei. This cell enters a second acytokinetic mitosis during which the chromosomes of both nuclei form a common metaphase plate. The resulting cell with two tetraploid nuclei undergoes an additional S-phase but does not enter a renewed mitosis. The functional significance of this genome multiplication may be an increased synthetic capacity of bovine TGCs, caused by an increased number of gene copies available for transcription. Since genome multiplication is a property of invasive trophoblast cells of different species, it may be advantageous for trophoblast invasion.