Abstract
IN rodents, the foetal part of the placenta contains giant trophoblast cells which are unique among mammalian cell types in that each nucleus contains several hundred times the haploid amount of DNA1–4. We have investigated the mechanism by which this DNA is accumulated, in order to understand its relation to trophoblast function. Galassi5 suggested that engulfment of maternal cells might be responsible for the formation of the giant nuclei, while Avery and Hunt6 raised the possibility that diploid trophoblast cells fused. Recent studies4,7 make both these possibilities seem unlikely. On the basis mainly of cytological observations, Zybina1 has proposed that giant trophoblast nuclei arise in the rat by a series of endoreduplications, that is replication of the genome without subsequent mitosis and cell division. Her claim8 that polytene chromosomes9 could be seen in these nuclei was not supported by our studies on mouse trophoblast4.
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