The fine structure of trophoblast giant-cells from the junctional zone of rat placentas was examined at intervals during two periods, namely, from 12 days of pregnancy (by which time the chorioallantoic placenta has differentiated into junctional zone and labyrinthine regions) until 21 days, and from term (at 23 days) until 4 days beyond term in living conceptuses experimentally restrained in utero beyond littermate delivery. Giant cells appeared viable throughout both periods in that they possessed numerous Golgi complexes, abundant free polyribosomes and mitochondria, and an euchromatic nucleus which was delimited by a nuclear envelop with prevalent nuclear pores. In that many free polyribosomes and few cisterns of endoplasmic reticulum were present giant cells had an undifferentiated appearance at 12 days; they appeared actively phagocytotic through day 18. Both forms of the reticulum were present, but variably so, throughout both periods that were examined; neither was markedly abundant at any stage. Correlation has been sought between the prevelence of agranular and granular cisterns of the reticulum and the relative amounts of secretion, respectively, of placental somatomammotropin and steroid. There was some correlation between development of granular reticulum and known somatomammotropic production, but none between agranular reticulum and what is known about placental progresterone levels during pregnancy in rats. A lack of an elaborately developed agranular endoplasmic reticulum in these giant elements suggests that the cholesterol necessary for progesterone synthesis is itself synthesized elsewhere.
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