Cytological events involved in protein synthesis in cellular and syncytial trophoblast of human placenta. An electron microscope autoradiographic study of [3H]leucine incorporation.

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Abstract
EM autoradiography was used to study protein synthesis in syncytial and cellular trophoblast of term human placental villi incubated in vitro with 3H-leucine ([3H]leu). Autoradiographs were analyzed using the hypothetical grain analysis of Blackett and Parry. Cellular and syncytial trophoblast have marked capacities for protein synthesis. Cellular trophoblast synthesized protein in its rough endoplasmic reticulum (RER) and its ground plasm, which contained abundant free ribosomes. The vast majority of 3H-proteins remained within the cell, with some of the proteins synthesized ultimately appearing in the nucleus. A small percentage of grains was ultimately associated with the trophoblast basement membrane. In syncytial trophoblast the RER was the dominant site for protein synthesis. As in the cellular trophoblast, the vast majority of 3H-proteins synthesized by the synctial trophoblast remained within the syncytial trophoblast throughout the incubation period. The major portion of [3H]leu-labeling present in the syncytial trophoblast of villi incubated the longest times (4 h+) remained in association with the RER. Labeled proteins did not become concentrated in synctial trophoblast Golgi apparatus, vesicles, or granules. In contrast to cellular trophoblast, the nuclei in the syncytium did not contain 3H-proteins at any time-point studied.