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

Electron microscope autoradiography has been used to study protein synthesis in syncytial and cellular trophoblast of term human placental villi incubated in vitro with tritiated leucine ([3H]leu). Autoradiographs were analyzed using the hypothetical grain analysis of Blackett and Parry (1973. J. Cell Biol. 57:9-15). The results of this study demonstrated that both cellular and syncytial trophoblast have marked capacities for protein synthesis. Cellular trophoblast synthesized protein in both 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. The autoradiographic data suggested that, as in the cellular trophoblast, the vast majority of 3H-proteins synthesized by the syncytial 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 syncytial 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.

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

Electron microscope autoradiography was used to study glycoprotein synthesis in cellular trophoblast (cytotrophoblast) and syncytial trophoblast of term human placental villi incubated in vitro with D-[1-3H]galactose ([3H]gal). Autoradiographs were analyzed using the hypothetical grain analysis of Blackett and Parry (1973. J. Cell Biol. 57:9-15). The results of this study indicated that [3H]gal incorporation into term placental villi was predominantly localized to cytotrophoblast. Utilization of [3H]gal by term syncytial trophoblast was extremely low and yielded too few grains for a quantitative grain analysis. This result is in striking contrast to that found in the preceding study of [3H]leucine incorporation (Nelson, D. M., A. C. Enders, and B. F. King. 1978). Within cytotrophoblast, the rough endoplasmic reticulum incorporated the most [3H]gal into glycoprotein. The Golgi apparatus was another site of [3H]gal incorporation. The vast majority of the [3H]gal incorporated into cytotrophoblast during the pulse incubation remained intracellular through the duration of the experiment. There was little autoradiographic evidence for secretion of tritiated macromolecules. Cytotrophoblast incubated for the longest time period studied (4 h+) showed a substantial concentration of tritiated macromolecules in the Golgi complex and in the ground plasm but not in the rough endoplasmic reticulum.

The Binding of Plasma Proteins to Human Placental Cell Membranes

1. To investigate the relative degree to which human IgG and other plasma proteins bind to cell membranes of the full-term human placenta, suspensions of membranes mixed with radio-iodinated proteins were incubated at pH 6·5 and subjected to Sepharose 2B column chromatography. The amount of labelled protein associated with membrane protein in membrane-containing fractions of the eluate was then determined. 2. Binding of IgG and each of the four subclasses of IgG was appreciable. Binding of IgG was markedly reduced if membranes were incubated at pH 8·0 rather than 6·5. Binding of labelled IgG was inhibited by excess of unlabelled IgG but not by excess of unlabelled albumin. Placental membranes bound much more IgG than did erythrocyte membranes. 3. Binding of insulin was relatively greater than that of IgG, whereas binding of IgE, IgM, IgA, albumin, transferrin and polyvinylpyrrolidone was much less than that of IgG and each of its subclasses. 4. The relative binding of IgG and fragments of the IgG molecule was, in decreasing order of magnitude: light chains, IgG, Fc, heavy chains and F(ab′)2. 5. The results are consistent with Brambell's hypothesis for the mechanism of the transmission of passive immunity from mother to young. In particular the data are consistent with the existence of a limited number of IgG specific receptors on the microvillus membrane of the syncytial trophoblast. Such receptors could protect IgG molecules from degradation during their transplacental transport. The submolecular region of the IgG molecule involved in specific membrane binding appears to be common to all four subclasses and to include the Fc region.