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

Abstract

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-aH]galactose ([SH]gal). Autoradiographs were analyzed using the hypothetical grain analysis of Blackett and Parry (1973. J. Cell Biol. 57:9-15 [aH]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.KEY WORDS placenta glycoproteins electron microscopy trophoblast 9 autoradiography Tritiated monosaccharides have been used in a number of studies to label glycoproteins whose synthesis and secretion are to be followed by light and/or electron microscope autoradiography. These studies have demonstrated that some glycoproteins that are synthesized remain within the cell (5,6,15,37,38,43) while others are added either to the surface coat (3,4,6,23,28,29,41) or to the basal lamina (12, 13). In other cases, the glycoprotein synthesized is destined for secre- 418

Galactosyltransferase Activity of the Microvillous Surface of Human Placental Syncytial Trophoblast

“…No plateau in tissue incorpora tion of 3H-galactose was observed. Tissues heated prior to incubation or incubated at 4 °C with UDP-3H-gal showed the absence of nonspecific adsorption of radioactivity to tissue indicating the need for an active enzyme system to have significant TCA-precipitable radioactivity in incubated tis-suc; (3) tissue incubated with 'H-gal contained significantly lower levels of TCA-precipitable radioactivity than tissue incubated with UDP-'H-gal at the same specific activity; (4) extending the autoradiographic studies pres ented in this paper, an electron-microscopic autoradiographic study of 'H-galactose incorporation into trophoblast has shown the galactosyltrans férase activity located in the cytoplasm of syncytial trophoblast to be ex tremely low, especially in comparison to that present in cytotrophoblast ( 12). It thus appears that !H-gal was utilized predominantly by intracellular glycosyltransfcrases of cytotrophoblast while the sugar of UDP-3H-gal (in the presence of 'H-gal) was incorporated into acceptors at the microvillous surface of syncytium.…”

“…The present study was undertaken to explore a possible mechanism of surface coat synthesis and/ 268 N elson Endf.rs/K ing or repair. The syncytial trophoblast has previously been shown to have minimal intrasyncytial galactosyltransférase activity at term [12]. The bio chemical and autoradiographic study reported here shows that syncytial trophoblast is capable of resynthesizing cleaved galactose moieties in the glycocalyx without dependence on cytoplasmically located galactosyltransferases.…”

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

“…First, the method takes into account the actual shapes and sizes of subceUular components observed in the set of autoradiographs being analyzed and thus avoids the need for constructing theoretical models of subcellular structures in order to take into account "cross-fire" effects of tritium emissions that cause grains to develop over organelles that are nonradioactive. An example of the usefulness of this aspect of the analysis is the fact that the grains overlying the compartment RER/ Mit could not be assigned to RER only in this study but could be assigned to RER in the following study of glycoprotein synthesis in trophoblast (41). The results of other autoradiographic (1,15) and biochemical (35,62) studies of a variety of tissues, including human placenta (38), are consistent with mitochondrial protein synthesis.…”

“…by RER are destined for secretion, as illustrated in the following study of cytotrophoblast (41) and by other investigators studying liver cells (2,3,21). The constitutive proteins most likely contribute to the process of division and/or differentiation of cytotrophoblast.…”

“…The hypothetical grain analysis, described by Blackett and Parry (5) and used in autoradiographic studies by Parry and Blackett (44) and Meyrick and Reid (36), was chosen to analyze the data in this study and the following one (41). This method of analysis was selected primarily because the organelles in both cellular and syncytial trophoblast are distributed throughout the cytoplasm in a pattern that is more nearly random than that in many secretory cells.…”