In the early 1960s, while analyzing electron micrographs of mosquito oocytes in search of hints about chromosomal structure, Thomas Roth's roving eye became more interested in the cell surface, which was covered with pit structures lined by a cytoplasmic coat of “bristles.” When he showed the find to his graduate advisor, Keith Porter, he noted that such structures had been seen in one or two other cell types but remained a mystery in those early years of EM. Indeed as Roth scoured other images in the lab and the literature, he saw the fuzzy-bordered pits in every cell type, “but nowhere had it been so in-your-face as it was for the oocyte,” he recalls.
Armed with only one brief reference to “concavities” in liver cells (Fawcett, 1955) and a key paper showing, by fluorescence microscopy, that moth oocytes took up yolk protein from the extracellular space (Telfer, 1961), Roth speculated that the exaggerated yolk protein uptake going on in the oocytes must be related to the exaggerated appearance of the pits all along the cell surface. He went on to describe how the coated pits filled with yolk protein and pinched off into the cell as coated vesicles. Following the dense yolk contents, he observed the vesicles losing their bristle coats (now known as clathrin) and fusing with one another to form yolk protein bodies (Roth and Porter, 1964).
“It was germinal in setting in motion the idea of highly specific endocytosis, now known as receptor-mediated endocytosis,” says Roth (University of Maryland, Baltimore, MD). “It suggested that specificity was involved in uptake [of proteins].” The schematic model figure in the paper has remarkably withstood the test of time even as others went on to describe other types of coated vesicles seen in the cell (Friend and Farquhar, 1967), the molecular mechanism of receptor-mediated endocytosis (Anderson et al., 1977), and the clathrin coat itself (Kanaseki and Kadota, 1969; Pearse, 1976).
But the authors' speculations that the coat “may have a mechanical function, giving…the spherical form to the base of the pit and the pit vesicles,” or determine “the specificity of materials absorbed,” are perhaps the most insightful. Scottie Robinson (University of Cambridge, UK) notes that their speculations were “spot on” as we now know that vesicle coats of all types exhibit both a mechanical and selective function. She marvels that Roth, Porter, and their contemporaries made such intuitive discoveries about the endocytic and secretory pathways from static EM images. “I gather Keith Porter had this extraordinary feel for how the cell did things,” she says.