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A. Armulik investigated how pericytes regulate the blood-brain barrier. In the 2010 mouse study, pericyte-deficient mutants showed increased passage of tracers across brain vessels. The increased permeability involved endothelial transcytosis; the study also linked pericytes to endothelial gene-expression patterns and polarization of astrocyte end-feet.^[ref]

This makes an important mechanistic distinction: a barrier depends on regulated transport through cells as well as seals between cells. Measuring tracer escape and identifying the route of escape answer different experimental questions. The primary paper identifies the author as Annika Armulik.

Richard Rodewald investigated selective transport of maternal antibodies across the newborn rat intestine. His 1973 study followed ferritin-linked immunoglobulins and antiperoxidase antibodies through intestinal epithelial cells, using tracer localization to distinguish selective uptake from nonspecific entry.^[ref]

In proximal intestinal cells, suitable antibodies bound at the apical surface and entered transport vesicles. Contrasting tracers and intestinal regions helped show that a molecule entering a cell is not necessarily carried into the circulation. The experiment made the route and selectivity of transcytosis visible, rather than relying only on a final blood concentration.

Transcytosis carries material through a biological cell from one surface to another by combining endocytosis, intracellular vesicle transport and exocytosis. In an epithelial sheet, cargo can enter at the apical surface facing a cavity and leave at the basal or lateral surface facing underlying tissue. This route crosses the cell rather than passing through the gap between neighbouring cells.

An illuminating example is transfer of maternal antibodies across the intestine of a newborn rat. Richard Rodewald followed antibodies carrying an electron-dense ferritin label, and independently followed antibodies recognizable by a peroxidase reaction. Electron microscopy located tracer at the apical surface, in internal tubular vesicles and near release sites at the opposite surface. Experiments used isolated, ligated intestinal segments and control tracers to distinguish selective transport from leakage between cells. The observations establish a route across the epithelial cell, not merely disappearance from the intestinal cavity.^[ref]