Modes of protein and peptide uptake in the pineal gland of the Mongolian gerbil: An ultrastructural study

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Abstract
The possible existence of either a blood‐brain barrier or a CSF‐brain barrier was examined in the pineal gland of the Mongolian gerbil using the ultrastructural tracers, horseradish peroxidase (HRP) and microperoxidase (MP). The mechanism of protein and peptide transport within the pineal gland and its possible relationship to pineal concretions was also considered. Gerbils were injected with either MP or HRP intravenously (IV), or they received intraventricular (IVT) injections of HRP. The IV injections resulted in both MP and HRP movement through the endothelial cells of the gland by vesicular transport and by diffusion through the endothelial intercellular junctions. Following the IVT injections, the tracer was demonstrated in the subarachnoid space as well as in the glial cells associated with the periphery of the gland. In addition, after the IVT injections, rounded enlargements of the intercellular space that resembled canaliculi were filled with reaction product. In both cases (IVT and IV), the reaction product was localized to the perivascular space, to the glial cells and pinealocytes, and to the intercellular spaces. More importantly, there was specific localization of the tracers in the vacuolated pinealocytes and in the pineal concretions. The results of this study demonstrate several significant findings: (1) neither a blood‐brain barrier nor a CSF‐brain barrier exists in the pineal gland of the gerbil, (2) localization of the tracers in pineal concretions indicates a relationship between these structures and protein and peptide storage within the gland, and (3) the presence of the tracers within canaliculi after the IVT injections suggests a possible mode of secretion of pineal substances into the CSF.