Macromolecular translocation--a possible function of astrocytes

We have used dense astrocytic cultures, which display a multilamellar geometry, to determine whether translocation of macromolecules can occur across astrocytic processes. Colloidal gold-labelled transferrin and serum albumin were allowed to bind to the most superficial of the astrocytic cell membranes at 4 degrees C, the temperature was then increased to 37 degrees C and the fate of these gold-labelled macromolecules was observed using the electron microscope. The gold-labelled macromolecules appeared to undergo receptor-mediated endocytosis followed by translocation and exocytosis, with the colloidal gold-labelled macromolecules moving from the apical to the more basal processes. After 1 h of incubation at 37 degrees C, colloidal gold became accumulated within secondary lysosomes of the basal-most layer of astrocyte processes. Since the extracellular space of the central nervous system (CNS) consists of narrow tortuous channels formed to a great extent by the multitude of processes extending from fibrous and protoplasmic astrocytes, these observations suggest to us that one function of astrocytes may be to facilitate transport of macromolecules from one cell to another.