Penetration of horseradish peroxidase into the extravascular channel system of in vitro European eel (Anguilla anguilla) pars distalis, with comments on the role of the nongranulated cells

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Abstract
Horseradish peroxidase (HPO) was used as an extracellular space marker to investigate the characteristics of the extravascular network in in vitro cultured hemipituitaries of Anguilla anguilla. HPO penetrated large extravascular channels in both the rostral and proximal pars distalis. In the rostral pars distalis the channels were associated with the base of the columnar prolactin cells and in the basal lamina between the corticotropes and anterior neurohypophysis in the dorsal region. In the proximal pars distalis the channels separated the cords of granulated cells. HPO also penetrated into fine extracellular spaces, which appeared to be continuous with the extravascular channels, in both rostral and proximal pars distalis. HPO penetrated only those extracellular spaces associated with the nongranulated (NG) cells, suggesting a related function between the movement of large molecules in the extravascular network and NG cell function. However, in in vitro situations which alter pars distalis cell activity (altered osmotic pressure of the incubation medium or incubation in the presence of metabolic inhibitors such as dinitrophenol or KCN) penetration of HPO did not appear to be affected, suggesting that the HPO penetration was passive, at least in the in vitro conditions used here.