Introduction

Seven small aggregates of tissue, that together account for less than 0.5% of the brain's volume (Gross and Weindl, 1987), have the vital role of transducing blood–borne, humoral signals into neural responses that maintain systemic homeostasis. Their location at the mid-sagittal surface of the cerebral ventricles suggests that the aggregates, designated as circumventricular organs (CVO), are in a position to participate in the exchange of substances between blood and the cerebrospinal fluid (CSF) (Fig. 29.1). They are able to receive solutes from blood and to release their secretory material into blood because they lie outside of the bloodbrain barrier. The morphological basis of the vessels’ permeability is like that for the capillaries of endocrine glands: a fenestrated endothelium, a constant feature of all but one CVO, the subcommissural organ. Accordingly, the structure, induction and physiology of fenestrated endothelium will be considered at length.

Fenestrated vessels

The endothelium of fenestrated vessels (FV) is passively permeable to hydrophilic solutes. FV are highly attentuated and perforated by clusters of holes or fenestrae, about 40-60 nm wide (Fig. 29.3), each spanned by a thin diaphragm, about 5 nm thick (Figs. 29.2 and 29.5).