ABSTRACT
Specialized areas of ependymal cells around the margins of the third ventricle are collectively termed circumventricular organs (Collins & Woollam, 1981), or ependymal organs of the third ventricle (Legait, 1942). The characteristic features of these regions are the presence of three main cell varieties (ciliated ependymal cells, secretory cells, and tanycytes), and the fact that most of them constitute special zones by which substances can pass from the nervous tissue and vascular supply into the cerebrospinal fluid within the ventricles. In most of these areas the blood brain barrier is modified. The circumventricular organs include the subfornical organ, the subcommissural organ, the median eminence, the collicular recess organ, the habenular ependyma and commissural organ, and the organum vasculosum of the lamina terminalis (Williams et al., 1995). By extension, both area postrema and funiculus separans have been sometimes considered as circumventricular organs, in spite of their location around the fourth and not third ventricle. This paper will focus on two of these circumventricular organs: the subfornical and subcommissural organs.
