Fine structure of synaptic contacts in the first order giant fibre system of the squid

Summary Scanning electron microscopy and the penetration of horseradish peroxidase, especially from the ventricular surface, has been utilized to determine the distinctive properties of the posterior portion of the area postrema. This part of the organ is characterized by a non-ciliated surface composed of flattened cells, which appear less permeable to cisternally injected peroxidase than the ciliated ependymal cells covering the anterior part of the area postrema. However, more diffuse and rapid penetration of peroxidase into the posterior region is achieved by way of the perivascular spaces which appear in direct communication with the CSF. No such filling is noted in the anterior area postrema. The posterior portion also contains cells which appear to be rapidly penetrated by horseradish peroxidase and which may be important as a sensing mechanism. The chief distinction of the anterior part of the area postrema appears to be the presence of vascular connections with the choroid plexus.This work has been supported in part by Grant NB08549-02 from the National Institute of Neurological Diseases and Stroke and Health Science Advancement Award F-304-FR06115.