Preliminary observations on argentophilic bodies in the human choroid plexus (Issidorides and Shanklin, '60), suggested that a secretion cycle may possibly occur in the epithelial cells of the choroid plexus. Cytochemical studies of the plexus of the rhesus monkey (Wislocki and Dempsey, '48) cast little light upon the question as to whether the epithelium secretes the cerebrospinal fluid; they did, however, suggest chemical regulation. Recent electron micrographs of the choroid plexus of the rabbit (Millen and Rogers, '56) showed large amounts of endoplasmic reticulum (RNA-rich substance.) in the epithelial cells, indicating protein synthesis. Blebs on the surface of the epithelial cells (Van Breemen and Clemente, '55) have been tentatively interpreted as secretion droplets. The morphological specialization of the epithelial cells in the rat, i.e., villi, protrusions, etc. as seen in electron micrographs (Maxwell and Pease, '56) also implied secretory activity.
Large vacuoles have been observed in the adult human choroid plexus. Some authors maintain that they indicate a disturbance in the water balance of the cells, others that they represent alcohol insoluble lipoid (Schaltenbrand, ' 5 6 ) or lipofuscin pigment (Volkmann, '33).
The observation that the larger argentophilic bodies in our material (Issidorides and Shanklin, '60) appeared to correspond in size and location to these large vacuoles suggested the necessity for further study of these bodies by additional histochemical and other staining methods. We also sought to determine to what extent the observed argentophilic inclusions could be regarded as a secretion product.
Agduhr ('32) published an excellent review of earlier work on the histology of the choroid plexus.