Ultrastructure of the choroid plexus and its development in the mouse.

The morphogenesis of the choroid plexus of the lateral ventricle of the mouse was studied from the 8th to the 13th post conception day (PCD) in semi- and ultra-thin sections. The choroid plexus appears on PCD 11 with the formation of a horizontal crest in the roof of the interventricular foramen. However, degenerative processes play an important role in the morpho- and histogenetic development of the choroid plexus from PCD 8. During the degenerative period the epithelial cells differentiate and stratify to form the choroid plaque. From PCD 11 the developing choroid plexus expands into the ventricular cavity taking with it mesenchyme and vessels. Ultrastructurally, the epithelial cells differentiate into cells which are more cylindrical and whose nuclei become more elongated. The cytoplasm exhibits numerous prolongations into the ventricular cavity. The golgi complexes, the endoplasmatic reticulum, the ribosomes and polyribosomes are found concentrated in the apical portions of these cells. The apical prolongations of three or more neighbouring cells form "lakes". Junctional complexes between the apical ends of the epithelial cells radiate out from the lakes in which there are microvilli. Macrophages, or "epiplexus cells", frequently give rise to thin cellular processes which encompass protrusions of the epithelial cells. These epiplexus cells contain many vacuoles and phagosomes; numerous mitochondria are localized adjacent to those epiplexus plasma membranes encompassing protrusions. It is proposed that the epithelial cells of the choroid plexus contribute to the formation of the embryonic encephalic liquid.