Regional surface changes during the development of the telencephalic choroid plexus in the chick. A scanning-electron microscopic study.

The surface morphology of the developing chick telencephalic choroid plexus (TCP) was examined by scanning electron and light microscopy. A blunt evagination develops rostro-cranially to the foramen of Monro on the medial telencephalic septum. The pseudostratified TCP epithelium differs in its surface morphology from that of the surrounding ependyma. Subsequently the TCP becomes elongated and branches. On the 9th embryonic day (ED) the pseudostratified epithelium progressively becomes high columnar epithelium in a distal to proximal direction along the branches of the TCP. The apical poles of the high columnar epithelial cells protrude into the ventricular lumen. Later, additional branches sprout at the base of the TCP, which then resembles a tree with a bush growing at its roots. Before the time of hatching, the high columnar epithelium changes to low columnar epithelium again in a distal to proximal direction. The surface of the TCP becomes flatter, in the process of which the number of cilia per unit surface area is reduced. On the developing TCP the epiplexus cells vary in shape, depending upon their functional state. It is proposed that not only the morphological but also the functional differentiation of the TCP proceeds in a distal to proximal direction along the branches of the choroid plexus. The surface differentiation of the TCP has a more regular character than that of the diencephalic CP (DCP), described previously, which seems to be influenced in its development by other anatomical structures.