The development of the diencephalic choroid plexus in the chick. A scanning electron-microscopic study.

The surface morphology of the diencephalic choroid plexus (Pl. ch. v. III) was investigated by light microscopy and scanning electron microscopy in chicks from the 7th embryonic day (ED) to the 8th week after hatching. Pl. ch. v. III develops on the anterior ventricular roof from a sagittally oriented fold and a few posteriorly located transverse folds. On the 7th ED no significant differences in the cell surface morphology between Pl. ch. v. III and the surrounding ependyma are observed: both are covered with cilia. During the next four days, long cell prolongations (one per cell) covered with microvilli develop first on the surface of the posterior ventricular roof and then on the posterior part of Pl. ch. v. III. These structures are transitory. On the 11th ED, round cell prolongations (one per cell) appear progressively on the entire plexus, also replacing the long ones. Now the plexus surface is distinct from the surface of the surrounding ependyma. During the last week before hatching and also after hatching, the round cell prolongations become less prominent. Simultaneously, the number of cilia per unit surface area diminishes. With consideration of earlier reports, this study suggests that the following factors are involved in the increase of the surface area of Pl. ch. v. III: (1) The pseudostratified epithelium changes into columnar epithelium. (2) Ependymal elements of the posterior roof of the 3rd ventricle contribute to the anlage of Pl. ch. v. III. In later stages, however, Pl. ch. v. III grows only by mitoses.