Ontogenetical study of the chick embryo subcommissural organ by lectin-histofluorescence and electronmicroscopy

The secretory activity of the subcommissural organ (SCO) was studied during embryogenesis of the chick (Gallus gallus) using two lectins labelled with fluorescein isothiocyanate, concanavalin A (Con A) and wheat-germ agglutinin (WGA). While WGA labels the apical or ventricular border of the organ, Con A labels both, the apical and vascular poles of the cells. Glycoproteinaceous secretory products, visualized by Con A appear early, at 5 days, in the ependymal epithelium and expand progressively in a rostrocaudal direction. A correlation is established between histofluorescence and the ultrastructural aspects of the ependymocytes. This throws light on the role of the endoplasmic reticulum in the synthesis, storage and transport of the material produced by the SCO, and points to the existence of two poles of exchange between the secretory cells and the extracellular medium, i.e., the ventricular and the vascular one. WGA reactivity at the apical border is linked up with the formation of Reissner's fibre and shows that the secretory product of the SCO cells undergoes at least partly modifications during its intracytoplasmic transport preceding apical discharge.