Evidence for apical secretion in the subcommissural organ of the chicken (Gallus gallus).

The apical surface and apical cytoplasm of the ependymal cells of the subcommissural organ (SCO) of the chicken, Gallus gallus were investigated by scanning and transmission electron microscopy. The presence of "secretory elements" formed by dilated and irregularly shaped RER cisternae in the proximity of the apical plasma membrane and located into ventricular protusions of apical cytoplasm, evidence an ependymal secretion towards the cerebrospinal fluid (CSF), which is responsible of the Reissner's fibre (RF) by means of an apocrine mechanism. The observed ependymal lysosomal population has been related with an endocytotic mechanism from the ventricular CSF which could also suggest an absorptive role for the SCO.