Ventricular ependyma of normal and hydrocephalic subjects: a scanning electronmicroscopic study.

Recent scanning electronmicroscopic studies of the ependymal surfaces of the lateral, third and fourth ventricles of a variety of animals have shown that most areas are covered by numerous cilia. In this paper, the density of the ciliary population in each of the ventricles is illustrated with material taken from human and rat brains. The authors' examination of Hy3 mice with hydrocephalus, and a number of other reports of examinations of animals with genetic and artificially induced hydrocephalus, have shown that the cilia are lost only from the ependymal surfaces covering those parts of the ventricular wall which are stretched and thinned by the raised intraventricular pressure. Thus the loss of the cilia is most probably the result of the hydrocephalus, and not its cause. Theories concerning the functions of the cilia are reviewed, and a new one accounting for why they are present in such large numbers is suggested. It is proposed that the cilia, together with the ventricular system and the cerebrospinal fluid, provide a cooling system for the brain.