Cerebrospinal fluid physiology of the developing fetus.

Formation of the ventricular system begins at the time the neural groove closes to form a neural tube. Fluid is present within the tube, even before the choroid plexus anlage appears. This fluid serves as a structural support for the tube, as well as a pathway for diffusion of metabolites prior to the formation of blood vessels. In the small, thin-walled fetal brain, fluid movement is characterized by noncommunication between the ventricles and the meningeal fluid spaces. Ciliary action inside the ventricles produces directional streaming and fluid mixing which aid in diffusing substances from the outer surface of the brain through the extracellular spaces of the wall of the neural tube into the ventricular cavity, and vice versa. The mesenchyme surrounding the brain thins out in a definite, organized pattern to form the pia-arachnoid membrane and the cisterns of the subarachnoid spaces. The residual mesenchyme forms the trabecular meshwork of the arachnoid.