Development of arachnoid villi and granulations in man.

The superior sagittal sinus and confluens sinuum of 27 fetuses and newborns ranging from postmenstrual intervals of 26-54 weeks were studied by scanning electron microscopy and histology. 26-week specimens showed oval depressions in the final portions of tributary veins to the sinus. Histologically there were arachnoid tissue clusters within the dural wall. The walls of the depressions were more irregular by the 30th week. Arachnoid villi were apparent by the 35th week and granulations were observed after the 39th weeks. The granulations increased in complexity as development proceeded.