Formation of the circle of Willis during human embryonic development

The circle of Willis (CW) is a circulatory anastomosis that supplies blood to the brain and adjacent structures. We examined the timing of formation of CW in 20 Japanese human embryo samples by using 3‐dimensional reconstruction of serial histological sections. The CW was closed in 1 ( n =  6), 2 ( n =  8), 2 ( n =  3) and 2 ( n =  3) samples at Carnegie stages 20, 21, 22, and 23, respectively. The CW was unclosed in 13 samples (unclosed at ACOM alone, 6 samples; ACOM and bilateral P1, 4; left PCOM and right P1, 1; right PCOM and right P1, 1; ACOM and left PCOM, 1). It was difficult to predict whether the circle would close during further development, as such variations frequently exist in adults.