At S5b, lacunae occur in the syncytium (lacunar stage) starting at the embryonic pole. They coalesce to form a ramified lacunar structure. Between the lacunae, plates of syncytiotrophoblast remain as trabeculae. The endometrium is transformed into the decidua (decidual reaction). The cells accumulate glycogen and lipids; the intercellular space fills with liquid and the tissue becomes oedematous. In the decidua, maternal capillaries widen to form sinusoids. Cells of the syncytiotrophoblast destroy the endothelial lining of the sinusoids. In this way, maternal blood can enter the lacunae. At S5c, uteroplacental circulation begins.