**2. Anatomical closure of the DA**

After birth, extensive remodelling of the DA wall occurs, leading to permanent closure of the DA. Although these rapid changes are readily apparent after birth, the structural remodelling of the DA has already begun in late gestation, under the control of the abovementioned unique differentiation system. Therefore, the DA has a distinct structural character from its neighboring arteries. For example, smooth muscle myosin isoform SM2, which is predominantly expressed in adult arteries, is highly expressed in the fetal DA (Kim, 1993).
