Behavior of smooth muscle cells during arterial ductal closure at birth

To determine which part of the smooth muscle cells (SMCs) of the ductus art eriosus (DA) contribute to duct closure after birth, we looked for areas in which SM2 myosin heavy chain (MHC) mRNA expression, which is associated wi th contraction of smooth muscle, and apoptosis could be detected in the DA during development. In situ hybridization revealed that the SM2 MHC mRNA wa s strongly positive in the longitudinally oriented SMCs and inner layer of the circularly oriented SMCs just before birth. Apoptotic cells were detect ed in the SMCs of the DA from 1 day after birth. Histochemical analysis usi ng terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end -labeling (TUNEL) revealed significant numbers of TUNEL-positive nuclei in the longitudinally oriented SMCs and the inner layer of the circularly orie nted SMCs. Masson-stained sections showed that the TUNEL-positive area in t he DA was replaced by connective tissue from 1 day after birth. These resul ts suggest that the increase in the SM2 MHC mRNA expression and the inducti on of apoptosis are present at the same site in the media of the DA. Theref ore, the SMCs in this area may play an important role in duct constriction and remodeling of the vessel wall after birth.