A STUDY OF THE ULTRASTRUCTURE OF DEVELOPING HUMAN UMBILICAL VESSELS

Electron microscopic techniques were used to examine the ultrastructur e of developing human umbilical arteries and vein (8-12, 13-17 and 37- 40 wk gestational age). These showed that with increasing age there is (1) an increase in the size of the lumen and the thickness of the med ia; (2) an increase in the ratio of contractile smooth muscle phenotyp ic cells; (3) an increase in the myofilament content of the smooth mus cle cells and the number of Weibel-Palade bodies; (4) a decrease in th e glycogen content; (5) an appearance of microvilli on the luminal sur face of the endothelium. Lipid vesicles, nerves and vasa vasorum were not observed in any region of the umbilical vein or arteries.