Blood Flow to the Yolk Sac Placenta as a Function of Arterial Oxygen Content
The yolk sac shrivels and disappears early in human development, but in other mammals it is retained until term and participates in fetomaternal exchange. The vitelline vessels which supply the inverted yolk sac placenta of the guinea pig pursue a different course from the blood vessels that supply the chorioallantoic placenta. The vitelline artery can, therefore, be catheterized without interrupting the umbilical blood flow, and can be used to obtain a reference sample of blood when determining blood flow to the lower body and placenta by the microsphere technique (Carter 1984). Evidently, tying a catheter into the vitelline artery precludes determination of blood flow to the yolk sac placenta. An alternative approach, which we have used when measuring cerebral blood flow (Carter and Gu 1988), is to catheterize the axillary artery. However, because of preferential streaming of blood and microspheres in the fetal circulation, a reference sample from a branch of the ascending aorta cannot be used to measure blood flow in organs supplied from the descending aorta. Blood flow to the yolk sac placenta of the guinea pig has, therefore, been estimated by an indirect approach.