Tissue sections from frozen yolk sacs were immunostained via the immunoperoxidase method, as previously described.²¹  Vessels were sized as small, moderate, or large in size, depending on diameter width: small, <25 μm; moderate, 25 to 75 μm; large, >75 μm. ND, not detectable on endothelium; PECAM-1, platelet endothelial cell adhesion molecule-1; VCAM, vascular cell adhesion molecule; ±, weak expression detected on a subset of vessel; PNAd, Peripheral node addressin.