Scanning electron microscopic study on the development of primitive blood vessels in chick embryos at the early somite-stage.

Primary vasculogenesis in chick embryos at the early somite stage (1-14 somites) was investigated mainly by scanning electron microscopy (SEM), with special reference to the development of primitive blood vessels such as the arteria et vena vitellina (AV, VV), aorta dorsalis (AD) and vena cardinalis (VC). After glutaraldehyde fixation, the endoderm or ectoderm was removed from the embryos to expose either the ventral (AV, VV, AD) or the dorsal (VC), vascular system. The mode of vascular formation was found to be identical in all these blood vessels, arising first in loco as isolated solid masses or cords composed of so-called angioblasts. The angioblasts at this developmental phase could be distinguished from underlying mesenchymal cells, exhibiting a relatively flat surface. The VV was recognized first on both sides of the anterior intestinal portal at the 4-somite stage, whereas the forming AD was identified on the ventral surface of the paired somites at the 6-somite stage, appearing almost simultaneously from the cranial to caudal somite regions. After the 8-somite stage, the AV was formed by transformation of one of the caudal plexuses spreading to the area vasculosa. In the 9-somite stage, the angioblastic cords of the VC appeared on the dorsal side of the mesoderm in the same manner as for other ventral vessels. This finding differs from the statement of a previous author that the VCis formed by longitudinal anastomosis of intersegmental diverticula of the AD.