Concerning the normal structure of blood vessels, the following is/are true:

a. In utero, hemangioblasts give rise to both vascular conduits and hematopoietic tissue

c. After birth, growth of large vessels does not change the number of elastic and smooth muscle layers

The earliest vascular primordia in the embryo are isolated hemangioblasts that give rise to both vascular conduits and hematopoietic tissue. Endothelial cells organize at sites of vessel development followed by mesenchymal cells that form the outer layers. The number of elastic and smooth muscle layers remains constant after birth, although wall mass increases due to proliferation. Adventitia lies outside the external elastic lamina