Final answer:
When a new tissue is initially differentiated, it is hemangioblasts, which secrete VGEF and signal endothelial cells to proliferate and grow new capillaries.
Step-by-step explanation:
During embryogenesis, blood vessels begin to form from the embryonic mesoderm. The precursor cells, known as hemangioblasts, differentiate into angioblasts, which give rise to the blood vessels. These cells form masses called blood islands, and the endothelial lining of the vessels arise from the angioblasts within these islands.
Vascular tubes also develop on the blood islands and eventually connect to one another, thus creating new blood vessels from existing ones, a process called angiogenesis. So therefore when new tissue is initially differentiated, it is hemangioblasts that secrete VGEF and signal endothelial cells to grow new capillaries. These cells form masses called blood islands and develop vascular tubes that connect to each other.