Have you ever wondered how blood gets into the blood vessels to start with? I mean, the vascular system is a closed system – there is no spout whereby blood gets poured in from a hose and a stopper placed to keep it in. And yet, there is blood pumping through the arteries and veins of itty bitty embryos.
There are 3 initial cell layers of the embryo – ectoderm (outside), mesoderm (middle) and endoderm (inside). The blood and blood vessels come from the mesoderm. In response to a bunch of chemical signals around day 13, some mesodermal stem cells are “drawn” to coalesce into “blood islands”. These endothelial type cells “form” into a vascular network, sprouting branches in response to even more chemical signals within the 3 layers of the embryo.
In the adult, red blood cells are made in the bone marrow. In a 13 day old embryo there are certainly no bones yet. It turns out that the cells on the inside of the primitive vascular cords are “signaled” to change into blood cells and plasma cells. The inner most cells within the blood vessels are “differentiated” into red blood cells and plasma cells and become a liquid that flows, able to carry oxygen and carbon dioxide and nutrients.
Embryology was my favorite subject in medical school. After learning how the morula divides and gastrulation occurs and then all the folding and differentiating happens, I found it nearly impossible to believe that anyone was born at all, ever. I was in awe of the profound complexity associated with even apparently “basic” aspects of embryologic development – like blood. I was equally awed by the gaping holes in our understanding despite the fact that what we already knew was extensive and intricate and detailed and divinely precise.