Vasculogenesis and Angiogenesis - Blood Islands - Vascular development during Third Week

In this Visual Medical Lecture Dr. Aizaz from MedicoVisual talks about difference between vasculogenesis and angiogenesis.
00:00 Review of Gastrulation and formation of Blood Islands
01:52 What are Blood islands and Hemangioblasts?
04:58 Formation of Blood vessels from Blood Islands; Vasculogenesis
06:27 3D Tour of Vasculogenesis
07:18 Vascular cords
09:24 Sprouting Angiogenesis
13:32 Splitting Angiogenesis / Non-Sprouting Angiogenesis
14:24 Quick Review

Attribution:
Heart vector (https://www.freepik.com/vectors/heart) created by mamewmy - www.freepik.com

In the lecture of gastrulation, we had seen that as the cells ingress downwards from the primitive streak they displace the primitive endoderm to the yolk sac endoderm.
Here this yolk sac endoderm, through certain chemical mediators that I won’t discuss here, will induce the overlying extraembryonic splanchnopleuric mesoderm to form the blood island. From these blood islands initial major blood vessels will be formed by the process vasculogenesis. Furthermore, primitive blood cells will also form from these blood islands.
What are these blood islands and how do they form blood vessels? Let’s talk about that in this visual medical lecture.
Blood islands are nothing more than clumps or aggregates of cells destined to form the blood cells and blood vessels. Thus, these cells that form the blood islands are called hemeangioblasts (heme; blood, angio; vessel, blast; to form) and they arise from the mesenchymal cells of this extraembryonic mesoderm. The yolk sac endoderm and allantois is almost fully covered with such blood islands in third week of development. Later on, these blood island will also form within the intraembryonic mesoderm but they do not form blood. They only form blood vessels.
On about 17 or 18, within these blood islands on the extraembryonic mesoderm plastering the yolk sac, small spaces begin to form due to programmed cell death (apoptosis) of the central cells. These spaces coalesce and in this way a lumen develops within each of the blood islands. As it happens, a vessel-like structure is now formed. Some of the cells within the lumen of this vessel-like structure remain alive and this isolated cell population is the precursor of blood cells. The peripheral cells of this vessel-precursor will flatten out and differentiate to form the endothelium, the inner most layer of the blood vessel. These peripheral cells from which endothelial cells are formed are called Endothelial precursor cells.
The neighbouring blood islands will then coalase together to form a long vascular cords. Later, the surrounding mesenchymal cells will surround this primordial blood vessel / vascular cord and differentiate into the vascular connective tissue and smooth muscles.
This process of de novo (new) synthesis of blood vessels from the embryonic mesenchyme is called vasculogenesis. Through this method most of the major vessels of the embryo as well initial vascular network of the yolk sac develops. Vasculogenesis is almost exclusively an embryologic process and rarely occurs post-Nataly.
After the initial vascular network is laid down by the vasculogenesis, some the endothelial cells under the influence of certain chemical factors mainly VEGF (Vascular Endothelial Growth Factor), grow towards these factors and the remaining cells behind these growing, tip cells, proliferate. In this way, a branch, at certain angle, arises from the main blood vessel. This branch, later acquires the smooth muscle and connective tissue and matures into a full fledge blood vessel. This process, by which the blood vessels are formed by sprouting from existing blood vessels is called angiogenesis, specifically, sprouting angiogenesis. Because there is another type angiogenesis called non-sprouting angiogenesis, that will just discuss. But before that, let’s talk about how can you remember that angiogenesis is origin of blood vessels from existing blood vessels? Well, it’s simple.
As the vascular branch sprouts, the new blood vessel is formed at an “angle”. So, its easy to remember. Angiogenesis = Anglogenesis.
Please note that the word angiogenesis does not mean angle. Its just a mnemonic to help you remember.
The word Angio came from the Greek word “ angeion” which means small vessels. Genesis means production or synthesis. So the word angiogenesis literally mean formation of small vessels. Because major vessels are laid down by the process of vasculogenesis then smaller branches arise from those major vessels by the process of angiogenesis.
Now lets talk about the other type of angiogenesis called non-sprouting angiogenesis. In this process the walls of a blood vessels in the centre come close to each other and a sort of pillar forms. The vessel then splits into two. It’s like mitosis of blood vessels. Another name of this non-sprouting angiogenesis is splitting angiogenesis.

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