Vascular development, structure of arteries, veins, lymphatic vessel, collateral circulation

Vascular Development:

There are 2 main forms of vascular development: VASCULOGENESIS AND ANGIOGENESIS

VASCULOGENESIS:Vasculogenesis is the process of blood vessel formation, occurring by a de novo production of endothelial cells. It is the first stage of the formation of the vascular network, closely followed by angiogenesis.

Steps of vasculogenesis:

1.Formation of Angioblasts:

Angioblasts, specialized endothelial precursor cells, differentiate from mesodermal cells in the embryonic mesoderm.

2. Angioblast Migration:

Angioblasts migrate to specific regions where new blood vessels will form, particularly around the yolk sac and embryonic tissues.

3.Angioblast Aggregation:

Angioblasts aggregate and align to form small clusters called blood islands.

4.Formation of Endothelial Tubes:

Angioblasts in the blood islands differentiate into endothelial cells that line the interior of the blood vessel. The endothelial cells join together to form primitive vascular tubes (the early blood vessels).

5.Lumen Formation:

The endothelial cells of the tubes organize to create a lumen, the hollow center of the blood vessel, allowing blood flow.

ANGIOGENESIS- the physiological process through which new blood vessels form from pre-existing vessels

Steps of angiogenesis:

1.Activation of Endothelial Cells:

Signals (such as growth factors like VEGF) activate endothelial cells (cells that line the inside of blood vessels).

2.Degradation of the Basement Membrane:

The endothelial cells break down the basement membrane of the existing blood vessels to allow them to move and grow.

3.Endothelial Cell Proliferation:

Endothelial cells divide and proliferate (increase in number) to form new blood vessel structures.

4.Migration of Endothelial Cells:

The proliferating endothelial cells migrate towards the area where new blood vessels are needed (e.g., to supply oxygen to tissues).

5.Formation of New Blood Vessel Tubes:

The endothelial cells form tubular structures, creating the lumen (hollow center) of the new blood vessel.

Structure of arteries, veins, lymphatic vessels:

Both arteries and veins have 3 layers:

TUNICA INTIMA-endothelium, in contact with blood

TUNICA MEDIA-made of smooth muscle cells and elastic fibers

TUNICA ADVENTITIA-made of collagen and elastic fibers with some smooth muscle cells

Differences between arteries and veins:

Thickest layer in artery: Tunica MEDIA

Thickest layer in vein: Tunica ADVENTITIA

Arteries have more elastic fibers, structure holds up more compared to veins.

Lymphatic vessel structure:

Lined with a single layer of endothelial cells. They have valves to keep the flow of lymph unidirectional.

Collateral circulation:

Collateral circulation is alternate or “backup” blood vessels in your body that can take over when another artery or vein becomes blocked or damaged. Your collateral circulation provides alternative routes for blood flow.

Collateral blood vessels connect to your main blood vessels and remain small and unused until you need them. Other collateral vessels develop throughout life in a process called angiogenesis (the formation of new blood vessels).

Angiogenesis may happen as a main blood vessel slowly becomes blocked. Your body senses the reduced blood flow and calls other blood vessels into action. Regular aerobic exercise, such as running, may also encourage new collateral blood vessels to form.

Examples of collateral circulation:

1. BRAIN-Blood flow to the brain in humans and some other animals is maintained via a network of collateral arteries that anastomose (join) in the circle of Willis, which lies at the base of the brain. In the circle of Willis so-called communicating arteries exist between the front (anterior) and back (posterior) parts of the circle of Willis, as well as between the left and right side of the circle of Willis.
2. RETINA-After central retinal vein occlusion, neovascularization may restore some blood flow to the retina, but the new vessels' bulk also presents a risk of causing acute glaucoma by blocking the drainage of aqueous humour. Collateral circulation is created (within months) around the blocked central vein via a generally winding path, usually from a branch vein to the choroid
3. HAND-The circulatory system of the human hand features extensive collateral circulation in the form of the deep and superficial palmar arches, and the proper palmar digital arteries of the digits, with the exception of the thumb, which has its blood supplied by the princeps pollicis artery. As each finger has two proper digital arteries, blood can be continue to be supplied to specific digits if one of the two is injured.