As it is challenging to reach drug into the brain due to BBB (blood-brain barrier) which contain Tight junctions. But other Blood vessels also have tight junction and no perforation. So why BBB is different from other capillaries?
Hello Alam Md Jahangir
There are three main structural classes of capillaries.
1.Continuous non-fenestrated capillaries of the skin and lung are joined together by cellular junctions, have a complete basement membrane (BM), and lack fenestra (pores) in their plasma membrane.
2. Continuous fenestrated vessels of the intestinal villi and endocrine glands have a similar continuous structure but contain diaphragmed fenestra throughout their membrane.
3. Discontinuous capillaries in the liver have large gaps throughout the cell and have an incomplete BM.
These classes of capillaries differ greatly in their regulation of movement of solutes between the blood and the tissues.
The capillaries in the CNS are continuous capillaries that lack fenestrations and have a continuous basal lamina. They contain only a few pinocytic vesicles, which distinguishes them from other continuous capillaries of the body and makes them well-suited to make up a selective barrier such as the blood-brain barrier (BBB).
So, drug molecules must traverse the endothelial cells, rather than passing between them, to move from circulating blood to the extracellular space of the brain.
As you mentioned, tight junctions exist between the endothelial cells of the blood-brain barrier, which permit the passage of only a select few types of substances between the cells. Furthermore, only a select number of substances can pass through the endothelial cells. Such substances include lipid-soluble substances (e.g., oxygen, carbon dioxide). Hydrophilic substances, for example, hydron and bicarbonate, are not permitted to pass through cells and across the blood-brain barrier.
Thus, not only the CNS vessels are continuous non-fenestrated vessels, but also contain a series of additional properties that allow them to tightly regulate the movement of molecules, ions, and cells between the blood and the CNS. This heavily restricting barrier capacity allows BBB ECs to tightly regulate CNS homeostasis, which is critical to allow for proper neuronal function, as well as protect the CNS from toxins, pathogens, inflammation, injury, and disease.
If you need more information on this topic you may also refer to the article attached below.
Article The Blood-Brain Barrier
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