Can a molecule 2.5 million daltons big enter the central nervous system?

Question

I know the blood brain barrier only allows molecules 500 daltons to pass through so it can't pass through there. However how do molecules enter the spinal cord? my biology isn't good at this point. Can anybody point me to any references as to how molecules enter the spinal cord? The bottom line is that I want to know if a hyaluronic acid molecule which is 2.5 million daltons big can or cannot pass into the central nervous system as any point, whether that be blood brain barrier or spinal cord. Thank you.

Answer 1

A molecule of this size could not enter the CNS through passive diffusion.

My old notes say that an enzyme necessary for producing this molecule is expressed in virtually every body cell, so essentially, the CNS cells make their own hyaluronic acid, and do not need to import what is made by other cells outside the CNS. This is a common feature with cells on the inside of the blood brain barrier. The energy cost in importing small molecules and energy, and building the necessary large ones is offset by the massive advantage of keeping the brain and spine separated from the rest of the body's metabolic function.

Notes say the enzyme is called hyaluronan synthase, if you'd like to search for more on it.