We discovered a fourth envelope around the brain

The brain is a well-protected organ: in addition to the cranial wall, it is enveloped by several membranes – three, it was believed: the dura mater, the arachnoid mater and the pia mater. But thanks to ultra-sophisticated imaging and molecular biology techniques, an American-Danish team has just brought to light a fourth, called SLYM. (subarachnoid lymphatic-like membrane). And its role in the protection of the brain would be major.

This membrane is located between the arachnoid and the pia mater, in a space filled with cerebrospinal fluid (CSF), the fluid that bathes the brain and spinal cord. The researchers detected it in mice, then confirmed its existence in humans, by analyzing brain tissue samples taken during neurosurgery operations. Its extreme thinness – it is only one or a few cells thick – explains why it has gone unnoticed until now.

A series of experiments on rodents then clarified the functions of this membrane. It would thus filter large molecules, contributing to the cleansing role of the CSF (which crosses the brain and eliminates waste). It also resembles the membranes which envelop the heart, the stomach or the lungs, and which attenuate the mechanical friction resulting from the movements of these organs: it therefore perhaps protects in the same way the brain, which constantly moves in the cranial box, especially when moving the head. The SLYM membrane also incorporates immune cells into its structure, which would monitor the cerebrospinal fluid, on the lookout for signs of infection. Finally, by its filtering power, it seems to protect the brain from the invasion of external immune cells. A dysfunction of this barrier is then perhaps involved in inflammatory diseases such as multiple sclerosis.

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