The brain and spinal cord are surrounded by three membranes: the meninges. The three meninges are the dura mater, the cobweb membrane (arachnoides) and the soft meninges (pia mater).
The main function of the meninges is to protect the brain. This is a very fragile organ that needs special protection. No other body needs it to that extent, at least not in the same way. In addition, these protective layers are part of the barrier between blood and the brain.
The meninges arise from another layer known as the primitive meninx, which in turn consists of elements derived from the mesenchyme and neural crest. The primitive meninx is divided into two distinct layers: the internal (endomeninx) and the external (ectomeninx).
The endomeninx is different from the cobwebs and the pia mater, and is derived from both the mesoderm and ectoderm. On the other hand, the ectomeninx forms the dura mater and the bones of the neurocranium and consists only of the mesoderm.
Structure of the meninges
The dura mater of the brain
The dura mater is the outermost layer of the meninges and consists of two layers. The outermost layer is the periosteum and this contains blood vessels and nerves. It attaches to the inner surface of the skull, with the joints specially adapted to the base of the skull.
The deepest layer of the dura mater is called the meningeal layer. This is responsible for the reflexes that divide the brain into compartments. The most notable compartments are:
- the falx cerebri
- tentorium cerebelli
In addition, there is no clear boundary between the dura mater and the periosteum. The layers are distinguished histologically, by the fact that the meningeal layer contains fewer fibroblasts and proportionally less collagen (2).
The cobweb is the middle layer of the meninges. Here is the subarachnoid space that stores cerebrospinal fluid. The depth of the subarachnoid space varies depending on the relationship between the cobweb and the pia mater.
Two separate cell layers together form the spider web. The barrier layer lies behind the edge of the cells of the dura mater (3). This layer is full of cells that are closely connected to each other through numerous desmosomes and tight junctions. This layer ensures that no liquid can move through it.
The reticular arachnoid layer is located deep in the spider web. The cells connect the subarachnoid space and attach to the pia mater. They also enclose the blood vessels that pass through the layer (1).
Villi arachnoideae are microscopic structures that play an important role in the absorption of cerebrospinal fluid. However, the way they work is unclear. Some experts believe they may also play a role in regulating cerebrospinal fluid volume.
The soft meninges (pia mater)
The pia mater is the innermost layer of the meninges. It is a delicate vascular structure that encloses, attaches and protects the surface of the brain and spinal cord.
It creates a continuous layer of cells that adhere to the surface of the brain and then become immersed in the crevices and grooves of the brain. Desmosomes and junctions connect the cells, allowing the layer to also function as a barrier.
The Virchow-Robin spaces are located around the vessels and surround the small arteries and the arteries. They break through the surface of the brain and project inward from the subarachnoid space (1).
These spaces increase in size with age without any apparent loss of cognitive function (4). In addition, the widening of these spaces is related to conditions such as:
- neuropsychiatric disorders
- multiple sclerosis
- trauma (5)
Finally, the authors Patel and Kirmi (2009) emphasized the importance of knowledge about the meninges. It is essential to understand the structure, functions and anatomy of the meninges, as it will also enable us to understand the pathologies of the meninges.