Meninges: Structure And Functions
The pia mater is the deepest layer of the meninges. It is a delicate, highly vascular structure of connective tissue that surrounds and protects the brain and spinal cord.
Three membranous layers surround the brain and the spinal cord: the meninges. These are the dura mater, the arachnoid and the pia mater. The conjunction of the last two, the pia mater and the arachnoid, form the soft meninges, or leptomeninges. The dura mater, on the other hand, forms the hard meninges, or pachymeninges.
The main function of the meninges is to provide a protective layer for the brain. It is, therefore, a very vulnerable organ that needs special protection that no other organ has. At least not in the same way. It is the brain that takes care of it. In addition, these protective layers participate in the blood-brain barrier.
The meninges develop from a layer known as the primitive meninges. It consists of elements derived from the mesenchymal and neural crest. Separate, thus, into two distinct layers: the internal endomeninx and the external ectomeninx.
The endomeninus derives from the mesoderm and the ectoderm. The ectomeninge forms the dura mater and neurocanial bones and is formed only from the mesoderm.
Structure of the meninges
The dura
This is the outermost layer. The cranial dura consists of two layers. The first, the outer layer, is the periosteum of the skull and contains blood vessels and nerves. It attaches to the inner surface of the skull, with particularly tight connections to the sutures and to the base of the skull.
The deepest layer of the dura is known as the meningeal layer. This layer is responsible for the formation of reflexes which divide the brain into compartments.
Among these compartments, the most important is the tent of the cerebellum. In addition, there is no distinct border between the dura and the periosteum. This only happens when they separate to form the venous sinuses of the dura. The layers are distinguished histologically by the fact that the meningeal layer contains less fibroblasts and proportionately less collagen.
The arachnoid, or the middle layer
The arachnoid is the middle layer of the meninges. It contains the subarachnoid space that stores cerebrospinal fluid (CSF). The depth of the subarachnoid space varies depending on the relationship between the arachnoid layers and the pia mater.
This layer is made up of two distinct cell layers. While the arachnoid cell layer follows the edge of the dura cells. This layer is full of cells closely linked by numerous desmosomes and narrow joints. Thus, they give the layer a barrier function which prevents fluid from flowing through it.
In the depths of the arachnoid is the reticular arachnoid layer. The cells of this layer join the subarachnoid space and the pia mater. They also contain the blood vessels that pass through the diaper.
Arachnoid granulations are microscopic structures that play an important role in the absorption of CSF. However, the mechanism is not clear. In addition, it is believed that arachnoid granulations may also play a role of regulating the volume of CSF.
The pia mater
The pia mater is the deepest layer of the meninges. It is, therefore, a delicate, very vascular structure of connective tissue that surrounds and protects the brain and spinal cord.
It forms a continuous layer of tightly attached cells to the surface of the brain that are submerged in cracks and furrows. The cells are, moreover, united by desmosomes and joints of interstices, which allows this layer to exert a barrier function.
Virchow-Robin spaces
Virchow-Robin spaces are spaces around (perivascular) vessels surrounding small arteries and arterioles. It should be emphasized that they pierce the surface of the brain and extend inward from the subarachnoid space.
The size of these spaces has been shown to increase with age. However, without there being any apparent loss of associated cognitive function. In addition, the dilation of these spaces corresponds to pathologies such as arterial hypertension, neuro-psychiatric disorders, multiple sclerosis and trauma.
In conclusion, the authors Patel and Kirmi (2009) stress the importance of knowing the meninges. It is, in fact, fundamental to understand its structure, functions and anatomy. Obviously, this will allow us to understand the spread and localization of pathologies related to the meninges. Thus, the most common pathology known is meningitis or subarachnoid hemorrhage.
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