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The pia is the innermost layer of the meninges. It is a delicate, highly vascular, connective tissue structure that surrounds and protects the brain and spinal cord.
The brain and spinal cord are surrounded by three membranous layers: the meninges. These are the dura mater, the arachnoid mater and the pia mater. The set of these last two, pia mater and arachnoid mater, form the leptomeninge. The dura mater, for its part, forms the pachymeninge.
The main function of the meninges is to provide a protective layer to the brain. This is a very vulnerable organ that needs special protection that no other organ has. Not at least in the same way. The meninges are responsible for this. Furthermore, these protective layers participate in the blood brain barrier.
The meninges develop from a precursor layer known as the primitive meninges. This is composed of elements derived from the mesenchymal and neural crest. It is separated into two distinct layers: an internal endomeninge and an external ectomeninge.
The endomeninge is differentiated into the arachnoid and pia mater and is derived from both the mesoderm and the ectoderm. The ectomeninge forms the dura mater and bones of the neurocranium and is formed only from the mesoderm.
Structure of the meninges
The dura mater
This is the outermost layer. The cranial dura mater is composed of two layers. The first, the outer layer, is the periosteum of the skull and contains blood vessels and nerves. It adheres to the inner surface of the skull, with especially tight attachments to the sutures and base of the skull.
The deepest layer of the dura mater is known as the meningeal layer. This layer is responsible for forming reflexes that divide the brain into compartments.
Among these compartments, the most prominent are the falx cerebri and tentorium cerebellum. Furthermore, there is no distinct border between the meningeal and periosteal dura. This only occurs when they separate to form the dural venous sinuses. The layers can be distinguished histologically by the fact that the meningeal layer has fewer fibroblasts and proportionally less collagen (2).
Arachnoid or intermediate layer
The arachnoid is the middle layer of the meninges. It contains the subarachnoid space which in turn stores cerebrospinal fluid (CSF). The depth of the subarachnoid space varies depending on the relationship between the arachnoid and pia mater layers.
This layer is made up of two different cell layers. Following the edge of the dura mater cells is the arachnoid barrier cell layer (3). This layer is filled with cells closely joined together by numerous desmosomes and tight junctions. Thus, these provide the layer with a barrier function that prevents fluid movement through it.
Deep in the arachnoid is the reticular arachnoid layer. Cells in this layer bridge the subarachnoid space and attach to the pia mater. They also enclose the blood vessels that pass through the layer (1).
Arachnoid granulations. These are microscopic structures that play an important role in the absorption of CSF. However, the mechanism is unclear. Furthermore, it is considered that arachnoid granulations may also have a role as a regulator of CSF volume.
Pia mater
The pia mater is the innermost layer of the meninges. It is a delicate, highly vascular, connective tissue structure that surrounds and protects the brain and spinal cord.
Form a continuous layer of cells closely attached to the surface of the brain that submerge into fissures and sulci. The cells are joined by desmosomes and gap junctions, allowing this layer to fulfill a barrier function.
Virchow-Robin spaces
The Virchow-Robin spaces are spaces around the vessels (perivascular) that surround small arteries and arterioles. They pierce the surface of the brain and extend inward from the subarachnoid space (1).
It has been shown that these spaces increase in size with age without apparent associated loss of cognitive function (4). Furthermore, the dilation of these spaces is associated with pathologies such as high blood pressure, neuropsychiatric disorders, multiple sclerosis and trauma (5).
To conclude, the authors Patel and Kirmi (2009) emphasize the importance of knowing the meninges. It is essential to understand the structure, functions and anatomy: this will allow us understand the spread and location of pathologies related to the meninges. The most common pathology known in relation to these is meningitis.
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All cited sources were reviewed in depth by our team to ensure their quality, reliability, validity and validity. The bibliography in this article was considered reliable and of academic or scientific accuracy.
Patel, N., & Kirmi, O. (2009). Anatomy and imaging of the normal meninges. In Seminars in Ultrasound, CT and MRI (Vol. 30, No. 6, pp. 559-564). WB Saunders.
Haines, D.E., Harkey, H.L., & Al-Mefty, O. (1993). The “subdural” space: a new look at an outdated concept. Neurosurgery, 32(1), 111-120.
Alcolado, R., Weller, RO, Parrish, EP, & Garrod, D. (1988). The cranial arachnoid and pia mater in man: anatomical and ultrastructural observations. Neuropathology and applied neurobiology, 14(1), 1-17.
Groeschel, S., Chong, W.K., Surtees, R., & Hanefeld, F. (2006). Virchow-Robin spaces on magnetic resonance images: normative data, their dilation, and a review of the literature. Neuroradiology, 48(10), 745-754.
Kwee, R.M., & Kwee, T.C. (2007). Virchow-Robin spaces at MR imaging. Radiographics, 27(4), 1071-1086.
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