1. From the perspective of the origin of meningeal development, the soft membrane and the arachnoid membrane are homologous. So learn together.
2. The related structures of arachnoid membrane are arachnoid granules and arachnoid trabecules, subarachnoid cavity and cerebral pool.
3. The soft membrane is close to the surface of the brain, protruding into the sulcus on the surface of the brain, and accompanied by the cerebral cortex artery into the space around the cerebral blood vessels. Therefore, I will also briefly review the ePEN membrane here. The ventricular membrane is lined with the inner wall of the ventricular system and is continuously connected to the leptomening membrane through the middle and lateral foramen of the fourth ventricle.
Pia mater and arachnoid mater
Overview of The Meninges Overview
The brain and spinal cord are surrounded by skulls and vertebralcolumns, respectively. This bone is intact except foramen (through which spinal nerves and associated blood vessels pass through the foramen) and foramina in the skull (which serve as channels for arteries, veins, and cranial nerve roots).
Meninges are three layers of membranes (membranous) that cover the brain and spinal cord. In general, the meninges are made up of fibroblasts and different numbers of extracellular connectivetissue fibrils. Each fibroblast in the meningeal layer is modified to perform a specific function. These include dura mater, arachnoid mater, and soft film.
These membranes, from the binding of mesoderm and neural crest cells, begin to form at weeks 4 to 5.
In the early stages of the fetus, the outer layer thickens to form the dura mater, and the inner layer forms a soft membrane - arachnoid mater (pia-arachnoidmater). The pia-arachnoidlayers make up the leptomeninges. Eventually, the soft membrane clings to the surface of the brain and spinal cord, and between the soft membrane and the arachnoid membrane is the subarachnoid space. Cerebrospinal fluid (Cerebrospinalfluid) circulates in the subarachnoid space. In the subarachnoid space there are arachnoid trabeculae that connect the soft membrane to the arachnoid membrane.
The main functions of the meninges are: (1) to protect the brain and spinal cord; (2) to serve as a supporting framework for important arteries, veins, and sinuses; and (3) to enclose fluid-filled cavities, i.e., subarachnoid cavities, which are essential for the survival and normal functioning of the brain and spinal cord.
Development of The Meninges Meninges
Sources of meninges:
Meninges develop from cells from neural crests and mesenchyme (mesoderm) that migrate around the developing central nervous system between 20-35 days of gestation (Figure 7.1a-c).
Proto-meninges:
Together, these crest and mesoderm cells form primitive meninges (promeningxprimitiva). At this stage, there are no obvious gaps (noobvious spaces) in the meninges (venous sinuses, subarachnoid space). Between 34 and 48 days of pregnancy, the primitive meninges differentiate into a more compact outer layer called the ectomeninx, a more reticulated inner layer called the endomeninx (Figure 7.1d).
Extraeplaneal meninges:
As development progresses (45-60 days of gestation), the outer meninges of the embryo become more compact, and gaps associated with the location of future venous sinuses appear at this layer.
Intraeplaneal meninges:
At the same time, the embryonic meninges become more reticulated, and the space that appears inside it corresponds to the subarachnoid spaces and cisterns of adults. In general, the outer embryonic meninges will develop into duramaters, and the inner embryonic meninges will develop into arachnoidmater and soft membranes (piamater) of the adult nervous system (Figure 7.1d).
Relationship of extraemonitional meninges to skulls and vertebrae:
The outer embryonic meninges around the brain (ectomeninx) are contiguous with the skeletogenouslayer that make up the skull. This relationship is maintained in adults, where the dura mater is closely linked to the inner surface of the skull. In the spine, the extraeplaneal meninges are also initially connected to the developing vertebrae. However, as development progresses, the spinal embryonic meninges (spinal ectomeninx) are separated from the vertebral bodies. A layer of cells remains on the vertebral body, forming the periosteum lining the vertebral canal in the spinal canal, and a larger part of the meninges outside the spinal embryo develops into a spinal dura. The median space becomes the spinal epidural space (Figure 7.2). In the spine, this space can be used for epidural anesthetics.
A brief introduction to the characteristics of each meningeum
Dura meninges (pachymeninx)
The outermost dura, also known as dura meningeal membrane (pachymeninx), attaches to the inner surface of the skull but is separated from the vertebrae through the epidural space (Figure 7.2). Around the brain, the inner layers of the dura mater create infoldings or septa, such as the falx cerebri and the tentorium cerebelli (Figure 7.2), which separate brain regions from each other. Major venous sinuses appear at the starting point of the diaphragm. When entering or leaving the central nervous system, the spinal and cranial nerves must pass through the cuffof the dura attached to the connective tissue of the peripheral nerve. Blood vessels cross the dura mater in a similar way. On the side of the mouth, the dural sac attaches to the edge of the large hole in the occipital bone. On the caudal side, the dural sac terminates at the level of the second sacralvertebrae and attaches to the coccyx (Figure 7.2) with an outer terminale externum (or dural part of the terminal wire).
Leptomeninges
The inner two layers of the meninges, the arachnoid mater, and the pia mater (Figures 7.2 and 7.3), collectively referred to as the leptomeninges. The term is also commonly used in clinical medicine (such as leptomeningeal cysts and leptomeningitis). Meningeal infections are often confined to the subarachnoid space and are therefore located within the leptomeninges.
arachnoid:
The arachnoid membrane is a thin layer of cells that attaches to the ductral membrane, but with the exception of arachnoid trabeculae, the subarachnoid space is separated from the soft membrane. The arachnoid membrane around the brain is directly connected to the arachnoid membrane on the inner surface of the dura mater (Figure 7.2). Thus, spinal subarachnoid spaces and cerebralsubarachnoid spaces of the brain are also directly connected at the foramen magnum of the occipital bone. The subarachnoid cavity contains cerebrospinalfluid (csf) and vessels, which are connected to each other by fibroblasts of different sizes and shapes to form arachnoid trabecules. The arachnoid membrane is vascular and does not contain nerve fibers.
Soft film:
The soft membrane is located on the surface of the brain and spinal cord, followed by their various grooves and protrusions (Figures 7.2 and 7.3). The pia mater forms denticulate ligaments and filumterminale internum (or the soft membrane part of the terminal membrane) around the spinal cord (Figure 7.2).
Arachnoid membrane-related structures
Arachnoid villi or arachnoid granulations
The only pathway that allows cerebrospinal fluid to be absorbed into the bloodstream is located mainly next to large venous sinuses (superior sagittal sinuses, transverse sinuses, etc.). A pressure trace of arachnoid granules is formed on the skull.
The above image is from Atlas of Normal Imaging Variations of the Brain, Skull, and Craniocervical Vasculature
Arachnoid trabecula and diaphragm
Present between the arachnoid membrane and the pia mater. Present in both the cerebral pool and the subarachnoid space. Separation of the cerebral pool, cleft brain, and sulcus during surgery is often encountered. If these trabeculars can be sharply cut, the damage to the soft membrane and cerebral cortex can be greatly reduced.
Subarachnoid space
It is believed that the space between the arachnoid membrane and the soft membrane contains cerebrospinal fluid, communicates with the cerebral pool and ventricles, and absorbs cerebrospinal fluid back into the bloodstream through arachnoid particles.
Cistern
There is a diaphragm between the cerebral pool and the subarachnoid space, as well as between different brain pools, relatively independent, but not completely isolated, cerebrospinal fluid can communicate with each other, open a brain pool, does not make the cerebrospinal fluid in the adjacent brain pool flow out rapidly but makes the entire subarachnoid cavity collapse. For the specific classification and types of brain pools, please refer to my previous notes
Summary of anatomical knowledge of the brain pool
Soft film-related structures
The spinal cord is suspended by filum terminales and left and right denticulateligaments in the dural sac.
Denticulate ligaments
The leptometrium extends outward on both sides of the spinal cord, in the form of a triangle, fibrous, partially extending outward through the arachnoid membrane and attaching to the meningeal layer of the dura mater to facilitate fixation of the spinal cord in the spinal canal. These triangles of the pia mater extend like shark teeth and are therefore called dentate ligaments. There are 21 (or 20-22) pairs of dentate ligaments, which are located between the posterior and anterior nerve roots. Arranged longitudinally from top to bottom, the first pair is located at the level of the foramen magnum, and the last pair is located on the caudal side of the 12th thoracic vertebra.
filum terminale
The spinal pia mater is closely connected to the spinal cord, and on the horizontal surface of the conusmedullaris, it aggregates into a very fine non-nerve fiber, the filum terminale, which extends about 20 cm from the tip of the spinal cone and connects to the periosteum of the first cococcygeal vertebra.
Ependymal membrane with ventricles
The pia mater acts as the membrane closest to the surface of the brain. The ventricular membrane is the membrane closest to the ventricular wall. It is continued with the soft membrane through the median and lateral foramen of the fourth ventricle. Several concepts related to the ependyctangle membrane and ventricular wall:
Choroidal tissue (tela choroidea): Choroidal tissue is present in the parietal wall of the third ventricle; the extension of the lower medullary sail in the lower half of the parietal wall of the fourth ventricle is choroidal tissue.
Taeniachoroidea:
Choroidalfissure
Choroidplexus
bibliography
《Langman’s Medical Embryology-LWW (2018)》
《Fundamental Neuroscience for Basic and ClinicalApplications-Elsevier (2017)》
《Clinically Oriented Anatomy Eighth Edition》、
《grant’s atlas of anatomy,14th edition》
Atlas of Sobotta Human Anatomy (21st Edition)
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