160. The Coverings and Vascularization of The Brain

  1. The meninges are membranes interposed between the bones of the skull and spine and the central nervous system (CNS) which completely enclose the CNS. Normal development of the meninges depends on that of the CNS. The loose mesenchyme around the neural tube condenses to form a covering or membrane called the primitive meninx. The innermost cells of this layer may be of neural crest origin
    1. THE LEPTOMENINGES (soft meninges) are the pia mater and the arachnoid
      1. They are derived from ectomesenchyme from neural crest cells
      2. The pia mater (innermost layer) lies directly on the nervous tissue
      3. The arachnoid, just outside the pia mater, surrounds the blood vessels
      4. The arachnoid trabeculations between the pia mater and the arachnoid attest to the fact that the leptomeninges form from a single layer
    2. THE DURA MATER (hard meninges) is the outermost layer and is derived from ordinary mesenchyme and differentiates after the leptomeninges
    3. FLUID-FILLED SPACES in the leptomeninges coalesce to form the subarachnoid space
  2. Role of the meninges is generally protection against mechanical shock
    1. VASCULAR ROLE: the pia mater and the arachnoid follow the vessels that penetrate the nervous tissue. They sheath the vessels at the level of the capillaries which are then in direct contact with the neuroglial cells. The combination of capillary wall and neuroglial cells make up the so-called blood-brain barrier
      1. The barrier is impermeable to some microbes and some compounds, like antibiotics, but is permeable to others, like sulfonamides
      2. The barrier is not present in the young embryo, but develops slowly
    2. CLEANING OR CARRYING OFF WASTES takes place in the subarachnoid space
  3. The choroid plexuses: primordia are found in the regions where the ependymal wall is thin, such as the roof of the third and fourth ventricles and the internal part of the lateral ventricles. Here the leptomeninges push the wall into the ventricles
    1. THE PLEXUSES form a highly vascularized meningeal axis covered with thin cuboidal ependymal epithelium. The first choroid plexus develops in the fourth ventricle between days 48 and 50 in a 20 mm embryo
    2. ROLE OF THE PLEXUSES is the production of cerebrospinal fluid
      1. In the fetus, the cerebrospinal fluid may furnish proteins to the CNS
      2. The cerebrospinal fluid circulates toward the fourth ventricle and the spinal canal. It then passes through the foramina of Magendie and Luschka and is resorbed in the subarachnoid spac The main site of absorption of the fluid into the venous system is through the arachnoid villi (Pacchionian bodies) projecting into the dural sinuses. The villi are a thin cellular layer derived from the arachnoid epithelium and endothelium of the sinus
  4. Vascularization of the brain
    1. CEPHALIC CIRCULATION begins to be established very early, at about week 3, even before closure of the neural tube. It develops rapidly, along with the brain
      1. The prosencephalon is vascularized first, by the internal carotid arteries which come off the dorsal aortas. Vascularization of the rhombencephalon and mesencephalon takes place later and is derived from the basilar artery, which is formed from the confluence of vertebral arteries
      2. The major arteries join together to form the circle of Willis, which is definitively formed by about 7 to 8 weeks. It has its own regulatory system to protect it from wide fluctuations and provides the brain with about 15% of its total blood
        1. The circle of Willis is nourished by blood-richer in oxygen than that of other parts of the body since it passes through the heart via the foramen ovale to the left side of the heart and then to the aorta and primitive carotids to the brain

the coverings and vascularization of the brain: image #1