CNS Histo

• Central Nervous System (CNS)


  Consists of the brain and spinal cord
• Meninges


  Connective tissue coverings of the CNS, consisting of 3 layers: Dura mater, Arachnoid mater, Pia mater

  
• Dura mater

  • Outermost layer, made up of 2 layers, periosteal/endosteal and meningeal layer, consists of dense, fibro-elastic connective tissue that is continuous with the periosteum of the skull, contains dural venous sinuses, covered by simple squamous epithelium

  
• Arachnoid mater

  • Middle layer, has two components: a sheet of connective tissue in contact with the dura mater and a system of loosely arranged trabeculae composed of collagen and fibroblasts, continuous with the underlying pia mater layer, subarachnoid space filled with CSF, arachnoid villi protrude into venous sinuses
• Pia mater

  • Innermost layer, closely adhered to the CNS organs, consists of flattened, mesenchymally derived cells
• Associated with the anterior fontanel of a newborn, the parietal and frontal bones are separated. There is a membrane covering the "exposed" area that one can feel if one touches the fontanel area lightly. This membrane would be the dura mater.
• Cerebrum

  Composed of grey matter externally (closest to pia mater) and white matter internally, contains deep nuclei
• Cerebellum


  Composed of grey matter externally (cortex) and white matter internally (arbor vitae)
• Cerebral cortex

  • Arranged in 6 layers, composed of several different cell types that perform various functions
  • Neuron types: Pyramidal (most abundant), Stellate/granule (less common), other (horizontal, fusiform, Martinotti)
• Pyramidal cells/neurons

  • Main neuron of cerebral cortex, multipolar neurons with characteristic triangular/pyramidal morphology
• Cerebellar cortex

  • Composed of 3 layers: Molecular layer, Purkinje cell layer, Granule cell layer

  
• Molecular layer of cerebellar cortex

  • Mainly contains basket and stellate cells (neurons)

  
• Purkinje cell layer of cerebellar cortex

  • Contains Purkinje cells/neurons, efferent/output cells of cerebellar cortex

  
• Granule cell layer of cerebellar cortex

  • Contains mainly granule cells and Golgi cells

  
• Neurodegenerative diseases often result in cortical atrophy (breakdown and cell loss), which can be seen macroscopically and microscopically.
• Choroid plexus

  Highly specialized tissue with elaborate folds and many villi projecting into the ventricles of the brain, each villus contains a thin layer of well-vascularized pia mater covered by cuboidal ependymal cells

  
• Meninges

  • 3 sequential connective tissue membranes covering the brain and spinal cord
  • Dura mater - outermost layer
  • Arachnoid mater - lies beneath the dura
  • Pia mater - delicate layer resting directly on the surface of the brain and spinal cord

  
• Development of pia and arachnoid

  1. Develop from a single layer of mesenchyme surrounding the developing brain

  2. Evident in adults: strands of connective tissue pass between pia & arachnoid- arachnoid trabeculae
• Dura Mater

  • Thick sheet of dense fibro-elastic connective tissue
  • 2 layers: Continuous at its outer surface with the periosteum of the skull, Dural venous sinuses between the 2 layers
  • Internal surface covered by simple squamous epithelium of mesenchymal origin
  • Sheet-like extensions of the inner surface form partitions between parts of the brain
  • In the spinal cord, forms a tube separate from the periosteum (creates epidural space)

  
• Arachnoid Mater

  • Delicate sheet of connective tissue adjacent to the inner surface of the dura
  • Extends arachnoid trabeculae to the pia mater on the surface of the brain and spinal cord
  • Subarachnoid space: bridged by trabeculae and contains cerebrospinal fluid
  • Arachnoid villi: areas where the arachnoid penetrates the dura mater and protrudes into venous sinuses

  
• Pia Mater

  • Lies directly on the surface of the brain and spinal cord
  • Thin squamous epithelial layer
  • Continuous with the perivascular connective tissue of the blood vessels that enter and leave the brain and spinal cord
  • Fuses with arachnoid around the opening for cranial and spinal nerves as they exit the dura mater

  
• Section of an area near the anterior median fissure showing the tough dura mater (D), epidural space (not shown), subdural space (SD), arachnoid mater (A), subarachnoid space (SA), connective tissue trabeculae (T), blood vessels (BV), pia mater (P), and white matter (WM) of the spinal cord

  
• Grey Matter - Cortex

  • Contains nerve cell bodies, axons, dendrites and central glial cells
  • Forms the outermost layer of the brain
  • Neuron types: Pyramidal cells, Stellate/granule cells, Horizontal cells, Fusiform cells, Cells of Martinotti
  • Composed of 6 layers superficial to white matter

  
• Grey Matter - Nuclei


  • Islands of grey matter found in the deep portions of the cerebrum
• White Matter


  • Contains only axons of nerve cells and associated glial cells and blood vessels
• Cerebellum - Cortex

  • 3 layers: Outer molecular layer, Middle Purkinje cell layer, Inner Granule cell layer
  • Purkinje cells: Large cell body, Large, ovoid, vesicular nucleus with prominent nucleolus, Nissl granules in cytosol extend only into dendrites, Apical dendrite ascends into molecular layer and then branches, Axon extends downward into granular layer

  
• Blood-brain Barrier

  • Functional barrier that allows much tighter control than that in most tissues over the passage of substances moving from blood into the CNS tissue. Composed of:
  • Capillary endothelium - cells are tightly sealed together with well-developed tight junctions,
  • Astrocytes - the limiting layer of perivascular astrocytic feet that envelops the basement membrane of capillaries in most CNS regions regulates passage of molecules and ions from blood to brain
• Choroid Plexus


  • Consists of highly vascular tissue, elaborately folded and projecting into the large ventricles of the brain
  • Found in the roofs of the third and fourth ventricles and in parts of the two lateral ventricular walls
  • Each villus contains a thin layer of well-vascularized pia mater covered by cuboidal ependymal cells
• Layers of the Cerebral cortex

  • I: Molecular layer
  • II: External granular layer
  • III: External pyramidal layer
  • IV: Internal granular layer
  • V: Internal pyramidal layer
  • VI: Multiform layer
• Molecular layer of cerebral cortex

  • Consists largely of fibres and relatively few cells
• External granular layer of cerebral cortex

  • Consists mainly of small pyramidal cells, and granule/stellate cells
• External pyramidal layer of cerebral cortex

  • Cells are not sharply demarcated from layer II, but the pyramidal cells are somewhat larger and possess a typical pyramidal shape
• Internal granular layer of cerebral cortex

  • Characterized by the presence of many small granule cells (stellate cells)
• Internal pyramidal layer

  • Contains medium-large pyramidal cells, in the motor area the pyramidal cells are extremely large and are called Betz cells
• Multiform layer

  • Contains cells with diverse shapes, many of which have a spindle or fusiform shape, called fusiform cells
• Choroid Plexus function

  • Function: to remove water from blood and release it as the CSF, which is clear, contains Na+, K+, and Cl– ions but very little protein, produced continuously and completely fills the ventricles, the central canal of the spinal cord, the subarachnoid and perivascular spaces,
  • Also provides the ions required for CNS neuronal activity
  • And in the arachnoid serves to help absorb mechanical shocks,
  • Arachnoid villi provide the main pathway for absorption of CSF back into the venous circulation