Central nervous system histology

The central nervous system or CNS consists of the cerebellum, cerebrum, brain stem, and spinal cord.The neuron is the basic working unit of the nervous system.And the neuroglia or glial cells are the non-neuronal cells that support and protect the nervous system.The central neuroglia includes astrocytes, oligodendrocytes, ependymal cells, and microglia.The peripheral neuroglia includes Schwann cells, satellite cells, and a number of cells associated with specific organs.Macroscopically the CNS is made of white matter and grey matter.The difference in appearance is from the lipid-rich myelin sheaths that cover the axons present in white matter.Whereas the grey matter consists mostly of neuron cell bodies, dendrites, astrocytes, and microglial cells.In this high power image of white matter from the spinal cord, the axons are surrounded by clear white space, which is where the myelin was present before the tissue was processed to create this slide.The outermost portion of the cerebrum and cerebellum consist of grey matter, with their white matter present mainly in the deeper regions of the brain.One the other hand, the spinal cord has the opposite arrangement, with white matter mainly in the periphery and grey matter mostly located closer to the center, forming an “H” or butterfly-shaped appearance when looking at a cross-section of the spinal cord.In this low power image, we can see the spinal cord’s two posterior or dorsal horns closer to the top of the image and two anterior or ventral horns at the bottom.The dorsal horns contain mostly sensory neurons and the anterior horns contain mostly upper motor neurons.A simple way of remembering the location of the motor neurons is to remember that most cars also have their motors in the front or anterior part of the car.The entire CNS is also covered by layers of connective tissue called the meninges.The portion of the meninges that can be seen in this low power image is the dura mater, which is the thickest and outermost layer of the meninges.The ventricles of the brain and the central canal of the spinal cord are filled with cerebrospinal fluid or CSF and have a lining of cuboidal or columnar cells called ependymal cells.These cells are responsible for producing CSF.If we take a closer look at the cells, we can also faintly see that the ependymal cells can have cilia, which helps circulate the CSF within the central canal.The ependymal cells also have microvilli, but they’re typically too small to be seen with light microscopy.Now, let’s take a closer look at the border between the spinal cord’s white matter and grey matter.On the left is the white matter, where we can identify many of the purple-stained axons by the myelin sheaths that leave a white space that surrounds each axon.On the right of this image is the grey matter.Although neuron cell bodies can vary a lot in overall appearance, they’re still usually recognizable by their prominent nuclei with distinct nucleoli.They are also usually much larger than glial cells.The different glial cells can be difficult to identify and differentiate from each other when using an H&E stain, except for oligodendrocytes.Oligodendrocytes can be identified by their small, round, and condensed nuclei.Their cell bodies are not easily seen because their cytoplasm is unstained because it contains a lot of Golgi complexes, which don’t absorb the stain.Oligodendrocytes have many processes that extend and wrap around axons to form the myelin sheath for axons within the CNS.The peripheral nervous system’s myelin sheaths are produced by Schwann cells instead.The grey matter surrounding all the cells is called the neuropil, which consists of a dense network of fibrous cellular branches or processes from glial cells and neurons.In the CNS, the star-shaped astrocytes are the most abundant type of glial cell.They have many long processes that extend in all directions from their cell body.Many of the processes have enlarged terminal ends called foot processes, which surround capillaries as part of the blood-brain barrier.The foot processes help induce and maintain the formation of tight junctions between endothelial cells.Astrocytes also produce structural proteins called glial fibrillary acid proteins or GFAPs.These proteins can be used as a unique marker for immunostaining to help visualize astrocytes in tissue samples.In this high power image, the GFAP-specific staining makes it easier to see the astrocytes in brown, as well as the foot processes that surround capillaries, as we can see near the top of this image.Microglia are small antigen-presenting cells of the CNS that are found in both white and grey matter.They have short branching processes that actively move and are the only glial cells that also migrate to different parts of the CNS, which helps them find and remove damaged tissue as well as function as part of the CNS’s immune response.Similar to astrocytes, microglia can be visualized more easily using techniques like immunostaining.Immunocytochemistry, is a form of immunostaining, where the cells are immunostained after they’ve been isolated and suspended in liquid.Which is why the microglia in this image are the only cells present and they’re also spaced out relatively evenly.The pyramidal cells are the most prominent neuron is the cerebral cortex.These efferent neurons integrate sensory information and initiate voluntary motor responses.They get their name from their large pyramid-shaped cell bodies that have an apex that points toward the cortical surface of the cortex at the top of this image.At the apex, there’s a large branching dendrite that extends up toward the cortical surface.The sides of the cell have much smaller dendrites that branch out to the sides, and it’s single axon extends from the base of the cell to synapse with other cells in the cortex or even deeper, into the white matter.Together, the direction of the axon and the large dendrite give pyramidal cells their appearance of being vertically arranged within the cerebral cortex.Moving to the cerebellum, at low power, the cortex has 3 layers seen with an H&E stain.The innermost layer that’s stained dark pink is the white matter.The purple middle layer and outer light pink layer are both a part of the gray matter.The outer layer is a molecular layer and the dark purple layer is the granular layer.If we zoom in closer to the border of these two layers, we can see that it actually has 3 layers because of a single layer of neurons called Purkinje cells that sit between the upper molecular layer and lower granular layer.Immunostaining was performed on this section of tissue in order to see the purkinje cells more clearly by staining them dark brown.These neurons have very large cell bodies with a highly branched dendritic system that extends into the molecular layer and an axon that extends down through the granular layer, which is often hard to see, even with immunostaining.