Nervous System Histology

Atlas:
Ross & Pawlina (6th ed.), Plates 27-31, pgs. 390-399
Text:
Ross & Pawlina (6th ed.), Ch 12, pgs. 352-389, Nervous System

Overview

  1. Spinal Cord
  2. Neurons
  3. Glia
    1. ependymal cells / choroid plexus
    2. astrocytes
  4. Cerebellum
  5. Cerebrum (neocortex)
  6. Hippocampal formation (archicortex)

 

I. Spinal Cord
Slide 65-1N (lumbar spinal cord, trichrome) [WebScope] [ImageScope]
Slide 65-2 (lumbar spinal cord, H&E) [WebScope] [ImageScope]
Slide 65-1 (lumbar spinal cord, H&E) [WebScope] [ImageScope]
Slide 66a (thoracic spinal cord, luxol blue & cresyl violet) [WebScope] [ImageScope]

Review the organization of the spinal cord using an anatomy atlas, then examine the cross section of the lumbar spinal cord in slide #65-2.  At low magnification, differentiate inner gray from outer white matter and identify dorsal and ventral horns of the gray matter. You should also identify the dorsal and ventral horns in Slide 65-1N stained with Masson trichrome. In these slides, dorsal happens to be "up," but you should be able to tell dorsal and ventral horns based on morphology and the cells present rather than the orientation. The perikarya of large somatic motor neurons [example] located in the ventral horn of the cord innervate the skeletal muscles of the limbs and trunk of the body (soma="body" in Greek, hence "somatic").

Slide 66a shows a section of thoracic spinal cord. In addition to the dorsal and ventral horns, two structures especially obvious in the thoracic cord are the dorsal nucleus of Clarke and the lateral horn. The dorsal nucleus of Clarke [example] is in the dorsal horn and contains relatively large, multipolar neurons that receive proprioceptive information from dorsal root ganglion cells innervating muscle spindles in the trunk and lower limb. The cells of Clarke's nucleus then relay this information via axonal projections that extend all the way up into the cerebellum (hence the reason why the cells are so large) where it is processed to allow for coordinated movement. The lateral horn [example] contains relatively large, multipolar visceral motor neurons of the intermediolateral cell column that extends from levels T1 through L2 of the spinal cord. The cells here are preganglionic sympathetic motor neurons whose axons terminate in either sympathetic chain ganglia or the "visceral" (or "pre-aortic") ganglia associated with the major branches of the abdominal aorta (e.g. celiac, aorticorenal, and superior/inferior mesenteric ganglia). Note that sacral levels of the cord (levels S2-4) also contain visceral motor neurons in the lateral horn, but these are parasympathetic motor neurons.

Many neurons in the spinal cord may appear shrunken and surrounded by an empty space due to poor fixation.  Cells that are well preserved show features characteristic of most neurons: large cell body, large pale nucleus, Nissl substance, and cell processes (most of which are dendrites). The delicate meshwork of dendritic processes and nerve fibers (axons) lying between cells in the gray matter is called the neuropil. The white matter contains nerve fibers (axons) entering and exiting the gray matter, and traveling up and down the spinal cord, linking it to the brain. Another feature commonly found in nervous tissue are starchlike granules known as "corpora amylacea" [example] (amylon = starch, Greek) which are aggregates of dead cells and/or proteinaceous secretions that may be found in either white or gray matter. These granules are of little pathological significance, but they generally increase with age.

Nervous tissue contains two basic categories of cells: neurons and support cells (glia). Both neurons and glia have fine processes projecting from the cell body, which generally cannot be resolved in the light microscope without special staining techniques. Astrocytes in the CNS provide metabolic support for neurons and play an important role in maintenance of the blood-brain barrier whereas oligodendrocytes (another type of glial cell) are responsible for myelination of CNS axons. Recall that Schwann cells are the glial cells responsible for myelination in the peripheral nervous system. Myelin is lipid-rich, and on gross inspection appears white. Thus, in the 'white matter' of the brain and spinal cord, myelinated axons are the predominant neuronal component whereas most of the the nuclei that you see in white matter are primarily of glial cells. The ‘gray matter’ contains relatively more neuronal and glial perikarya as well as non-myelinated (e.g. dendritic) processes. The other major glial cell type you should know about are microglia which are small cells derived from blood monocytes. They are considered part of the mononuclear phagocytic system and will prolifereate and become actively phagocytic in regions of injury and/or inflammation.

 

 

II. Neurons (Slide #65)

Neurons are characterized by a large cell body or perikaryon containing a large, pale (active, euchromatic) nucleus with a prominent nucleolus.  Scattered in the cytoplasm are the characteristic clusters of ribosomes and rough ER termed Nissl bodies  or Nissl substance [example].  One or more cell processes may also be seen emerging from the neuronal perikaryon. The dendrites receive neural input from other neurons via synapses (or they are specialized to receive sensory stimuli), and they transmit neural information toward the perikaryon. A single axon (often called a nerve fiber) leaves the perikaryon and transmits neural signals to other neurons or to the effector organ (e.g., skeletal muscles) via synapses. The axon may be identifiable by the notable ABSENCE of Nissl substance at the axon hillock; however, this is not always so easy to see by light microscopy.

 

 

III. Glial Cells

A. Ependymal cells / Choroid Plexus
NP004N
 hippocampal region, coronal section, luxol blue [Webscope] [Imagescope]

Many types of glial cells require special histological stains and can’t be unambiguously identified in regular histological slides.  Ependymal cells, which are (low) columnar epithelial cells lining the ventricles of the brain the central canal of the spinal cord, however, are rather easy to discern.  In this section, lining ependymal cells can be seen all along the interface of the gray matter and the ventricles [example]. Also present in this slide is the choroid plexus [example], which is the source of CSF. Here, specialized ependymal cells actively transport ions into the ventricular space (and water follows). This process relies on membrane-associated Na/K ATPases; thus the cells are quite eosinophilic due to the highe concentration of membrane and mitochondria. The connective tissue immediately below the choroidal cells is also very richly vascularized. 

 

B. Astrocytes
Slide 13270
astrocytes, Gold-staining [Webscope] [Imagescope]

The links above should open to a view of a lighter stained area of the slide where you can look for typical star-shaped cells, which represent astrocytes.  Many of these astrocytes send out processes that contact and wrap around nearby capillaries, which are also clearly recognizable as tube-shaped segments.

 

 

IV. Cerebellum
Slide 77 20x (cerebellum, H&E) [WebScope] [ImageScope]
Slide 77 40x (H&E) [WebScope] [ImageScope]
Slide 77a 40x (luxol blue/cresyl violet) [WebScope] [ImageScope]

Using slide 77, determine that the cerebellar cortex is organized into an outer molecular layer [example] containing basket and stellate cells (not distinguishable by routine light microscopy) as well as axons of granule cells found in the deeper, highly cellular granule layer [example].  Still deeper is the white matter [example] of the cerebellum, which contains nerve fibers, neuroglial cells, small blood vessels, but no neuronal cell bodies.

Examine the boundary between molecular and granule cell layers.  Here you will see the Purkinje cell bodies [example].  In these slides you will not be able to discern the amazing dendritic tree that extends from the Purkinje cell bodies into the molecular layer, nor will you be able to see their axons, which extend down through the granular layer into deeper parts of the cerebellum.  The dendritic tree and axon or each Purkinje cell can only be seen in thicker sections stained with special silver stains.  Most of the nuclei visible in the granular layer belong to very small neurons, granule cells, which participate in the extensive intercommunication involved in the cerebellum’s role in balance and coordination.

 

 

V. Cerebrum
Slide 76
(cerebrum, luxol blue/cresyl violet) [WebScope] [ImageScope]
Slide 76b (toluidine blue & eosin) [WebScope] [ImageScope]
  
Unlike the highly organized cerebellar cortex, the cerebral cortex appears to be less well-organized when viewed with the light microscope.  Nonetheless, it is loosely stratified into layers containing scattered nuclei of both neurons and glial cells.  Examine the layered organization of the cerebral cortex using slide 76 stained with luxol blue/cresyl violet [ORIENTATION] (which stains white matter tracts and cell bodies) or toluidine blue and eosin [ORIENTATION] (TB&E, toluidine blue stains the nuclei and RER of cells whereas eosin stains membranes and axon tracts).  Typically one or more sulci (infoldings) will extend inward from one edge of the section.  Examine the gray matter on each side of the sulcus using first low and then high power.  Neurons of the cerebral cortex are of varying shapes and sizes, but the most obvious are pyramidal cells.  As the name implies, the cell body is shaped somewhat like a pyramid, with a large, branching dendrite extending from the apex of the pyramid toward the cortical surface, and with an axon extending downward from the base of the pyramid.  In addition to pyramidal cells, other nuclei seen in these sections may belong to other neurons or to glial cells also present in the cortex.  You may be able to see subtle differences in the distribution of cell types in rather loosely demarcated layers. There are 6 classically recognized layers of the cortex:

  1. Outer plexiform (molecular) layer: sparse neurons and glia
  2. Outer granular layer: small pyramidal and stellate neurons
  3. Outer pyramidal layer: moderate sized pyramidal neurons (should be able to see these in either luxol blue [example] or TB&E-stained [example] sections)
  4. Inner granular layer: densely packed stellate neurons (usually the numerous processes aren’t visible, but there are lots of nuclei reflecting the cell density)
  5. Ganglionic or inner pyramidal layer: large pyramidal neurons (should be able to see these in either luxol blue [example] or TB&E-stained [example] sections)
  6. Multiform cell layer: mixture of small pyramidal and stellate neurons

Pyramidal cells in layers III and V tend to be larger because their axons contribute to efferent projections that extend to other regions of the CNS –pyramidal neurons in layer V of motor cortices send projections all the way down to motor neurons in the spinal cord!

Deep to the gray matter of the cerebral cortex is the white matter that conveys myelinated fibers between different parts of the cortex and other regions of the CNS. Be sure you identify the white matter in both luxol blue [example] and TB&E-stained [example] sections, as it will appear differently in these two stains. Review the organization of gray and white matter in cerebral cortex vs. spinal cord.

 

 

VI. Hippocampal Region
Slide NP004N (hippocampal region, coronal section, luxol blue) [WebScope] [ImageScope] [ORIENTATION]

This coronal section includes the hippocampus (hippocampus = sea horse), dentate gyrus, and adjacent temporal lobe gyrus (entorhinal cortex). Above the temporal (ventral or inferior) horn of the lateral ventricle the lateral geniculate nucleus is present. Lateral to this structure is the tail of the caudate. The medial surface of the section is the posterior portion of the thalamus and a small portion of the cerebral peduncle. Look at the margins of the ventricle at higher magnification and note that it is entirely lined by ependymal cells. Just medial (to the right) of the tail of the caudate, note the choroid plexus [example], which consists of highly convoluted and vascularized villi covered by ependymal cells which are specialized for the production of cerebrospinal fluid, or CSF.

The hippocampus and dentate gyrus function in what is known as the "limbic system" to integrate inputs from many parts of the nervous system into complicated behaviors such as learning, memory, and social interaction beyond the scope of what can be described here. For now, focus just on the morphology of these regions and observe the presence of three distinct layers rather than the six layers found in the cerebral cortex (evolutionarily speaking, the three-layered organization is considered to be "older," so this type of cortex is also known as "archicortex" whereas the "newer" six-layered cerebral cortex is "neocortex"). In the hippocampus [ORIENTATION], observe:

  • ("1" in the orientation figure) a polymorphic layer containing many nerve fibers and small cell bodies of interneurons,
  • ("2" in the orientation figure) a middle pyramidal cell layer containing hippocampal pyramidal cells [example], and
  • ("3" in the orientation figure) a molecular layer containing dendrites of the pyramidal cells.

In the dentate gyrus [ORIENTATION], observe:

  • ("4" in the orientation figure) a polymorphic layer containing nerve fibers (known as "mossy fibers") and cell bodies of interneurons,
  • ("5" in the orientation figure) a middle granule cell layer containing the round, neuronal cell bodies of dentate granule cells [example], and
  • ("6" in the orientation figure) a molecular layer containing dendrites of the granule cells.

The "hilus" is the region where the head of hippocampus abuts the dentate gyrus. The multipolar neurons in this area are known as "mossy cells" [example] and they primarily receive input from mossy fibers of the granule cells of the dentate gyrus and then relay those signals back to other cells in the dentate. In terms of clinical significance, the pyramidal cells of the hippocampus are particuarly vulnerable to damage in severe circulatory failure and by anoxia of persistent severe seizures. You may see small calcific bodies in part of the hippocampus, which occur as a normal part of the aging process. Calcific bodies are also present in the choroid plexus, another common site of accumulation as the years pass.

 

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Updated 4/10/12 - Velkey