1. Distinguish between gray and white matter in the CNS.

2. Distinguish cerebellum from cerebrum in histological sections. Identify the layers of the gray matter in the cerebellum.

3. Understand the organization and structure of the spinal cord. Identify multipolar and pseudounipolar neurons and satellite cells.

4. Identify nerve cell bodies, Nissl substance, neuroglia and neuropil.

5. Identify ganglia and nerves, the endo-, peri-, and epineurium of nerves, and the structure of nerve fibers.

6. Identify different encapsulated sensory receptors (lamellar and tactile corpuscles).

slides:

s6 spinal cord, Nissl stain
s9 "sciatic" nerve, osmium tetroxide fix with eosin counter-stain
s10 mesentery
s12 sympathetic trunk ganglion
s13 cerebellum
s14 cerebrum
s15 "peripheral" nerve (H&E, trichrome)
s17 spinal cord, H&E
s31 palmar skin
s41 tongue, anterior
s48 esophagus and trachea
s95 jejunum
m7 finger (Medical Collection, Demonstration slide)

Index of images

Nerve tissue is involved in communicating information within you (between organ systems for example) and outside of you (seeing for example). The main cell involved in this is the neuron or nerve cell. A phenotypic feature that differentiates this cell from other tissue types is their large, euchromatic nucleus with a prominent nucleolus. The nucleus is so large that Cooter often mis-identifies the nucleus as the cell and the nucleolus as the nucleus. The nucleus is in the so-called nerve cell body or perikaryon. Another feature is their long cell processes that extend from the cell body and these are classified into dendrites and axons that receive and send information, respectively. There are other cells found in nerve tissue that are involved in maintaining the neurons and essentially electrically insulating the axons. Nerve tissue is typically subdivided into that which is encased within the cavities of the cranium and spine (central nervous system) and that which is not (peripheral nervous system).

  The Central Nervous System

We will begin our study of nervous tissue by investigating the central nervous system (CNS). In non-stained preparations, the CNS can be visually divided into gray and white matter. The gray matter consists primarily of nerve cell bodies and neuroglia (support cells) while white matter consist primarily of neuroglia, nerve cell processes and myelin. When stained with H&E, the gray matter is usually more basophilic owing to the greater number of cell nuclei present. The white matter usually has a foamy appearance because of the high lipid content of myelin. We will look at sections of the cerebellum, cerebrum and the spinal cord.

The parenchyma of the cerebellum (s13) is arranged in lobes that are further divided into lobules with foliate projections from the lobules. Each lobule and folia has a core of white matter and a cortex of gray matter. The gray matter can be subdivided into three layers. The innermost (nearest the white matter) is called the granular layer in that it contains numerous granular cells, the most abundant type of neuron in the human brain. The next layer is the Purkinje cell layer which is directly next to the granular layer. The Purkinje cells have a large nucleus and cell body that extends processes into the molecular layer. These cells are not abundant but can readily be observed at the microscopic level. The outermost layer of the gray matter is called the molecular layer. It is has few nuclei and contains dendrites from the Purkinje cells. Be able to distinguish the layers and identify Purkinje cells.

The cerebrum (s14) is also arranged into lobes but the gray and white matter are not as readily distinguished as in the cerebellum. The outermost layer is the gray matter and can be divided into a number of layers. We will not try to distinguish the layers but focus on distinguishing neurons, neuroglia, and neuropil. The neurons are distinguished by their large size and distinguishable cytoplasm. In general, only the cytoplasm around the nucleus (the perikaryon or cell body) is distinguishable.  The neuroglia are the support cells, and we cannot easily classify the neuroglia without other staining methods. The neuroglia appear as primarily round nuclei without a defined cytoplasm. The neuropil is essentially the material that fills the space between the nerve cell bodies and neuroglia and consists of neurites and glial cell processes which gives this material a fibrous, felt-like appearance, hence its name.

The CNS also includes the spinal cord. It differs from the brain in that the gray matter is innermost while the white matter is outermost. We will first observe a Nissl-stained section of the spinal cord. Study s6 without optical assistance and you will observe an H or butterfly shaped region with the most staining. This outlines the gray matter. Observe the section near the center with optical assistance and you will observe a central vessel that is the central canal and the white and gray matter. The lumen of the central canal is lined with cuboidal to columnar cells called ependymal cells. Move into the gray matter and study the cells. You will note large cells with large nuclei, prominent nucleoli, and punctate staining in the cytoplasm. These are multipolar neurons and the punctate material is Nissl substance or rER and polyribosomes. Note that the Nissl substance is not present in the distal regions of the nerve cell processes.

Next we will observe the spinal cord stained with H&E. A diagram of the spinal cord region which we will study is shown in this illustration.  Study this diagram to understand the topology and wiring of the spinal cord and the location of the dorsal root ganglion.

Study the spinal cord (s17) at low power and distinguish the gray from white matter. Study the gray matter more carefully and identify multipolar neurons, neuroglia and neuropil. Identify the dorsal root and follow it out to the spinal (dorsal root) ganglion. The dorsal root contains myelinated nerves. Note that the nerves have a foamy appearance because of the myelin. The spinal (dorsal root) ganglion (a collection of nerve cell bodies outside of the CNS, see below) contains readily identifiable neurons which are unipolar (also called pseudounipolar). These cells are large, round, have a prominent nucleolus and are surrounded by layer of flattened cells. These latter cells are called satellite cells. The section may contain some of the ventral root which runs just beneath the spinal  ganglion. It also contains myelinated nerves.

A ganglion is essentially a gang of nerve cell bodies outside of the central nervous system. The cluster of nerve cell bodies is usually encapsulated with connective tissue and acts as a synaptic (relay) station between the CNS and the effector organs for the autonomic nervous system. We will study sympathetic ganglia which are located in specific regions within the body (close to the lumbar and thoracic regions of the spinal cored) and parasympathetic-like ganglia which are located within an effector organ.

 More representative (“truer(?)”) parasympathetic ganglia can be found in your slides containing the trachea (s48) and tongue (s41; below), however, they are much more difficult to locate than the enteric ganglia (plexi).  Start first with the trachea (s48) and examine the periphery of the fibromusculocartilagineous coat of the trachea.  You might observe a cluster of neurons and their support cells, i.e., a ganglion.  These autonomic neurons likely innervate the smooth muscle and glands of the trachea.  Note that these autonomic neurons exhibit the classic morphology of neurons:  abundant cytoplasm, euchromatic nucleus and prominent nucleolus. We will look for another example of parasympathetic ganglia in the tongue, below.

We have previously studied the "sciatic" nerve focusing on staining and the difference between longitudinal and cross sections. Now, we want to focus more on the detailed structure of this nervous tissue.  Study s9 ("sciatic" nerve, osmic acid fixation with eosin counter-stain) at low power and focus on the cross section of the nerve. We'll start with the three connective tissue coats whose names parallel those of skeletal muscle organs (i.e., epi-, peri- and endo-neurium; schematic).  Re-examine this low power image of s9 concentrating on the nerve in cross-section.  Note that it is comprised of 4 circular structures (two large, two small) which are nerve fascicles.  [The number of nerve fascicles in your slide may differ.]  Each of the fascicles is surrounded by a perineurium, about more of which later.  The material surrounding the fascicles and loosely holding them together is the epineurium.  At higher magnification the epineurium can be seen to consist of irregular connective tissue (sometimes dense, sometimes loose) and adipocytes.  Blood vessels are often found in the epineurium of large nerve trunks such as this and are called collectively the vasa nervorum.  Surrounding each of the fascicles (schematic) is the perineurium which at higher magnification appears to be a thin layer of dense regular connective tissue, although its actual structure is a bit more complex.  The perineurium is functionally important as it maintains the fluid environment necessary for nerve fiber conduction.  In many fascicles, extensions from the circumferential perineum can be seen penetrating into the nerve fascicle.  These are called perineurial partitions or septa and are of unknown significance.  Finally, the endoneurium is a loose connective tissue interspersed among the individual nerve fibers.  The nerve fibers are seen here as darkly staining myelin sheaths of varying intensities surrounding eosinophilic axons of varying dimensions.  Owing to the absence of hematoxylin staining in this slide (despite the slide label), the cells comprising these structures are largely obscured.  The nodes of Ranvier can be observed in the longitudinal section at high power. They present as a bulbous junction of the myelin sheaths along an axon.

A more “typical” view of nerves is seen in s15 (“peripheral” nerve); this slide comes in two stains, a trichrome and H&E.  The latter are less common and slides with this stain have been placed at the demonstration slide trays for your examination.  Examine the nerve in cross-section (trichrome, H&E) and re-examine the connective tissue coats (epi-, peri- and endo-neurium; trichrome, H&E).   In the absence of osmic acid (osmium tetroxide) fixation, the appearance of the nerve fibers (trichrome, H&E) is much different.  Surrounding the centrally placed axon, the myelin sheath (or more properly myelin space) is represented by a circular deposit of neurokeratin, a sparse proteinaceous network that is the remnants of the myelin sheath following removal of fatty material.  External to the ring of neurokeratin is a thin rim of densely staining material, variably interpreted as either cytoplasm or basement membrane.  In a few select fibers, the nucleus of the Schwann cells can be seen as crescent shaped nuclei conforming to the shape of the myelin sheath.  Other nuclei within the fascicle represent fibroblasts and endothelial cells which you should now be able to distinguish.  Finally, examine the nerve in longitudinal section and look for nodes of Ranvier. 

Lab:  Nervous Tissue