HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 10 NERVOUS ELEMENTS

The nervous system provides for the control and coordination of all the body's activities. It spreads out widely to all organs from central nervous organs of a complex and discriminating nature, permitting a multitude of finely graded responses to changes in the external and internal states. It makes use of millions of nerve cells having especially the properties of excitability and conductivity. Information is conducted along long nerve cell processes as an electrical excitation generated across the cell membrane.
The nervous system comprises the central parts (CNS) of brain and spinal cord, and neural parts of the eye, and the peripheral (PNS) of nerves, ganglia, receptors, and neural endings on effector structures - muscle and glands. Powerpoint

A ELEMENTS OF NERVOUS SYSTEM

l  Nerve cells/neurons
                  _
                 | Oligodendrocytes, Astrocytes, Ependymal cells,
                 | Microglia, Special glial cells - (CNS)
2 Glial cells ---|
                 | Schwann cells, Satellite/Capsule cells,
                 |_Enteric (gut) glia - (PNS)
                 
3  Blood vessels
4  Connective tissue enclosing sheaths

B NEURONS

l Shape
Neurons are characterized by having long processes extending from a cell body/soma. One of these is the axon transmitting information; the others are receptive dendrites.
l Unipolar have one process, e.g., neuroblast.
2 Pseudounipolar have one process branching into two a short way from the cell soma, e.g., dorsal-root ganglion cell.
3 Bipolar have two processes, e.g., bipolar cell of the retina.
4 Multipolar have many processes. Shapes include:
... (a) stellate or star-like,
... (b) pyramidal with apical and basal dendrites, or
... (c) Purkinje with a plump body tapering to an espalier-oriented dendritic tree.

Note that the dendrites branch repeatedly, becoming finer. The axon retains its diameter along most of its length. The axon, though, may give off side branches or collaterals, and will usually divide into many fine branches, telodendria or the preterminal axonal arborization, near to its terminal structures.
The Golgi method which delineates only a few (roughly l in 70) of the neurons allows the full dendritic spread and the axon to be seen in a thick (l50 µm) section. Thus the shape of the neuron could be determined and classified, e.g., Golgi type l cells with long axons (projecting neurons) distinguished from type II cells with short axons (non-projecting neurons). No intracellular detail was seen.
Current methods yielding Golgi-like detail are to fill the neuron with Lucifer Yellow which can be lighted up with fluorescence microscopy, or with horseradish peroxidase which produces a visible product by acting on a substrate.

2 Nerve cell structure
l Soma contains a large central nucleus with much sap, but little visible chromatin. The nucleolus is prominent because the neuron has to synthesize organelles and much cytoplasm to fill its long processes.
2 Around the nucleus is the perikaryon with:

(a) Nissl bodies/granules - basophilic, cytoplasmic structures are concentrations of granular ER.
(b) Neurofilaments - a variety of intermediate filament - are aggregated into neurofibrils visible in the cytoplasm after silver impregnations.
(c) Surrounding the nucleus are elements of the Golgi apparatus, mitochondria, lysosomes, and microtubules. Actin filaments move vesicles in the zone directly under the neuron's plasmalemma.
(d) Pigment is sometimes present, e.g., melanin in substantia nigra neurons, and lipofuscin in old neurons.
(e) Cell membrane has specialized receptive areas, the subsynaptic membranes of synapses.

2 Dendrites

(a) Contain mitochondria, microtubules, and granular ER.
(b) Membranes have receptive subsynaptic membrane areas.
(c) Some dendrites have spine-like side processes, also receptive,
(d) Dendrites integrate the excitatory influences along them, and modify their responses and morphology in learning.

3 Nerve fibre (includes the axon and its myelin sheath, if present).

(a) Contains axoplasm flowing centrifugally from the somatic starting-point of the axon - the axon hillock.
(b) Has mitochondria, neurofilaments, microtubules, travelling vesicles, and, in some neurons, secretion droplets, in the axoplasm.
(c) Membrane of the tube is the axolemma, swelling out into a bag at its terminal/synapse which holds vesicles/microvesicles. (The axon is also termed the axis cylinder.)
(d) Myelin sheath of lipoprotein around the axolemma is interrupted at regular intervals to leave the axolemma `bare' at nodes of Ranvier. The membrane propagates the action potential.
(e) EM reveals myelin to have lamellae with alternate dark (major dense) and light (intraperiod) lines, apparently concentric around the axon.
(f) Closer examination reveals a mesaxon from the innermost lamella by the axolemma, and an outer mesaxon from the outermost lamella to join the enclosing Schwann or glial cells' plasmalemma.
(g) To control the neuron's ionic and transmitter environment, enclosure of the axon (and myelin) by Schwann or glial cells is complete, including nodes of Ranvier and the terminal bags (synapses). At intervals along the myelin, Schwann cell cytoplasm penetrates into conically oriented separations of the myelin lamellae, constituting the incisures of Schmidt-Lantermann, perhaps giving the myelin some flexibility and aiding molecular turnover.

Fig. 3 Staining methods for CNS neurons and glia(Stain #s in Table 3 below)

               \/   Full extent of dendritic tree
          \    /    shown by 3 GOLGI
           \  /
            \/
            /                                 5  GLIAL
         \ /                          \    /  for glial cell 
          \                            \  /   processes
           \                       ____ OO____
         / /                            OO
 \ \    / /                            /  \
  \ \  / /                            /    \
    \ / /           OO
     | |            OO             _
     | |           Glial nuclei     |
     | |                            |
     |*|Base of dendrites           |____ 1  NISSL
     |*|                            |
    /   \ Soma with Nissl granules* |
   /  *  \    and the nucleus      _|
 /*   * * \
/ *  ___   \     
* * | 0 | * \______########################_ _###################_________
 *  |___|    _______________________________|___________________________axon
*  *  *  *  /      ######################## | ################### myelin   /
\  NEURON  /                                |          |                  /
 \ *   *  /                                 |          |                 /
  \ *  * /                                  |          |                /
   \  * /                                   |      4  MYELIN           /
    \ */                                    |                         /
    |* |                                    |                        /
    |* |                                    |                       /
    |  |                                   Axon                    /
   / /\ \                                   |        Synapses O---/
 / /    \ \                                 |                    /
/ /      \ \                                |             &     O
          \ \ --------First dendrites-- 2 SILVER    terminal axonal 
                                       for neurofibrils    branchings

3 Neuron staining
l Fundamental to an understanding of nerve cell histology is the knowledge:
(a) that most neurons' processes are so extensive that only part of the cell is present in a 8 µm-thick section;
(b) that different parts of the neuron contain different elements, and staining for one of these elements reveals only the part of the cell containing it.
For example, a basic stain like toluidine blue will stain only nuclei of nerve and glial cells and Nissl bodies of nerve cells, leaving the large areas of surrounding tissue pale and apparently structureless, although other stains reveal that these areas of neuropil are packed with dendrites, axons, and processes of glial cells.
2 The staining methods for normal neural tissue, numbered l to 5 in Table 3, just below, reveal correspondingly numbered elements in Fig. 3 showing details of a CNS neuron and glia cell. Table 4 later lists the kind of information obtained by applying these techniques and degeneration-specific ones to the normal and pathological CNS.

Table 3 (a) and (b). Histological methods for the central nervous system.

Staining or
impregnation                                  Elements of nervous tissue
method              Nature of reagent          revealed


1  Nissl      Basic, e.g. methylene blue,  Nuclei of nerve cells, glia
              cresyl violet, thionine.     and blood vessels. Nissl
              Haematoxylin.                granules in nerve cell bodies (blue)

2  Silver     Reduced silver nitrate       Nerve cell bodies and larger
              methods of Cajal, Biel-      dendrites, axons and synapses,
              schowsky and Glees           because of their neurofibrillar
                                           content. (Soma, yellow: axons
                                           and synapses, black.)

3  Golgi      Silver nitrate.              Complete outline of only a 
                                           few (1/70) nerve cells - soma,
                                           dendrites and axon (black).

4  Myelin     Mordanting followed by       Myelin sheaths (blue).
              haematoxylin - Weigert-
              Pal technique. Luxol blue.

5  Neuroglia  (1) Cajal's gold-sublimate.  Astrocytes, oligodendroglia;
              (2) Hortega's silver         Microglia
                carbonate.

Degeneration-specific methods

6  Nauta,     Reduced silver nitrate,      Nerve fibres experiencing
   Fink-      after suppressive pre-       Wallerian degeneration
   Heimer     treatment.                   (black); pale, but identifiable
                                           background of normal nerve
                                           cells and fibres (yellowish-
                                           brown).
                                                 
Staining or
impregnation      Used in combination    Elements of nervous
method            with other methods     tissue not revealed

1  Nissl          Yes. Myelin            Nerve fibres (axon and
                                         myelin) and synapses

2  Silver         No                     Myelin of nerve fibre.
                                         Synapses without neuro-
                                         fibrils. Glial cell processes.

3  Golgi          No                     Most nerve and glial cells.
                                         Intracellular structures of
                                         the few cells revealed.

4  Myelin         Yes. Nissl             Axons, synapses and nerve
                                         cell somas. Glia.

5  Neuroglia      No                     Most nerve cells and
                                         processes.

6  Nauta          No                     Glial cell processes.

C GLIAL CELLS AND MYELINATION

Consult Chapter 11.C. . . . . . . . . . . . . 11.C

D CELL LINEAGES OF THE NERVOUS SYSTEM

1. For the PNS, the problems are: (i) the large number of neural-crest-derived cell types, including many non-neural ones (mostly because the crest is the major constructor of the head); (ii) that additions are still being made to the list; (iii) the evidence is chiefly from birds; and (iv) some head structures, e.g., receptors and ganglion neurons for hearing and balance come from ectodermal placodes.

2. In the CNS, the story is coming to resemble that for haematopoiesis, with a multipotent neural stem cell giving rise to a self-propagating progenitor pool. From this pool, self-sustaining populations of neuroblasts and glioblasts derive. Further specifications, under the control of neural 'growth factors', are for transmitter type, shape, and axon length, and for glioblast derivatives, whether to be type 1 or 2 astrocytes, or oligodendrocytes. Microglia are regarded as invaders of haematopoietic origin, but is this true for all of them, always? Other questions are: do neural progenitors live on in the adult CNS? (They are present in olfactory mucosa.) And how well does the astrocyte 2 correspond to the fibrous astrocyte, and the 1 to the protoplasmic?

Neural and other cell derivatives of neural tube and crest

NEURAL TUBE
CNS: Neurons, Astrocytes, Oligodendrocytes, Ependymal cells, Special central glia

NEURAL CREST
PNS: Sensory- & autonomic-ganglion neurons, Adrenal neurons, Satellite cells, Schwann cells, Enteric glia
OTHERS: Chromaffin cells, C-cells, Melanocytes, some Cardiac (outflow tract) & Carotid-body cells

NEURAL CREST via Mesectoderm
ANTERIOR CRANIAL SKELETAL TISSUES: Osteoblasts, Chondroblasts
DENTAL TISSUES: Odontoblasts, Cementoblasts, Ligament fibroblasts
HEAD MUSCLES & CONNECTIVE TISSUES: Smooth & skeletal muscle cells, Fibroblasts, Adipocytes, Meningeal cells

William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159