CHAPTER 1. NEURONS AND NEUROGLIA

A. HISTOLOGICAL IDENTIFICATION

OBJECTIVE:
    BE ABLE TO DISTINGUISH NEURONS AND THE VARIOUS TYPES OF NEUROGLIA IN HISTOLOGICAL PREPARATIONS.

Slide 61 This is a silver preparation of human dorsal root ganglion. The staining capacities of unipolar ganglion cells vary considerably. These neurons have, with few exceptions, centrally located nuclei with prominent nucleoli. The cells vary in size from 15-100 microns and have only one protoplasmic process, the axon. Each axon bifurcates into a peripheral and a central branch. The peripheral branch enters the spinal nerve and ends as a somatic or visceral receptor. The central branch enters the spinal cord.
Slide 62 This is a silver preparation of human celiac ganglion. These multipolar neurons vary in size from 15-60 microns and contain centrally or eccentrically located nuclei with prominent nucleoli. Within the perikarya are varying amounts of lipofuscin pigment. The dendrites and cell bodies receive synaptic terminals from preganglionic sympathetic neurons. The axons of these multipolar ganglion cells become postganglionic sympathetic fibers and terminate on smooth muscles and glands.
Slide 63 This is a silver preparation of the gray matter of the human spinal cord. The two multipolar cells are motor neurons. Their cell bodies may be as large as 100 microns in diameter and may give rise to as many as 20 dendrites plus the axon. They contain large, centrally located nuclei.
Slides 64 & 65 These are protoplasmic (Sl. 64) and fibrous (Sl. 65) astrocytes in silver preparations of the human cerebrum. The protoplasmic astrocytes are predominantly in gray matter and are referred to as "mossy cells" due to their short, thin, heavily-branching processes. The fibrous astrocytes are predominantly in white matter and are characterized as "spider-like"; their processes are thicker, longer, and have fewer branches.
Slide 66 These are fibrous astrocytes in the white matter of the human cerebrum. These astrocytes have processes that extend toward the larger and smaller blood vessels passing horizontally through the field. The processes have terminal expansions, the vascular end-feet, which ensheath the blood vessels. Astrocytes in proximity to the external and ventricular surfaces of the brain and spinal cord have processes whose terminal expansions form the external (pial-glial) and internal (ependymal-glial) limiting membranes.
Slide 67This is a hematoxylin-eosin preparation of the gray matter of the human spinal cord. The conspicuous large cells are motor neurons. Their nuclei are spherical, large, and usually centrally located and they contain prominent nucleoli. The perikaryon is filled with coarse Nissl bodies. The dendrites are large, taper and branch, and contain Nissl bodies at their origins.

The neuropil surrounding the neurons contains axons and dendrites (which cannot be distinguished in this type of preparation), neuroglial cells, and capillaries. The glial nuclei are scattered throughout the field and should be distinguished. The oligodendrocyte nucleus is small, spherical, and stains darkly due to its dense chromatin. The astrocyte nucleus is larger, oval or elongated, and stains lightly.

Slide 68This is a hematoxylin-eosin preparation of gray and white matter in the human spinal cord. Several large multipolar neurons are in the gray matter close to its border with the white matter. These neurons contain centrally or eccentrically located nuclei with prominent nucleoli and dust-like chromatin. The perikarya contain variable numbers of coarse Nissl bodies some of which extend into the dendrites.

The white matter contains numerous glia among its axons. The oligodendrocyte nuclei in both the gray and the white matter are distinguished by their small, spherical, and darkly staining nuclei, whereas the astrocyte nuclei are larger, oval or elongated, and lighter staining.

B. ULTRASTRUCTURAL IDENTIFICATION OF CNS

OBJECTIVE:
    BE ABLE TO IDENTIFY THE FEATURES OF NEURONS, AXONS, DENDRITES, SYNAPSES, ASTROCYTES, AND OLIGODENDROCYTES IN ELECTRON MICROGRAPHS.

Figures 1-1 to 1-8 are electron micrographs of the mammalian CNS. Be aware that a variety of factors, such as speed and type of fixation, thickness of sections, and choice of photographic material, among others, may alter the appearance of biological tissues and cause departure from the original, in vivo, condition to a varying extent. Shrinkage or swelling of mitochondria, dispersion or particulate clumping of chromatin, and shape and electron lucency (or density) of vesicles represent only a partial list of components vulnerable to the insults of tissue processing.

Figure 1-1. This is the neuropil from the cat thalamus. It contains axons, dendrites, and astrocytic processes sectioned in various planes. Small axons (1) usually travel through the neuropil in bundles in which they are arranged parallel to each other. Their contours are regular except where they expand to form boutons (2) which contain abundant synaptic vesicles and mitochondria. On the other hand, small dendrites (3) travel through the neuropil individually and tend to change directions frequently. Their contours are usually irregular and their surfaces frequently bear spines (4).

The dendrite cut longitudinally contains neurotubules (5) and long, slender mitochondria (6). Several synaptic junctions (7) are seen on its surface. The myelinated axon at the bottom of the field (8) contains abundant neurofilaments. Astrocytic processes (9) occur randomly throughout the neuropil and can be distinguished from dendrites by their relative absence of organelles. X 12,900 (Courtesy of Dr. Kristy Kultas-Ilinsky)

Figure 1-2. This is the cyton of a small neuron in the rat thalamus. The oval nucleus (1) is indented and includes homogeneously dispersed chromatin. The nuclear envelope is bilaminar and it possesses pores (arrows). The perikaryon contains small Nissl bodies (2), Golgi apparatus (3), mitochondria (4), and lysosomes (5). The neuropil surrounding the neuronal cell body is densely packed with bundles of unmyelinated axons (6), dendrites (7), and astrocytic processes (8). Several terminal boutons packed with vesicles (9) and a few small myelinated axons (10) are scattered in the neuropil. X 10,500 (Courtesy of Dr. John J. Taylor)

Figure 1-3. This micrograph is from the neuropil of the cat thalamus and shows a protoplasmic astrocyte lying next to an oligodendrocyte. The main ultrastructural features of an astrocyte are its watery or translucent cytoplasm and its irregular shape. The nucleus (1) is irregular or elongated and the nuclear envelope is indented. The chromatin is distributed relatively evenly throughout the nucleus although some clumping does occur, especially beneath the nuclear envelope. The cytoplasm (2) is very lucent and it is confined to a narrow rim surrounding the nucleus. It contains mitochondria (3) and scanty amounts of rough endoplasmic reticulum and other organelles. Numerous processes (4) emanate from the cell body and ramify among the axons and dendrites in the neuropil.

The main ultrastructural features of the oligodendrocyte are its dark appearance, small size, and round shape. The nucleus (6) usually lies in an eccentric position, is round or oval, and contains dense clumps of chromatin. The cytoplasm (7) appears very dark and contains abundant amounts of rough endoplasmic reticulum, numerous free ribosomes, and Golgi apparatuses. Mitochondria are also numerous but they are inconspicuous because of the density of the cytoplasm. X 11,625 (Courtesy of Dr. Kristy Kultas-Ilinsky)

Figure 1-4. This micrograph is through a portion of a capillary (1) in the neuropil of the rat thalamus. The endothelial cytoplasm (2) contains pinocytotic vesicles. The cytoplasm of a pericyte (3) is enclosed in the basal lamina (4) of the capillary. External to the basal lamina is a vascular end-foot (5) of an astrocyte. It contains a bundle of glial filaments (6) and several mitochondria (7). X 28,800 (Courtesy of Dr. John J. Taylor)

Figure 1-5. This is a large motor neuron from the rat hypoglossal nucleus. The oval nucleus (1) is surrounded by a bilaminar membrane and contains a prominent nucleolus (2) and homogeneously dispersed chromatin. The perikaryon contains numerous large Nissl bodies (3) and several perinuclear Golgi apparatuses (4). In addition there are elongated and Y-shaped mitochondria (5), lysosomes in various stages of their cycle (6) and microtubules (7). A capillary (8) with its endothelial cell nucleus and basal lamina lies close to the surface of this neuron. X 8,500 (Courtesy of Dr. Margaret H. Cooper)

Figure 1-6. This micrograph shows an axon hillock and initial segment of a neuron in the cat thalamus. The neuronal nucleus (1) contains a nucleolus (2) and chromatin which has tended to clump slightly. The perikaryon contains Nissl bodies (3), a Golgi apparatus (4), and bundles of neurotubules (5) that enter the initial segment of the axon (6). Besides the longitudinally arranged bundles of neurotubules, other characteristics of the initial segment are the dense layer situated beneath the axolemma and scattered clusters of ribosomes. The neuropil above the axon hillock and initial segment contains numerous unmyelinated and a few small myelinated axons, vesicle-filled boutons next to a dendrite (7), and astrocytic processes. Beneath the initial segment and adjacent to the neuronal cell body is an astrocyte containing an indented nucleus (8). The cytoplasm is watery or translucent (9) and it contains mitochondria and bundles of glial filaments (10). X 11,610 (Courtesy of Dr. Kristy Kultas-Ilinsky)

Figure 1-7. This micrograph shows asymmetrical (type I) synapses in the cat thalamus. The axon terminal (1) contains numerous mitochondria (2) and synaptic vesicles (3). The dendritic or receptive component (4) of the synapse contains a mitochondrion and filamentous material. At the synaptic junction, the pre- (5) and postsynaptic (6) membranes are separated by a synaptic cleft that contains intercellular material. Note the round synaptic vesicles clustered at the active zone of this synapse (7). Also note the asymmetric nature of the membranes, with the postsynaptic possessing a prominent coating of dense material on its cytoplasmic surface. Partially surrounding this axodendritic synapse are astrocytic processes (8). X 39,000 (Courtesy of Dr. Kristy Kultas-Ilinsky)

Figure 1-8. This micrograph shows a symmetrical (type II) and an asymmetrical (type I) synapse in the cat thalamus. In the symmetrical synapse the axon terminal is densely packed with mitochondria (1) and synaptic vesicles (2). The dendritic component (3) contains scattered mitochondria, rough endoplasmic reticulum (arrowheads), neurotubules, and filamentous materials. At the synaptic junction the pre- (4) and postsynaptic (5) membranes are separated by the synaptic cleft. Note that the densities and thicknesses of the pre- and postsynaptic membranes are equal and that the synaptic vesicles are oval in this synapse. A thin astrocytic process (6) partially surrounds the bouton.

The axonal component of the asymmetrical synapse (7) contains round vesicles and the pre- and postsynaptic membranes are of unequal densities and thicknesses as described in Figure 7. Next to this synapse is an axon (8) surrounded by a myelin sheath composed of concentric spiral lamellae. X 39,000 (Courtesy of Dr. Kristy Kultas-Ilinsky)