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
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:
3 Nerve fibre (includes the axon and its myelin sheath, if present).
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.
Consult Chapter 11.C. . . . . . . . . . . . . 11.C
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
CNS: Neurons, Astrocytes, Oligodendrocytes, Ependymal cells, Special central glia
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