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Wallerian degeneration
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    Neurologists of the early 19th century relied on gross anatomy. The development of the compound microscope, techniques for tissue fixation and embedding, and the use of a microtome were essential inventions for the microscopic study of the nervous and other systems. Histology applying these methods to neural tissue was in large part the achievment of Robert Remak1 and his former student Rudolph Albert von Kölliker (1817–1905). Until the 1830s, the microscopic anatomy of the neuron, axon, and their connecting processes were not understood. Remak (1838) had noted “primitive bands” that probably were nerve sheaths, but it was Theodor Schwann (1810–82) who first described in 1838 the myelin sheath as a fatty deposit on the inner surface of the structureless cell membrane (called the neurilemma by Bichat). By the use of a compound microscope, Remak was able to prepare a histological monograph2 on the nervous system. Kölliker's text3 included accounts of various forms of nerve cells as well as:

    “a good many fine, pale fibres, like the processes of cells,...whether they are nerve tubes or are to be referred to as the processes of cells.”

    Deiters, in 1865 gave an important account of the nerve cell and its processes—“protoplasmic” (dendrites) and “nervous” (axon). He died aged 30 of typhus, but his findings were published in Schultze's book4 that also included Deiter's account of the vestibular nucleus that commemorates his name.

    Before the essential structure of nervous tissue could be shown, specific staining techniques and serial experiments were needed. Carmine was applied with success and used by Joseph von Gerlach in 1858. Nissl introduced methylene blue in the same year. Waldeyer used haematoxylin to stain axis cylinders in 1863. The myelin sheath had to await the Carl Weigert stain in 1884. The next year, Vittorio Marchi (1851–1908) with Algeri showed the black, degenerative products of myelin by mordanting with chromium salts and staining with osmic acid.

     The demonstration of degenerative processes of the myelin sheath was of course a crucial tool with which to reveal degenerative pathology.

    Augustus Volney Waller (1816–70) developed a vital technique in the study of the nervous system. He was born on a Kent farm, but spent his childhood in France, and qualified in Paris (1840). He then practised in Kensington (1842–51) and later went to Europe to become an outstanding experimental physiologist. In 1850, Waller5 reported his findings to the Royal Society. He showed that if the glossopharyngeal and hypoglossal nerves are cut, the distal segment containing axis cylinders cut off from the nerve cell undergoes degeneration. Since the cell body and proximal stump remained intact for a long period, Waller inferred that the nerve cells nourish nerve fibres. His studies made it possible to trace nerve fibres and their diseases. Combined with Forel's seminal studies, Waller's work was a necessary foundation for the neuron theory:

    “It is my intention at present to describe various alterations, as seen under the microscope, which take place in the structure of the same nerves after their continuity with the brain has been interrupted . . .at the end of the third or fourth day, we detect the first alteration...about five or six days after section, the alteration of the nerve-tube . . .has become much more distinct by a kind of coagulation or curdling of the white substance and axis cylinder . . .the disjointed condition . . .is greater toward the extremities . . .as we ascend toward the brain the disorganisation appears to decrease . . .On the 7th, 8th and 9th days... the curdled particles of medulla become still more disconnected, and in parts are removed by absorption The tubular sheath is also ruptured and disorganised, . . .collected into oval or circular coagulated masses . . .”

    Wallerian degeneration was born and has been inculcated into medical students ever since. Copious references and extracts of these investigations are contained in McHenry6, and in Clarke and O'Malley.7

    Waller ceased clinical practice in 1851, returning to Bonn where he worked with Budge on pupillary innervation that he showed derived from the T1-T2 segments (cilio-spinal centre) and the sympathetic trunk that exerted a vasoconstrictor action. He travelled to Paris in 1856 and fell sick, probably with rheumatic fever. He recovered and took the chair of physiology in Birmingham, but further illness hastened retirement. He died with severe angina in 1870.

    His son Augustus D Waller became a distinguished physiologist, who on mention of father's name, remarked: “I am the Wallerian degeneration.”

    The varied terminations of axons was studied by Willy Kuhne, and Nissl had advanced descriptions of the cell body constituents by using aniline dyes as cell stains. Rudolf Virchow in 1846 first observed the neuroglia, as an elevated membrane of what he realised was connective tissue, beneath the ventricular ependyma.. In 1870, Bernhard Aloys von Gudden (1824–1886), professor of psychiatry at Zurich and later Munich, produced secondary atrophy of the central neural structures by the removal of sensory organs or cranial nerves in young animals. In 1889, he showed the crossed and uncrossed fibres of the optic nerve and the occurrence of secondary atrophy in the thalamus after removal of specific cortical areas—transneuronal degeneration.

    This vast field of work7 culminated in the work of Camillo Golgi8 who used silver stains and demonstrated structures previously invisible, which were to be the basis of his axonal net theory that displaced Gerlach's notion of a neural plexus.


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