AXONAL DEGENERATION

Axonal degeneration inevitably follows the death of the neuron of which it is a part. A severed or severely damaged axon undergoes distal degeneration without usually provoking death of the proximal part of the neuron, although this does undergo a series of structural and metabolic changes (i.e. axon reaction and chromatolysis, see below). Within days, the distal part of the axon fragments and the surrounding myelin sheath breaks up into ovoids. Over the next 3 weeks or so, the axon and myelin debris are taken up by macrophages, which infiltrate the degenerating fiber tracts. Several methods can be used to demonstrate degenerating nerve fibers in the CNS. These include specialized silver impregnation techniques and stains for degenerating myelin and for lipid (Fig. 1.1).

Marchi’s method, and oil red O or other stains for neutral lipid are particularly useful. During the first 2–3 weeks after injury when most of the products of fiber degeneration are still extracellular, their demonstration by Marchi’s method requires staining of unembedded tissue that has not fixed long in formalin. If there has been a longer period of time since the injury and much of the debris has been taken up by macrophages, Marchi’s method can be used on frozen sections and the staining is not affected by the duration of formalin fixation. Degeneration of fiber tracts can be demonstrated for several months with stains for neutral lipid, and for several years by Marchi’s method. In addition, immunohistochemistry for macrophage markers (e.g. with antibodies to CD68 and HLA class 2 antigen) is useful for detection of degenerating fiber tracts.

AXON REACTION AND CHROMATOLYSIS

Damage to the axon provokes a series of morphologic and biochemical changes in the neuronal cell body. These are collectively referred to as the axon reaction (Figs 1.2, 1.3). The changes include disruption and dispersion of Nissl bodies (chromatolysis) associated with rearrangement of the cytoskeleton and marked accumulation of intermediate filaments. The axon reaction is conspicuous in large neurons with axons that project into the peripheral nervous system and in some of the larger neurons with central projections. Chromatolysis is not visible on conventional light microscopy of small neurons or certain large neurons such as the cerebellar Purkinje cells, but changes can be demonstrated in these cells by electron microscopy and immunohistochemistry.

SWOLLEN NEURONS

Swollen or ballooned neurons are a feature both of the axon reaction and of a variety of diseases in which perikaryal changes occur independently of axonal damage (Table 1.2). Histologically, they appear as distended, weakly-staining cells with large, relatively clear nuclei (Fig. 1.4). Occasionally the cells contain small vacuoles. In conventionally processed tissues an artefactual lacuna is often present around the abnormal cell. Swollen or ballooned neurons can be demonstrated with several immunohistochemical markers (Fig. 1.5, Table 1.3).

TRANS-SYNAPTIC NEURONAL DEGENERATION AND OLIVARY HYPERTROPHY

Neurons within some nuclei in the CNS atrophy and degenerate in response to deafferentation. Examples are neurons in the lateral geniculate nucleus, which degenerate after optic nerve or tract lesions, and neurons in the pontine nuclei, which degenerate after interruption of descending frontopontine afferent fibers. Neurons in the inferior and accessory olivary nuclei undergo an unusual form of trans-synaptic degeneration after a destructive lesion (such as an infarct) of the ipsilateral central tegmental tract (Fig. 1.6). The olivary ribbon as a whole becomes thickened and neurons show marked enlargement, cytoplasmic vacuolation, and some dispersion of Nissl bodies. Olivary hypertrophy is associated with the development of palatal myoclonus in some patients.

HYPOXIC CELL CHANGE

Neurons are especially vulnerable to damage from hypoxia, which causes the following distinctive histologic changes (Fig. 1.7):

Histologic changes identical to these can be induced in neurons by hypoglycemia or by exposure to excessive amounts of excitotoxic neurotransmitters.

Cell stress proteins are expressed at an early stage of hypoxic injury and are demonstrable by immunohistochemical techniques.

Neurons in certain parts of the brain that are especially vulnerable to hypoxic damage are:

The pattern of regional susceptibility to hypoxia differs between infants and adults (see Chapters 2 and 8).

FERRUGINATION

Although dead neurons usually undergo liquefaction or are removed by phagocytosis, they may instead become encrusted and replaced by mineral salts, a process termed ferrugination. This is particularly prominent in the infant brain in response to hypoxic–ischemic damage (Fig. 1.8).

MARINESCO BODIES

These are small spherical nuclear inclusions that are brightly eosinophilic and are often seen in neurons of the adult substantia nigra (Fig. 1.9a). They may also occur in other neurons, such as the pyramidal cells of the hippocampus and neurons in the tegmentum of the brain stem. Marinesco bodies are most common in the elderly and in dementia with Lewy bodies. They have an increased prevalence in neurons containing Lewy bodies.

Ultrastructurally, Marinesco bodies are composed of filaments with the same diameter as intermediate filaments, and may be derived from the nuclear lamins. They are immunoreactive for ubiquitin, an 8 kD polypeptide involved in the degradation of many abnormal or short-lived proteins (Fig. 1.9b).

VIRAL INCLUSIONS

Nuclear inclusions are a feature of infection by several viruses (see Chapters 13 and 14), for example cytomegalovirus infection, which produces particularly striking nuclear inclusions (Fig. 1.10).

Hirano bodies

These are brightly eosinophilic rod-shaped or elliptical cytoplasmic inclusions that may appear to overlap the edge of a neuron (Fig. 1.11). They are immunoreactive for actin, actin-associated proteins, and caspase-cleaved TDP43.

Ultrastructurally, Hirano bodies consist of a regular lattice of multiple layers of parallel 10–12 nm filaments, the filaments in one layer being transversely or diagonally oriented with respect to those in the adjacent layers.

Hirano bodies are most numerous in the CA1 field of the hippocampus. Their density in the stratum lacunosum increases until middle-age and declines gradually thereafter, except in chronic alcoholics, in whom the density may continue to increase. In the elderly, the number of Hirano bodies increases in the stratum pyramidale, and they are particularly numerous in this region in Alzheimer disease, Pick’s disease, some sub-types of Creutzfeldt–Jakob disease, and Guam parkinsonism–dementia.

EOSINOPHILIC THALAMIC NEURONAL INCLUSIONS

These are smaller than Hirano bodies, but are otherwise similar in shape and tinctorial staining characteristics. They occur in thalamic neurons in normal middle-aged and elderly individuals, but are commoner in patients with myotonic dystrophy.

ROD-LIKE CYTOPLASMIC INCLUSIONS

These eosinophilic cytoplasmic inclusions occur in large neurons of the caudate nucleus. The inclusions resemble loose bundles of twigs and, like the thalamic neuronal inclusions described above, are common in patients with myotonic dystrophy. However, they are sometimes an incidental finding in elderly patients who do not have neurologic or muscular disease. The rod-like inclusions stain strongly with phosphotungstic acid/hematoxylin (Fig. 1.12).

OTHER FILAMENTOUS NEURONAL INCLUSIONS

There are several other types of neuronal inclusion that comprise or include elements of the cytoskeleton and usually occur in the context of specific neurodegenerative diseases (Table 1.4). These inclusions are described in more detail and illustrated in the sections concerned with the relevant diseases.

LAFORA BODIES

These are composed of polyglucosans (polymers of sulfated polysaccharides) and are similar to corpora amylacea in composition and staining characteristics (see below). They are present in large numbers in Lafora’s disease (see Chapter 7), both in the CNS and in certain peripheral tissues such as sweat glands, liver, and skeletal muscle. Lafora body formation has been linked to aberrant glycogen hyperphosphorylation. The inclusions usually have a round core that is intensely periodic acid–Schiff (PAS)-positive (Fig. 1.13). Spicules of the core may radiate outwards, into a surrounding zone of less intensely PAS-positive material.

EOSINOPHILIC INCLUSIONS IN THE INFERIOR OLIVES

A characteristic feature of aging is the development of ill-defined eosinophilic inclusions in the neurons of the inferior olivary nuclei. These inclusions are intensely immunoreactive with antibodies to ubiquitin (Fig. 1.14) and should not be confused with lipofuscin, which also accumulates with age (see below) and is particularly abundant in neurons of the inferior olivary nuclei.

COLLOID INCLUSIONS

Colloid inclusions are round eosinophilic inclusions that usually occur in neurons in the hypoglossal nuclei (Fig. 1.15), but may be seen in other large neurons, particularly in the elderly. Electron microscopy shows that these inclusions result from dilatation of the endoplasmic reticulum by amorphous material. The importance of recognizing colloid inclusions, which do not have any clinical significance, is that they are occasionally confused with inclusions that are clinically significant such as Lewy bodies, pale bodies, and hyaline inclusions of motor neuron disease (see Chapter 27).

BUNINA BODIES

These are small beaded eosinophilic inclusions seen in motor neurons in motor neuron disease. Cystatin C, transferrin, and peripherin have been detected in Bunina bodies. Ultrastructurally, they appear as electron-dense membrane-bound bodies (see Chapter 27).

INCLUSIONS DERIVED FROM THE ACID VESICLE SYSTEM

The acid vesicle system consists of endosomes, lysosomes, and lysosome-derived dense bodies.

Lipofuscin (Fig. 1.16) is produced by oxidation of lipids and lipoproteins within the lysosomal system. It appears as orange-brown granular material in sections stained with hematoxylin and eosin, and is acid-fast (as demonstrated by the long Ziehl–Neelsen method) and autofluorescent under ultraviolet light. The granules are also sudanophilic and stain with PAS and Schmorl’s stain. Lipofuscin accumulates with aging in neurons and glia, particularly in:

The lipofuscin accumulation is increased in some neurodegenerative disorders such as Alzheimer disease and motor neuron disease.

Granulovacuolar degeneration. This term describes the accumulation of vacuoles containing small round dense bodies (granulovacuoles) (Fig. 1.17). The dense bodies are ubiquitinated and react with antibodies to some epitopes of the microtubule-associated tau protein. The immunohistochemical data have been interpreted as suggesting that dense bodies are derived from partial degradation of tau protein within lysosomes. Although anti-phosphoTDP43 antibody is sensitive for detecting granulovacuolar degeneration, TDP43 proteinopathies are not associated with an increase in granulovacuoles. Granulovacuolar degeneration is seen in normal aging after the sixth decade, predominantly in the hippocampal formation. Neurons in the CA1 field are most severely affected and, in descending order of severity, those in the prosubiculum, CA2, CA3, and CA4 fields. The density of hippocampal neurons showing granulovacuolar degeneration is increased in patients with Alzheimer disease and Pick’s disease, in whom granulovacuolar degeneration may also occur in neurons in the subcortical nuclei.