Structural abnormalities in neurones.

A wide variety of neuronal alterations ascribed to the effect of hypoxia of different types has been described in man and in several species of experimental animals (Spielmeyer, 1922; Gildea and Cobb, 1930; Tureen, 1936; Hoff et at, 1945; Morrison, 1946; Grenell and Kabat, 1974; Krogh, 1952; Courville, 1951; Meyer, 1963). In a comprehensive review of the literature, Hoff et al (1945) concluded that the changes were remarkably alike whatever the cause of hypoxia. These changes, probably best seen with the Nissl stain, range from non-specific forms where the cells appear very pale or are undergoing swelling and loss of stainable substance to the classical descriptions given by Spielmeyer (1922). These latter changes, which include 'coagulation necrosis' or 'ischaemic cell change', 'homogenizing cell change' and 'liquefaction necrosis', have been discussed in terms of more modern views by Greenfield and Meyer (1963) and Brierley (1976). Nissl's 'acute cell disease' (Spielmeyer's 'acute swelling') and Nissl's 'chronic cell degeneration' must be added to the above neuronal alterations because they are so often mentioned as evidence of hypoxia and ischaemia. From this wide range of neuronal alterations Spielmeyer's ischaemic cell change, the term originally used to describe the alteration in nerve cells after general or local circulatory arrest, is now recognized as the degenerative response that is common to all types of hypoxia. Thus it is also seen in substrate deficiency (hypoglycaemia), anaemic hypoxia represented by carbon monoxide intoxication, histotoxic hypoxia as exemplified by cyanide poisoning and the complex situation represented by status epilepticus. However, it is important to stress that the patterns of distribution of ischaemic cell change in the brain may differ widely among the categories of hypoxia. The great diversity ofthe morphological alterations in neurones attributed to hypoxia is probably the outcome of postmortem autolytic changes unavoidable in human material, where there is a variable delay between death and necropsy coupled with slow penetration of the fixative, and to the use of immersion-fixation in experimental animals with the inevitable introduction of histological artefacts. Thus Hicks (1968) described diminished staining of Nissl bodies and other eosinophilic changes as the earliest neuronal alterations in anoxic necrosis in human material but considered them to be indistinguishable from postmortem changes. There are two common cytological artefacts encountered in human and experimental animal brains each of which has been interpreted as evidence of hypoxic damage and reported as such. The hyperchromatic neurone or 'dark cell' has been the subject of numerous studies, including those of Scharrer (1938), Wolf and Cowaen (1949), Koenig and Koenig (1952), Cammermeyer (1960, 1961, 1962) and Cohen and Pappas (1969). This artefact is most frequently observed after a fresh neurosurgical biopsy specimen is fixed in formaldehyde, when the brain of a normal animal removed immediately after death is immersed in a fixative, or when a perfusion-fixed brain is removed immediately from the skull. In paraffin and celloidin sections the dark cells appear heavily stained and unevenly shrunken (fig 1). It is generally agreed that dark cells result from postmortem trauma incidental to removal of the brain. The second type of artefact is known as 'hydropic cell change' or 'water change' (Jakob, 1927; Cammermeyer, 1960, 1961; Coimbra, 1964). In contrast to the hyperchromatic neurone the hydropic cell is swollen and the cytoplasm stains less intensively than normal (fig 2). It is sometimes seen in human material and particularly in infants. It occurs in immersion-fixed animal brain and is a particular hazard in those that are incompletely or inadequately perfusion-fixed. Both the above artefacts and also artefactual perineuronal and perivascular spaces can be excluded in the experimental animal by the employment of carefully controlled in-vivo intravascular perfusionfixation and delayed removal of the brain. We have employed this method of fixation for our light and electron microscopic studies of the evolution of ischaemic cell change in rodents and primates in the following experimental models (1-6). 1 The preparation developed by Levine (1960) in the rat which allowed ischaemic neuronal alterations to be produced unilaterally when ligature of a common carotid artery was followed by intermittent exposure to nitrogen was modified by Brown and 155 coright.