Common Spontaneous and Background Lesions in Laboratory Animals

Chapter 4
Common Spontaneous and Background Lesions in Laboratory Animals

Elizabeth McInnes

Cerberus Sciences, Thebarton, SA, Australia

Tyro study personnel will be rapidly introduced to the presence of background lesions in laboratory animals used for safety studies and will be confronted by the fact that sometimes these lesions are ignored, sometimes they are exacerbated at the end of a study (and thus become test article-related findings) and sometimes they are unique to a particular animal or procedure. This chapter will attempt to define and discuss the causes of background lesions, before looking at a few changes that are unique to mice, rats, beagle dogs, non-human primates, rabbits and minipigs. Common causes of death or euthanasia of older rodents on carcinogenicity studies will be discussed, as will the significance of test article-related background findings. Clear histopathological and macroscopic photographs illustrating some of these changes will be provided, although histopathology is not meant to be a skill for study personnel. The changes described here are those seen most commonly; further detailed information is provided in Johnson et al. (2013).

Background (incidental or spontaneous) lesions are pathology findings that are usually thought of as a change in tissue morphology outside of the range of normal variation for a particular species or strain (Long and Hardisty, 2012). Background changes can be congenital or hereditary; normal variations of findings that are unique to the histology of an animal species; related to trauma or normal ageing; or physiologic or hormonal changes (McInnes, 2012a; McInnes and Scudamore, 2014).

Congenital lesions are present at birth and probably represent abnormalities in normal embryogenesis and organ migration of the unborn animal. Squamous cysts – often containing keratin, since they are lined with the same epithelium found on the skin surface – are common congenital background lesions in the stomach and central nervous system (Figure 4.1). Minor developmental anomalies can also include ectopic tissue (tissue found in unexpected places; Figure 4.2).

TEM image of Squamous cyst in a mouse spinal cord.

Figure 4.1 Squamous cyst (*) in a mouse spinal cord.

TEM image of Bluish thymus within the thyroid follicles of a mouse.

Figure 4.2 Bluish thymus within the thyroid follicles of a mouse.

Background lesions also include normal ageing changes and degenerative conditions. Ageing background changes include chronic nephropathy (chronic kidney disease in rats and mice), cardiomyopathy (chronic heart disease in rats and mice) and polyarteritis (chronic inflammation of blood vessels in mice) (McInnes and Scudamore, 2014). All of these findings can be exacerbated by certain treatments, so a knowledge of what is normal for the strain and age of rodent being evaluated is crucial to interpretation of these changes. Study personnel should also be aware that age lesions can include younger animals and that the thymus is always bigger in younger animals as it has not yet undergone regression.

Background lesions influenced by ageing also include spontaneous tumours (not induced by external treatments or compounds) in older laboratory animals. It is helpful for study personnel to know about the incidence of these naturally occurring tumours in control animals, in order to assess whether an external treatment has caused an increased incidence of a particular tumour type (Hardisty, 1985). There are extensive references on the expected background incidence of spontaneous tumours and their descriptions in the various strains of rodents; many of these are summarised on the goRENI website (

Some background lesions are caused by infectious agents. For instance, lesions consisting of alveolar macrophages and lung wall thickening are reported in the lungs of immunocompetent young Han Wistar rats and have recently been shown to be associated with the presence of Pneumocystis carinii DNA (Livingston et al., 2011; Henderson et al., 2012). These lesions may mask or confuse research findings, particularly in inhalation studies.

Background findings may be traumatic in origin and can include fractures, bite wounds and foot lesions. Gavage injury is not a background change as such, but is a commonly encountered incidental, non-drug-related finding in rats and mice. It can consist of damage to the oesophagus and surrounding pleural tissues due to the penetration of the gavage tube through the wall of the oesophagus (Bertram et al., 1996).

Artefacts are a subgroup of background changes. They are important because they may occur as a result of faulty collection and processing of tissue specimens (McInnes, 2012b). Knowledgeable study personnel can recognise these artefacts in reports (such as air bubbles trapped below the cover slip on a glass slide) and take steps to eliminate them, as artefacts are produced at different stages of the harvesting and processing of tissue samples.

Background lesions are also associated with physiological variability, including reproductive senescence and sexual maturity, since significant variability in the appearance of the reproductive organs occurs as a result of normal reproductive cyclicity. Examples include changes associated with oestrus, sexual maturation and reproductive ageing in both sexes.

Normal physiology in rodents may also result in background changes such as haematopoiesis (the presence of small clusters of immature red blood cells) in the spleen and liver (Figure 4.3), failure of growth plate closure in the long bones, continuous eruption of incisor teeth or thymic involution. Stress is a normal physiological response, and stress during animal studies may be caused by noise, changes in temperature, handling, dosing, restraint, sample collections, transport or group housing of animals (Everds et al., 2013). It can lead to reduced total body weight or body weight gain, reduced food consumption, changes in organ weights (e.g. decreased thymus and spleen weight, increased adrenal weight), lymphocyte depletion in the thymus and spleen, increased or decreased white blood cell levels in the blood, gastric ulceration and reproductive organ atrophy; Everds et al., 2013).

TEM image of Haematopoiesis in the liver.

Figure 4.3 Haematopoiesis (*) in the liver.

4.1 Rats

Rats display similar background lesions to mice, including mineralisation of the kidney papilla (the presence of small foci of calcium in the medullary papilla) and inflammatory cell foci in the liver. Rats have a specialised sebaceous gland situated at the base of the external ear canal called the Zymbal’s gland, as well as Harderian glands behind the eye, for which there is no human equivalent. The rodent stomach subdivides into the glandular and nonglandular stomach; lesions in the nonglandular part may not have relevance to humans, who only possess a glandular stomach. In addition, the preputial or clitoral glands situated between the penis and the rectum in the mouse and rat have no human equivalent and may not be relevant to human patients.

Chronic progressive nephropathy (CPN) is a common spontaneous kidney disease of ageing rats, including Fischer 344 rats (Dixon et al., 1995) and mice. It is more common in male rats, and is exacerbated by a high-protein diet (Montgomery and Seely, 1990). CPN is also directly influenced by total food consumption over a lifetime (Keenan et al., 2000). Administration of certain drugs may exacerbate or hasten the lesions of CPN, particularly in long-term studies. CPN is observed commonly in ageing mice, and is characterised by thickening of the glomerular basement membranes and periglomerular fibrosis (Percy and Barthold, 2007) (Figure 4.4). The age of onset is later in mice than in rats, and the incidence is not as high (Frazier, 2013).

TEM image of Thickening of the glomerular basement membranes and fibrosis in an ageing mouse, indicative of CPN.

Figure 4.4 Thickening of the glomerular basement membranes and periglomerular fibrosis (*) in an ageing mouse, indicative of CPN.

The presence of alveolar macrophages in the lung alveoli is a background change in rats and mice, and thus interpretation of a drug-induced alveolar macrophage increase may be difficult. Spontaneous alveolar macrophage aggregates are generally observed randomly dispersed throughout the lung tissue, whereas induced macrophage responses tend to be located at the bronchoalveolar junction (Lewis and McKevitt, 2013).

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May 8, 2017 | Posted by in GENERAL | Comments Off on Common Spontaneous and Background Lesions in Laboratory Animals
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