Following the external examination, the investigator can proceed to the dissection of the animal. In some cases the interest of the researcher is focussed on a specific area or organ; in this case the investigator would be tempted to limit his macroscopic examination to that particular region, without trying to have a general picture of all the organs of the animal. This type of limited survey is unadvisable. It is always better to carry out a complete necropsy of the mouse possibly with a fast and accurate method, which reveals the particular characteristics of all the organs and their evident lesions.
We will therefore describe a systematic method, designed to provide and accurate autopsy of a laboratory mouse.
Fig. 4 Opening of the mouse skin: external examination of subcutaneous tissues, muscles, lymph nodes and mammary glands (Click on the image for a larger version)
Once the animal is fixed on the appropriate table in the position described previously, the investigator will make a small cut at the level of the pubis, such as to allow the entrance of one point of the scissors. Then, the investigator will perform a median longitudinal cut superiorly to the chin, having care to accurately seperate the skin from the underlying musculature.
The skin is then dissected and turned on one side and then on the other, so as to obtain an examination field as wide as possible (Fig. 4). The characteristics of the subcutaneous, superficial lymph nodes, mammary glands and of the skeletal muscles will then be apparent.
The presence of a glossy, gelatinous and clear liquid filled subcutaneous space confirms the diagnosis of oedema, also noticeable during the external examination, while a more or less extended collection of blood, with enlarged margins, indicates the presence of haematoma. Small, more or less numerous, haemorrhagic petechia can moreover be observed more frequently in mice affected by an infectious disease. Variations of the normal colour can also be found for the presence of particular pigments, as it happens in icterus.
Anatomical outline. In normal conditions, the lymph nodes of the mouse are easily detectable. They are numerous, of variable size in different strains of animals, greyish, and shaped as a small pea or bean. According to the localization, they can be classified as superficial lymph nodes, situated in the subcutaneous area and near the skeletal muscular masses, and as deep lymph nodes, situated inside the thoracic and abdominal cavity or close to the organs. Figure 5 shows a general picture of the morphological characteristics of every lymph node and its localization in the normal mouse.
Fig. 5 Scheme reporting localization of the lymphatic system (from T. B. Dunn, 1954, courtesy of the Author) (Click on the image for a larger version)
All superficial lymph nodes are bilateral and can be classified as: cervical superficial lymph nodes, situated immediately above the submandibular salivary glands; axillary lymph nodes, present in the axillary fossa; brachial and retroscapular lymph nodes, in proximity to the angle of the scapula; inguinal lymph nodes situated closed to the bifurcation of the superficial epigastric vein.
The main deep lymph nodes are: the deep cervical lymph nodes, often difficult to localize, the more superficial ones are found in the cervical plane, hidden in the connective tissue that encircles the trachea; mediastinum or thoracic lymph nodes situated on the posterior face of the two lobes of the intimately connected thymus; the pyloric or pancreatic lymph nodes near the margin of the pancreas; the renal lymph nodes situated between the median margin of kidneys, more often at level of the hilum and in correspondence of the abdominal aorta; the mesenteric lymph node, of lengthened shape, that lies between the mesentery membranes, close to the ascending portion of the colon; the lumbar and caudal lymph nodes localized in proximity to the bifurcation of the aorta.
Examination. Of the several lymph nodes, we will describe the particular characteristics as the shape, the volume, the consistency and the eventual relationships between them and the underlying plans.
The lymph nodes near the centre of an inflammatory process frequently show increase of volume and sometimes haemorrhagic characteristics.
Fig. 6 Observation of the mesenteric lymph node
When, on the contrary, the whole lymphatic system of the animal is affected, this can be ascribed to a lymphoma. But also in these cases, the only character that can be observed macroscopically is enlargement which frequently is also of remarkable degree. Therefore, it will be more prudent to make a diagnosis of lymphoma, even if at this stage enough indicative, only when the necropsy is finished; alterations of the deep lymph nodes and of other organs, like the thymus, spleen and the liver, can be of help.
The investigator must also examine, with particular attention, the state of the mesenteric lymph node. This lymph node in the mouse is found intimately connected, by means of mesentery, to the ascending colon (Fig. 6), and is easily detectable when the small intestine is reflected on the left side. In some diseases, the mesenteric lymph node can show remarkable variations in colour, consistency, and volume until becoming various times larger than the normal. This happens both in the lymphomas that in the mouse it is believed to originate just from the mesenteric lymph node, and in the so-called mesenteric syndrome described by Dunn (1954), in which you observe a marked proliferation of endothelial tissue, which forms numerous vessels overfilled by blood.
In normal conditions, the mammary apparatus is formed by five pairs of glands (Fig. 7). Three pairs are situated in the thoracic and two in the abdominal region. This glandular tissue is formed by a system of lobules and excretory ducts. When it is completely developed, it is extended through nearly all the subcutaneous region, except in some areas of the back.
Fig. 7 Scheme reporting the localization of mammary glands (Murphy E.D., chapter 27 Characteristic Tumors, in E.L. Green Ed., "Biology of the Laboratory Mouse", reproduced by permission of McGraw-Hill, New York 1966).
The most frequent lesions of the mammary glands are the tumours. The benign or malignant tumours that originate from the duct epithelia are adenomas and adenocarcinoma. These tumours appear macroscopically as plates or nodules of various consistency, haemorrhagic, sometimes of a cystic aspect, of diameter variable from a few millimetres to some centimetres. Less frequent are tumours that have a connective tissue origin, like fibroma or fibrosarcoma, which show a wooden consistency, are markedly invasive, and often ulcerated at the skin surface.
In some strains of mice, in particular in many C3H lines, a spontaneous incidence as high as 100 percentage of these neoplasms is observed. In other strains, the increase of incidence of these neoplasms can be the consequence of chemical carcinogen, radiation or hormone treatment.
The various sections of the skeleton may be examined in a systematic way only when there are specific clinical-scientific interests. Otherwise, the inspection will be limited to those skeletal sections that appear as the autopsy proceeds. A complete examination of the skeleton will be possible only by means of one good X-ray (Fig. 8).
Fig. 8 Mouse X-ray picture
With this technique, it will be possible to detect, in a more detailed way, both the neoformations of the bone (tumours), in many cases macroscopically evident by their mass and for invasion of the adjacent soft parts, as well as thinning or atrophic lesions, particularly frequent in old animals. The state of the skeletal muscles must be then taken into consideration. A marked atrophy is a consequence of an infectious disease, often caused by viruses (dermatomyositis).
Once the observation of the superficial organs is over, the investigator will proceed with the examination of the inner cavities of the animal, starting from the abdomen, then the thorax, and, finally, the skull.