The Anatomy of the Laboratory Mouse
Margaret J. Cook

INTRODUCTION

The anatomy of the laboratory rat has been fairly extensively studied and the anatomical atlas by Eunice Chace Green was published as long ago as 1935. However, very little has been written on the anatomy of the most commonly used laboratory mammal, the mouse, and it was suggested by Dr. Lane-Petter, Director of the Laboratory Animals Centre, Carshalton, that this omission should be rectified.

In 1961 a study was commenced, and it was decided to use the random-bred strain maintained at Carshalton, the LAC Grey mouse. This strain was founded by Dr. Lane-Petter using a litter of pet mice obtained from a schoolboy. The LAC Grey is a large mouse which has been selected since its origin for high reproductive performance and extreme docility.

Throughout the investigation sexually mature mice were used; for all dissection purpose large animals were selected; i.e. mice of 3 months or over.

Drawings were made direct from the dissection rather than from photographs. As a general rule all structures so drawn were visible to the naked eye, but in the case of the skeleton it was necessary to use a low-power, wide-field binocular microscope.

The skeletons were dyed with alizarin red and macerated with potassium hydroxide according to the method of Crary ( 1962) of The Roscoe B. Jackson Memorial Laboratory, Bar Harbor.

Wherever possible in this study fresh preparations were used, mice being obtained direct from the animal house, killed with ether and dissected immediately. However, it was found impossible to use fresh material for the study of the vascular system as the blood vessels were too fragile for successful dissection. For this work the vascular system was injected, the animal preserved in formalin and acetic acid for about 10 days and stored in propylene phenoxetal until required.

In preliminary injection experiments colored gelatin was used, but latex (Revultex latex, obtained from Revertex Ltd.) was found to be more satisfactory for subsequent dissection and was thereafter used routinely. This latex is white and was colored with red, blue and yellow dyes (obtained from Imperial Chemical Industries Ltd.) just prior to injection. Throughout the work the arterial system was injected via the left ventricle using red latex, the venous system via the right ventricle using blue latex, and the hepatic portal system via the hepatic portal vein using yellow latex. The dyes were mixed thoroughly with the latex, and this was then filtered through glasswool to prevent any lumps from blocking the tubes of the injection apparatus. It was found to be essential to re-filter the latex before a series of injections was commenced and also to renew the plastic tubing on the apparatus at regular intervals.

The injection method used was suggested by Dr. H.F. Steedman and Mr. P. Anderson of the Zoology Department, University of Glasgow (personal communication), and the injection apparatus was designed by Mr. R.C. Emery and built under his direction in the M.R.C. Instrument Workshop, Carshalton (Figures 1 and 2).

Before latex injection was commenced, citrate saline solution was injected into the vascular circuit to prevent clotting of the blood in the vessels. This can be injected using a syringe and hypodermic needle, but it was found to be more satisfactory to include a bottle of saline in the injection apparatus, the saline being introduced via a glass cannula into the right ventricle. This cannula was then rapidly withdrawn and replaced by the blue latex cannula (Figure 3), care being taken to ensure that there were no air bubbles in the cannula or tubing. Blue latex was then injected into the venous circuit using 2 - 5 lb/in² (0.14 - 0.35 kg/cm²) pressure (the pressure required varied very much from mouse to mouse, but it was always kept as low as possible to prevent damage to the blood vessels.) When injection was complete the cannula was removed and red latex was injected as before into the left ventricle. In order to prevent excessive outflow of latex when the cannula was removed, a cotton-wool swab moistened with acetic acid was applied to the site of injection; this proved sufficient to solidify the latex. The yellow latex was then injected into the hepatic portal vein, the pressure being kept very low (under 5 lb/in² (0.35 kg/cm²)) to prevent rupture of the vessels in the liver. Excess latex was then washed off, the mice were immersed in a 10% formalin, 5% acetic acid mixture for about 10 days. After this time the animals were removed, rinsed thoroughly in tap water and stored in a solution of 1% propylene phenoxetol. It has been found in practice that propylene phenoxetol is preferable to formalin as it leaves the tissues in a more flexible condition, thus giving greater ease in dissection; moreover propylene phenoxetol does not have the unpleasant smell associated with formalin.

On some occasions the latex would not penetrate to the extremities of the animal however high the air pressure, and no satisfactory explanation can be given for this. It was found necessary under these conditions to discard the animal and begin again with another one.

As already mentioned, only mice of the random-bred LAC Grey strain were used in this work and it may be that this strain differs considerably in its anatomy from other strains, especially some inbred ones. Professor H. Grüneberg and his coworkers at University College, London, have carried out extensive studies on the skull, the axial, and the appendicular skeleton, and it has been well established by them that there are genetically controlled variations in the morphology of bones, certain variations occurring characteristically in some inbred strains. Less study has been made of the soft tissues of the mouse, but Fround ( 1959), for example, has studied the arterial system of three inbred strains and has found constant differences in arrangement which may well be genetically controlled.

Certain variations in the arrangement of some blood vessels, notably veins, were foreseen since the animals were random-bred, and in fact this proved to be the case. One particularly striking example of this was the junction of the internal spermatic veins and the inferior vena cava.

This book covers work on the skeleton, viscera and circulatory system. The section on the skeletal system includes drawings of the skull, the vertebral column, thorax and the fore and hind limbs. Drawings of the viscera include both the thoracic and abdominal regions, and also some drawings of the endocrine organs. Reproductive organs are also drawn in some detail. Blood vessels supplying the brain, fore and hind limbs, the neck, thorax and abdomen are illustrated. As the injection medium did not as a general rule penetrate to the feet of the animals, few drawings have been made of their blood supply. Some blood vessels were found in the mouse which did not appear to be recorded for other species; where no names could be found for them, they have been left unnamed. The book does not include work on the nervous or muscular systems.

ACKNOWLEDGEMENTS

Grateful thanks are due to many members of the staff at the M.R.C. Laboratories, Carshalton. Special mention must be made of Miss Josephine Lumbers who has assisted throughout the great part of this work, Mr. R.F. Legg who has prepared the photographs and Dr. Marjorie Dinsley for her help and advice.

REFERENCES

Crary, D.D. (1962). Stain Technol. 37, 124-125.

Froud, M.D. (1959). J. Morph. 104, 441-478.

Green, E.C. (1935). Anatomy of the Rat. Haffner, New York.