Nonrodent Species Gross Anatomic and Histologic Differences – Pathology Considerations


Anatomic species differences are an important consideration at every step of preclinical toxicity studies including the pathology phase. Not taking species differences into account can result not only in time-consuming and costly procedural problems but could affect study results and interpretation. The impact of these issues can be magnified by the often small sample (group) sizes in nonrodent studies.

This article focuses on the most common nonrodent laboratory species: domestic rabbit,  dog (laboratory Beagle), monkey (cynomolgus and rhesus macaques), and domestic pig (including minipigs) but many points are also applicable to other nonrodent species such as cats, ferrets, marmosets and other monkey species, ruminants (sheep, goat, cow), and aquatic animals (fish and amphibians).

Though beyond the scope of this article, it should not be forgotten that nonmurine rodents such as guinea pigs and hamsters also have anatomic and histologic differences compared to murine rodents like the rat [Rattus norvegicus] and/or mouse [Mus musculus].

In nonrodent studies, it is especially important to avoid the “fallacy of rat/mouse extrapolation” – that is, the blanket assumption that nonrodent anatomy  will be exactly as in the more familiar rats/mice, and that all rat/mouse necropsy/histology methods can be used without modification in nonrodent species.

Protocol lists of tissues to be collected at necropsy are best vetted in advance to ensure that they are accurate for the species. EPL has received study protocols listing “seminal vesicles” and “Harderian glands” in dogs; cecal “appendix” in macaques; and “nonglandular stomach” in rabbits as required tissues – though none of these are anatomically present in the respective species.

On the other hand, depending on the study design and goals, it may be scientifically appropriate to augment “standard” protocol lists with tissues that occur only in certain species, such as the sacculus rotundus and cecal appendix of rabbits, the forestomach compartments (rumen, reticulum, omasum) of ruminants, and so on. Obviously, such additions cannot even be considered without knowledge of the anatomic “options” available in a given species.


Not taking species differences into account can also result in errors at necropsy, many of which can never be corrected. For example, the lacrimal glands of macaques and dogs are small, inconspicuous, and are often missed at necropsy (perhaps because of a fallacious expectation that lacrimal glands in these species are just like the relatively large lacrimal glands of rats and mice). To compound this issue, dogs and rabbits, (but not macaques) possess extraorbitally located zygomatic salivary glands which are frequently mistaken for and collected as “lacrimal gland” ” (perhaps in some cases because of an erroneous assumption that rabbit and dog  zygomatic glands are exact analogues of the extraorbital lacrimal glands of rats/mice).  As another example, male rabbits have large, prominent vas deferens ampullae which have been collected at necropsy as “seminal vesicles”. In many of these real-life situations, the uncollected correct tissues have been lost forever because they were unwittingly discarded after necropsy along with the rest of the carcass. As a result, tissue accountability was essentially 0%, a deficit which might be regarded unfavorably during regulatory audits.  Even worse, such studies were de facto inadequate for determination of any possible treatment-related effects in the missing tissues.

Some nonrodent-species tissues are functionally analogous to those in rats and mice but have different gross anatomy. For example, seminal vesicles in rats, mice, and most nonrodent species are paired organs, but in rabbits, the seminal vesicle is a conjoined singular structure. The rabbit Harderian gland is bilobed, but only the “white” lobe grossly resembles the rat/mouse Harderian gland while the “red” lobe has been mistaken for a “lacrimal gland”. As in humans, the uterus of macaques is a singular pyriform structure rather than bicornuate or duplex as in nonprimate laboratory species.

Other analogous tissues in nonrodent species are found in different, sometimes distant locations than in rats and mice. These tissues can and have been missed at necropsy because they are not sought in the appropriate sites in the nonrodent species. The most notorious examples are probably in the pig, whose thyroids are located at the thoracic inlet rather than near the larynx and whose parathyroids are separate from the thyroids in the cranial thymus near the larynx. Other examples include the rabbit sublingual salivary glands, which are small, inconspicuous and located distant from the other major salivary glands.

To ensure that tissue collection is accurate and complete, EPL encourages study directors and sponsors to solicit pathologist input during the protocol review phase of studies that utilize nonrodent species.





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