Anatomical study of a temporal bone from a non-human primate (Callithrix sp)

Summary The search for experimental (animal) models is essential to the development of clinical studies. Aim To demonstrate, by means of micro dissection techniques, the anatomical structures of temporal bones from the primate Callithrix sp. Study design Experimental. Methods Dissection of temporal bone structures of Callithrix sp and photographic documentation. Results We identified the main constituents of the temporal bone (external, medium and inner ear and facial nerve). Conclusion The non-human primate Callithrix sp. is an adequate experimental model for the studies of temporal bone structures given its close anatomical similarities to that found in humans.


INTRODUCTION
The investigation on new animal models in biological sciences represents a fundamental step in the search for scientific progresses that can be applied in day-to-day medicine. The study of physiology, physiopathology and the effects of new therapeutic approaches in experimental animals has become a mandatory precedent before carrying out clinical trials in humans. Otology also requires such progress, and many experimental models are reported in the literature. Small size mammals such as rats, mice and guinea pigs [1][2][3][4][5] are commonly used in studies of temporal bones, and their anatomy and physiology are broadly described. Other mammals such as racoons 6 and pigs 7 are also well defined experimental models. However, the phylogenetic distance of such animals to human beings does not allow direct inference of the results obtained 8 . Moreover, factors such as body balance based on the quadruped position, a not very differentiated cochlear structure, a difficulty in assessing specific facial movements, amongst others, makes it even more difficult to study temporal bones in these animals. Anatomical-surgical studies in primates are being carried out in the literature to overcome these difficults [8][9][10] . The sagui used in the present investigation (Callithrix sp) is a native primate in Brazil, of small size, which is not under risk of extinction, and can reproduce in a cage and represents a low cost alternative to maintain studies in the many fields of knowledge. The saguis, like human beings, are primates, thus being animals very near each other in the phylogenetic scale.
Our proposal is to present a preliminary study on the temporal bone anatomy of the Callithrix sp with the goal of making feasible future efforts to define a new experimental model in otology.

METHOD
We used four skulls (8 temporal bones) of species which were previously slaughtered as part of other studies on the central nervous system, previously approved by the Ethics in Research Committee of our Institution (Protocol # 1113/01). These animals were reared in our own institution. With the help of a surgical microscope (DF-Vasconcelos® M90) coupled to a digital camera (FUJI® F420), we then obtained corresponding images to the different stages of the dissection of such skulls, trying to prove the feasibility of carrying out the necessary surgical access in order to apply experimental paradigms to the temporal bone. Figure 1 depicts the macroscopic aspects of the skull and face of animals in dissections with and without the skin, connective tissue and muscles. Figure 2 represents an upper view of the skull base, showing the cranial nerves and the internal acoustic meatus. Figure 3 shows the middle ear dissection with visualization of the mastoid, anterior and posterior antrostomies, tympanic membrane and ossicles. Figure 4 shows the facial nerve in its intrameatal trajectory, labyrinthine, tympanic, mastoid and extratemporal. Figure 5 corresponds to the inner ear dissection, with the opening of the vestibules, the lateral canals, oval and round windows, promontory, cochlear turns, Eustachian Tube entry and carotid.

DISCUSSION
Several experimental paradigms about the temporal bone have been proposed in different species of animals in the study of the ear (external, middle and internal) and the facial nerve (intra and extra-temporal). The Callithrix sp is promising in the application of these study models since surgically we have been able to locate the same anatomical structures mentioned in these papers [1][2][3][4][5][6][7][8][9][10] . However, some details must be stressed.
Apparently, the facial nerve in these species has its extratemporal portion located under the parotid and not within it, as it happens to humans. This has been already described in rats 1 , and as we see it, facilitates its experimental handling. We did not see a stapedial artery that crosses over the round window niche as described in the rat5, absent in humans, and this would also represent an easiness to work in this region. Through posterior antrostomy we have access to the vestibule windows, where we could, for example, carry out gene transfer to the inner ear, as proposed in rats 5 . We see a cochlea made up of 2.5 turns, similar to that in humans (2.5-2.75 turns), and different from that of mice (1.5 turn) and guinea pigs (4.5 turns)6, and this means another advantage.
We have also noticed some difficulties. We were unable to open the internal acoustic meatus through the middle fossa without damaging the inner and middle ear structures, as it happens in humans. Apparently, the