Timing of mammalian peripheral trigeminal system development relative to body size: A comparison of metatherians with rodents and monotremes

Abstract

Specializations of the trigeminal sensory system are present in all three infraclasses of mammals (metatheria, eutheria, prototheria or monotremata). The trigeminal sensory system has been suggested as a critically important modality for sampling the path to the pouch and detecting the nipple or milk patch, but the degree to which that system may be required to function at birth varies significantly. Archived sections of the snout and brainstem of embryonic and postnatal mammals were used to test the relationship between structural maturity of the two ends of the trigeminal nerve pathway and the body size of mammalian young in metatherians, rodents and monotremes. A system for staging different levels of structural maturity of the vibrissae and trigeminal sensory was applied to embryos, pouch young and hatchlings and correlated with body length. Dasyurids are born at the most immature state with respect to vibrissal and trigeminal sensory nucleus development of any available metatherian, but these components of the trigeminal system are also developmentally advanced relative to body size when dasyurids are compared to other metatherians. Vibrissal and trigeminal sensory nucleus development is at a similar stage of development at birth and for a given body size in non-dasyurid metatherians; and trigeminal sensory nucleus development in monotremes is at a similar stage at birth to metatherians. Rodents reach a far more advanced stage of vibrissal and trigeminal sensory nucleus development at birth than do metatherians, and in the case of the mouse have a more developmentally advanced trigeminal system than all available metatherians at any given body length. Precocious development of the trigeminal sensory pathway relative to body size is evident in dasyurids, as might be expected given the small birth size of those metatherians. Nevertheless, the trigeminal sensory system in metatherians in general is not precocious relative to body size when these species are considered alongside the pace of trigeminal somatosensory development in rodents.

• Dasyurids
• didelphids
• diprotodontids
• peramelids
• trigeminal nuclei
• vibrissae

Acknowledgements

I would like to thank Dr Peter Giere of the Museum für Naturkunde, Berlin for access to the MfN collections and for all his kind help during the work. I would also like to thank Dr Lauren Marotte, formerly of the Australian National University, for giving her collection of tammar wallaby material to me. I am very grateful to Professor U Zeller for allowing access to the 7 platypus young in his collection.

Declaration of interest

The author reports no conflicts of interest. The authors alone are responsible for the content and writing of this article.

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