Human semicircular canal form: Ontogenetic changes and variation of shape and size

Abstract

The semicircular canals (SCCs) transduce angular acceleration of the head into neuronal signals, and their morphology has been used to infer function. Once formed, the bony labyrinth, that surrounds the canals, is tightly regulated and has a very low bone turnover. However, relaxed postnatal inhibition of bone remodelling later in ontogeny may allow for some organised adjustments of shape and size or for greater stochastic variation. In the present study, we test the hypotheses that after birth, the shape and size of the bony canal changes or becomes more variable, or both. We study microCT scans of human perinatal and adult temporal bones using a combination of geometric morphometric analysis and cross‐sectional measures. Results revealed marginal differences of size (<5%), of cross‐sectional shape and of measurement variability. Geometry of the three canals together and their cross‐sectional areas were, however, indistinguishable between perinates and adults. These mixed findings are indicative of diminutive levels of relaxed inhibition superimposed over a constrained template of SCC morphology.

The bony labyrinth of the Miocene boselaphin bovid Miotragocerus pannoniae: insights into ontogeny

“…This pattern seems to hold across ontogeny, since Bos taurus fetuses before and around mid-gestation show the same feature as in adults albeit with slight variation in the course or curvature of the aqueduct (Costeur et al, 2017). Bone remodeling may still influence the shape of these bony structures before mid-gestation (Cardenas and Jefferey, 2022) and this may explain morphological differences. There is only a slight difference between the course and shape of the fetal endolymphatic duct and that of the adults studied.…”

“…Previous studies on artiodactyls, ruminants or other mammals (Maier, 2013; Billet et al, 2015; Mennecart and Costeur, 2016a, 2016b) indicate that the area of the petrosal bone surrounding the bony labyrinth is fully ossified around parturition. Studies on humans and diverse artiodactyls (Jeffery and Spoor, 2004, Cardenas and Jefferey, 2022, Costeur et al, 2017, Maier, 2013, Mennecart and Costeur, 2016a) show that the bony labyrinth acquires its full size at around mid-gestation time. Even though the gestation time of Miotragocerus pannoniae is not known, it is likely that the bony labyrinth reached its full size around mid-gestation time in this species as well.…”

“…The bony labyrinth of mammals is a classic example for early development in taxa in which the bony encasement features accommodating hearing and balance fully ossify by mid-gestation stages. This includes humans (Jeffery and Spoor, 2004; Cardenas-Serna and Jeffery, 2022), mice (e.g., Nojiri et al, 2021b), bats (Nojiri et al, 2021a, 2021b), ruminants (Costeur et al, 2017), and cetaceans (Thean et al, 2017). Full ossification before birth does not necessarily imply functionality (see Vater et al, 2000 for bats) but enables paleontologists to use the bony labyrinth for phylogeny and evolutionary studies whatever the ontogenetic stage once ossification is achieved.…”