Orbital growth in perinatal primates: a CT and histological study of soft tissue‐bone interfaces

Abstract

Extant primates are notable for their relatively convergent, or forward‐facing orbits, although some other mammals exhibit similarities, such as a postorbital bar supporting the eye laterally. Here, we use a novel approach of co‐registering micro‐computed tomography (CT) scans of late fetal nonhuman primates with serial histological sections, to infer how orbital morphology emerges during ontogeny. We use three primates (loris, tarsier, tamarin) that have absolutely or allometrically large eyes, and draw a contrast to the tree shrew (Tupaia glis), which may more closely resemble ancestral primate cranial form. Serial slices were annotated to create three‐dimensional cranial reconstructions that reveal the distribution of osteoclasts and osteoblasts. Reconstructions revealed that the superomedial orbit is resorptive in all primate species, although the amount of medial resorption is greatest in the tarsier and tamarin. In contrast, the orbit of Tupaia is depositional except for small inferomedial foci adjacent to infraorbital nerves. Broadly, orbital growth remodeling patterns in primates may be explained by the degree of orbital convergence, which far exceeds that in Tupaia. However, the three primates vary in the degree of orbital convergence and frontation, suggesting that a different factor (e.g., proportional eye size) may influence the similarities in orbital growth patterns.