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Lateral Growth in the Basicranium in Bushbabies: Role of Synchondroses and Brain Growth

Recently, we described bilateral synchondroses (alisphenoid‐basisphenoid synchondrosis, ABS) within the middle cranial fossa of strepsirrhine primates (lemurs, bushbabies, lorises). These exist as growth centers in late fetal or newborn strepsirrhines, whereas in similarly‐aged monkeys, they are not present. It is unclear whether these joints are critical for active bilateral growth, especially with our current lack of understanding of perinatal brain growth. In the present study, two genera of bushbabies (Otolemur spp., Galago spp.) were studied using paraffin histology and computed tomographic (CT) imaging software (Amira). Endocasts of the cranial cavity were produced by manually tracing the endocranial margin of the cranial cavity of a late fetal and a one‐month‐old specimen. Both genera of bushbabies are confirmed to possess the bilateral ABS. The synchondroses were “bipolar,” with proliferating and hypertrophic chondrocytes at both medial and lateral sides. CT reconstructions of late fetal, newborn, and one‐month‐old Otolemur illustrate that the ABS narrows dramatically in mediolateral width, suggesting a decrease in growth rate. Endocasts suggest that contours of the late fetal brain are rounded. By comparison, the endocast of one‐month‐old suggest greater differentiation of cerebral lobes and a more rostrally projecting olfactory bulb. We hypothesize that bilateral synchondroses of middle cranial base may be important to expansion of basicranial width during fetal growth, during which there is a relatively generalized expansion among all brain regions; postnatally, differential growth of brain regions renders bone modeling a more effective means of lateral basicranial expansion. However, further investigation is required to determine if synchondroses may grow differentially among basicranial regions.