John Abramyan PhD

The assembly of the upper jaw is a pivotal moment in the embryonic development of amniotes. The upper jaw forms from the fusion of the maxillary, medial nasal, and lateral nasal prominences, resulting in an intact upper lip/beak and nasal cavities; together called the primary palate. Due to the risk of craniofacial clefting, this process requires a balance of proper facial prominence shape and positioning, whilst still accommodating the vast phenotypic diversity of adult amniotes. As such, variation in craniofacial ontogeny is not tolerated beyond certain bounds. We identified two categories of primary palatogenesis in amniotes, depending on whether the nasal and oral cavities remain connected throughout ontogeny or not.

In mammals and crocodilians, the tissue demarcating the connection between the oral and nasal cavities (the internal nares or choanae) thins and becomes the bucconasal membrane, which eventually ruptures and allows for the essential connection of the two cavities to form. However, the majority of reptiles (birds, turtles and squamates) maintain a patent connection between the oral and nasal cavities via open choanae during primary palate formation.

Subsequent to primary palate development, select lineages within both categories develop a secondary palate that either completely or partially separates oral and nasal cavities in adults. The only group which has lost the ability to form a secondary palate are the turtles, which we found to have lost expression of the BMP2 gene in the embryonic mesenchyme around the maxillary prominences, thus losing the palatal shelves which are the precursors to secondary palate formation.