Clinically Silent Calvarial Defects in a Dog: Developmental Failure of Endochondral and Intramembranous Ossification
*Eli B. Cohen Department Of Bio Sciences And Diagnostic Imaging, College Of Veterinary Medicine, University Of Georgia, Athens, Greece
*Corresponding Author: Eli B. Cohen
Department Of Bio Sciences And Diagnostic Imaging, College Of Veterinary Medicine, University Of Georgia, Athens, Greece Email:email@example.com
Published on: 2017-01-20
A 3-year-old Shih Tzu was examined for bilateral nasal discharge and sneezing. The dog had no apparent neurological deficits. Computed tomography (CT) of the nasal cavity and remaining head was performed. Imaging findings were consistent with a non-destructive rhinitis. Multiple well-defined, round to ovoid defects in the calvarial bones were an ancillary finding of the CT imaging. A thin layer of soft tissue attenuation was present spanning these defects. Severe hydrocephalus was also identified, with no evidence of parenchymal herniation through the calvarial defects. Findings are consistent with a congenital failure of both endochondral and intramembranous ossification involving cells from both neural crest and mesoderm origin.
The craniofacial bones of higher vertebrates form by both intramembranous and endochondral ossification. With intramembranous ossification osteoblastic differentiation from mesenchymal cells is direct. Endochondral ossification begins with formation of a cartilage template, with subsequent ossification. The skull is also separated into the neurocranium and the viscerocranium. The neurocranium envelops the brain and special sense organs, while the viscerocranium scaffolds the face and jaws. The neurocranium is further divided into those bones formed by intramembranous ossification, and those formed by endochondral ossification. The membranous bones of the neurocranium compose the calvarial vault (frontal, parietal, temporal, ethmoid), and the bones formed by endochondral ossification compose the base of the skull (occipital, sphenoid, pterygoid). The viscerocranium is entirely formed by membranous bones (nasal, maxillary, vomer, palatine, mandibular). Craniofacial bones are also classified by their embryonic cell of origin. The bones formed from cephalic mesoderm via somitomeres include the supraoccipital, temporal, and a portion of the sphenoid. The exoccipital bone is contributed by skeletal somite cells. The remainder of the neurocranium and viscerocranium is formed from neural crest cells of ectodermal origin.