On the Fila Olfactoria and the Cribriform Region of the Crocodylia

In mammals the fila olfactoria, fascicles of axons coursing from sensory neurons in the olfactory epithelium to the glomeruli of the olfactory bulb, not only have a topographic projection pattern but also serve as routes for cerebrospinal fluid (CSF) drainage from around the brain. Les is known about the fila olfactoria in nonmammalian taxa. This work explores the fila olfactoria of the American alligator (Alligator mississippiensis) using a combination of gross dissection, histology, Diffusible Iodine-based contrast-enhanced computed tomography, latex corrosion casting, and India ink tracers. In Crocodylians, as in other nonmammalian vertebrates, the fila olfactoria courses through a foramen in the nasal capsule rather than an ethmoidal cribriform plate. In Alligator this foramen is filled by dense connective tissue; prominent perineural spaces extend through the connective tissue, effectively making it perforate like the cribriform plate. Latex or India ink introduced into the cranial CSF pass through the dense connective to reach the submucosa of the olfactory epithelium, suggesting that Crocodylians have the same cranial CSF drainage pattern as mammals. In Alligator, the fila olfactoria is asymmetric, with more fascicles entering the ventral and lateral surfaces of the olfactory bulb than the dorsal or medial surfaces. If individual fascicles of the fila olfactoria are traced in Alligator, a clear topographic projection emerges; with medial and lateral polarity maintained between olfactory epithelium and olfactory bulb, and a rostral-caudal polarity projecting as dorsal-ventral on the olfactory bulb.