Olfactory and Solitary Chemosensory Cells: Two Different Chemosensory Systems in the Nasal Cavity of the American Alligator, Alligator mississippiensis

As discussed in lecture, olfactory receptors and the vomeronasal organ detect a variety of odors that are related to food/hunting and reproductive pheromones. In addition, these odors can serve as social stimuli. Due to extremely sensitive olfactory receptors that are odor-specific, the olfactory receptor gene family contains more copies than any other gene family in mammals. Separate from these olfactory receptors is the vomeronasal organ, which is specifically stimulated by nonvolatile chemicals like proteins in the urine and secretions of conspecifics.

Adult American alligators have a seemingly unique olfactory reception system that lacks a vomeronasal organ. To enhance their ability to detect unique odors while hunting, alligators displace the floor of their pharynx; this action is referred to as “gular pumping”. Chemoreception in this semi-aquatic species is particularly interesting because the mechanism by which they detect both airborne and water-soluble compounds in terrestrial and aquatic environments is unknown. Many of the olfactory organs in semi-aquatic animals are compartmentalized to house different cells for both environments. However, alligators only have one olfactory chamber. Researcher Anne Hansen investigated how alligators manage to detect airborne and water-soluble chemicals without a compartmentalized olfactory organ. She hypothesized that an alligator’s olfactory receptor contains different types of olfactory sensory neurons (OSNs) that are modified to function either above or below water.

First, Hansen confirmed previous research that had found the entire alligator nasal to be lined with olfactory epithelium. In comparison to the olfactory epithelium of other groups of animals, the number of gator OSNs is relatively scarce and randomly distributed. Hansen observed an increased density in OSNs around the turbinate bones and a decrease in OSN density towards the front of the nasal cavity.

Hansen discovered that two types of OSNs exist within alligator olfactory epithelium. OSN types I and II differed in both electron density and cell organelle type. She believed that this varied morphology is responsible for the alligator’s ability to detect both airborne and underwater stimuli without a compartmentalized olfactory chamber.

In addition, she identified solitary chemosensory cells (SCCs) mixed in among the OSNs. SCCs represent a chemosensory system that is separate from olfaction altogether. Commonly found in the skin and gills of fish and amphibians, the only other species (besides alligators) known to have SCCs within their olfactory epithelium are goldfish. Hansen speculated that the presence of SCCs in alligators, in addition to a presence in rodents, large mammals and amphibians, suggests that it is a conserved trait that evolved early in vertebrate history.

Hansen, A. (2007) Olfactory and solitary chemosensory cells: two different chemosensory systems in the nasal cavity of the American alligator, Alligator mississippiensis. BMC Neuroscience, 8, 1-10