Wednesday, August 22, 2012

Twists and turns on smell's evolutionary road

Smell is a complicated sense and its evolutionary path is a convoluted one. Olfactory receptor cells developed different shapes and different chemical receptors and were sometimes divided into separate organs and sometimes not.

Rainbow Goldfish: experimental animal (source)

A research group from the Rocky Mountain Taste and Smell Center (not affiliated with Coors) decided to research the olfactory cells of the noble goldfish. Goldfish are an interesting vertebrate because they, like humans, do not have the pheromone-sensing vomeronasal organ (though rats, a much closer evolutionary relative, do have it).

This group published a paper analyzing the morphology and chemical signature of the different types of smell cells in the goldfish olfactory epithelium (basically the back of the nose). Since Form and Function is one of my favorite topics, this paper sparked my interest. 

Hansen et al. (2004) show that there are three main shapes for the goldfish smell cells.

Hansen et al., 2004 Figure 3 (3 types of cells in the goldfish olfactory epithelium)
There are the Ciliated, the microvillous, and the crypt cells. 

"Ciliated ORNs are tall cells, with their nuclei usually located in the lower half of the OE. The cells possess a narrow dendrite and long apical processes radiating from an olfactory knob at the distal end (Fig. 3). Crypt receptor cells are obvious because of their typical ovoid shape and location in the upper half of the OE (Fig. 3). These ORNs possess microvilli that border the apical rim of the cell. At the same time, they possess cilia that are located in a “crypt”-like invagination." Hansen et al., 2004
Hansen et al. wanted to see whether these morphological characteristics correlated with the chemical signature of the cell. More specifically, they wanted to see which type of receptors these cells had and which g protein they expressed. 

They found that there was a direct correlation between the shape of the neuron and the type of smells it was sensitive to (as indicated by the receptors and g proteins it expresses). 

The most interesting finding was that the microvillous and crypt cells in the goldfish have very similar characteristics to the cells in the rat vomeronasal organ, and probably also serve the function of sensing pheromones. The paper inspires questions about why rats might have evolved a separate organ to house their pheromone receptors, while goldfish have all their receptors packed into one organ. Why would a separate organ be necessary if a range of informative odors can be sensed using one organ?

Eisthen (2004)
In her commentary on the paper, Eisthen presents an evolutionary tree showing the animals that have the vomeronasal organ and those that do not.  (I've blogged about her work on the olfactory sense of the axolotl here)

Even though goldfish have all these cells in one organ, the cell types aren't evenly intermixed.  The microvillous and crypt cells are concentrated closer to one end. The authors speculate that the differential location of these cells within the goldfish olfactory epithelium might be an intermediate evolutionary step towards an actual separate organ.

© TheCellularScale

ResearchBlogging.orgHansen A, Anderson KT, & Finger TE (2004). Differential distribution of olfactory receptor neurons in goldfish: structural and molecular correlates. The Journal of comparative neurology, 477 (4), 347-59 PMID: 15329885

Eisthen HL (2004). The goldfish knows: olfactory receptor cell morphology predicts receptor gene expression. The Journal of comparative neurology, 477 (4), 341-6 PMID: 15329884

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