Olfactory mucosa was excised from the heads of embryonic day 15 (E15) rat fetuses, prior to the appearance of olfactory marker protein (OMP), and cultured with or without the presumptive olfactory bulb. Specific immunohistochemical staining for the OMP was detected in both groups of explants 6 days after explantation. At no time do the supporting cells and basal cells show positive OMP staining. Results of radioimmunoassay on the two groups of cultures showed that about twice as much OMP was present in organ cultures grown with presumptive olfactory bulb. Estimates of the area of olfactory mucosa in the explants revealed no significant differences between those cultured with or without the bulb. Cell counts revealed that the increased amount of OMP in explants with the bulb was due to a higher number of OMP-positive receptor neurons. Explants of olfactory mucosa cultured with other tissues (cerebrum, cerebellum, cervical spinal cord, and heart) contained approximately the same amount of OMP as explants of mucosa alone; i.e., these other tissues did not enhance OMP levels. Similarly, there was no enhancement of OMP level when the olfactory mucosa was separated from the bulb by a Millipore filter of 0.45 micron pore size. Thus the results of the study indicate that the presumptive olfactory bulb specifically increases OMP level by a mechanism that requires direct contact between the receptor cells and bulb.
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