The reciprocal synapse in the external plexiform layer of the mammalian olfactory bulb

It has recently been claimed by Ramon-Moliner29, that reciprocal synapses between mitral (and tufted) cell dendrites and granule cell gemmules, which for more than 10 years have been thought to represent a major feature of the structural and functional organization of the olfactory bulb28, are non-existent or, at best, extremely rare. The challenge is based on the contention that there is little, if any, morphological evidence to substantiate the existence of the gemmulofugal component of the reciprocal synapse. We have re-examined by electron microscopy the external plexiform layer (EPL) of the olfactory bulb of adult rabbits and rats. The study was carried out in conventionally prepared material, using individual and serial sections, goniometry, and quantitative analysis of montages, and also in material stained by the E-PTA and BIUL methods. According to currently accepted morphological criteria, gemmulofugal synapses can be identified consistently and with confidence at all levels in the EPL: they are particularly clearly and unambiguously resolved in E-PTA and BIUL preparations. At least the majority of gemmulofugal and mitrofugal synapses are closely associated in reciprocal pairs, as many previous authors have reported. Furthermore, there are approximately as many gemmulofugal as mitrofugal synapses in the EPL, which supports the long-standing proposal that the overall synaptic relationship between granule cells and mitral cells is a reciprocal one. We conclude that the challenge of Ramon-Moliner to the reality and significance of the reciprocal synapse is not well-founded.