A sexually dimorphic group of atypical glomeruli in the mouse olfactory bulb

Atypical glomeruli (AtG) are clearly distinguishable from typical ones beca use of their strong cholinergic innervation. AtG are located in defined pos itions in the caudal half of the main olfactory bulb of rodents. The AtG pa rtially overlap with other specialized olfactory subsystems, such as the mo dified glomerular complex, which is close to the accessory olfactory bulb. So far, possible sex differences in these specialised olfactory systems hav e not been investigated. In this work we have identified AtG in the mouse b y means of acetylcholinesterase histochemistry and compared the number and size of these glomeruli between the sexes and also between the two strains that demonstrate intraglomerular synaptic differences, i.e. BALB/c and CD-1 mice. First, we divided the AtG into three types according to their positi on (I, rostral-most; II, around the accessory olfactory bulb; III, caudal-m ost) or their reactivity to acetylcholinesterase histochemistry (AtG type I I being the least reactive glomeruli). ANOVA analyses revealed differences in the maximum diameter of glomeruli among the three types, but not in thei r sectional areas, indicating that all three types have different shapes. M oreover, both morphoplanimetric parameters were seen to be different betwee n the two strains studied and also between the sexes: male mice and BALB/c animals had the largest glomeruli. The number of AtG was also significantly different between the sexes and strains, although these factors presented a strong interaction. Thus, the males had higher numbers of AtG in the CD-1 strain whereas in the BALB/c mice males demonstrated fewer AtG than female s. These differences in number were largely due to AtG type II. The present work is evidence that AtG type II is a sexually dimorphic group of special ized glomeruli located in the main olfactory bulb.