Alerations in the development of the main olfactory bulb of the mouse after ethanol exposure

The effect of ethanol on the development of the main olfactory bulb (OB) was examined by light and electron microscopy in 21-day-old Swiss mice (CD-1), exposed to ethanol from embryonic day 13 to postnatal day 21. Two control groups were included in the study to rule out changes due to malnourishment: a normal control group, whose dams were fed rodent lab chow throughout the study and a pairfed group, whose dams were fed the same amount of liquid diet as the dams in the ethanol group but with sucrose replacement of ethanol. Somatic growth, measured by length and body weight, was retarded to the same degree in ethanol and pairfed mice, indicating a similar level of malnourishment in both groups without an additional ethanol effect. The most striking change in olfactory bulb development was the stunted growth of this structure in the ethanol-exposed animals with reductions in the volume of the glomerular. external plexiform and granular cell layers. The laminar organization and cellular cytoarchitecture were not substantially altered by ethanol or malnourishment except for a retarded migration of the periglomerular cells in the ethanol-treated group. Examination of synaptogenesis in the external plexiform layer (EPL) of olfactory bulb from 21-day-old mice in the three groups revealed no treatment-related changes in synaptic structure, appositional length or ratios of the two dominant type of EPL synapses, which are formed between dendrites of the output neurons (mitral and tufted cells) and granule cells. However, morphometric analysis revealed elevated densities of both synaptic types in the ethanol-exposed group but with no change in the total synaptic number per olfactory bulb. The results suggest that ethanol exposure of mice after the latter half of gestation and during the postnatal period primarily affects the growth of the olfactory bulb probably by decreasing neuronal cell growth, and/or proliferation but has little affect on other aspects of maturation such as formation of cortical structure, differentiation and synaptogenesis.