Morphometric study of the glomerular population in the mouse olfactory bulb: numerical density and size distribution along the rostrocaudal axis.

A morphometric study of the glomerular population in the olfactory bulb of the mouse has been carried out by using stereological methods. On the basis of the assumption that the glomerular population is a polydispersed system of spheres, glomerular profile distributions obtained from profile measurements were subjected to a mathematical unfolding procedure to obtain the actual glomerular size distribution. We used a distribution-free method to account for the combined effects of overprojection due to section thickness and truncation (two missing profile mechanisms). Results proved better than those obtained directly from profile measurements without stereological analysis. Several new findings were obtained. First, significant variations of the glomerulus sizes were found along the rostrocaudal axis. The glomeruli are larger in the middle region of the olfactory bulb, whereas their numerical density decreases in the same region. Moreover, the profile density is homogeneous along the rostrocaudal axis. In other words, the relative surface occupied by the periglomerular cells in the glomerular layer is invariant. As a consequence, it may be concluded that the variations in size and numerical density are inversely correlated. Thus, since the glomeruli are larger in the middle region, their number per unit volume is logically smaller in this same area. Finally, the computerization of all these data has led us to estimate the number of glomeruli (1,810 +/- 41) in the olfactory bulb of the mouse. In order to get a comparative idea of their advantages and disadvantages, other standard stereological methods were used in the present study to determine this number. Functional interpretations of the variations of the size and numerical density along the rostrocaudal axis of the olfactory bulb are discussed with respect to ontogenetic and morphofunctional data obtained elsewhere.