A quantitative study of the effects of early unilateral olfactory deprivation on the number and distribution of mitral and tufted cells and of glomeruli in the rat olfactory bulb.

Anatomic effects of early unilateral olfactory deprivation on the developing olfactory bulb were investigated in the albino rat. Unilateral anosmia was experimentally induced by neonatal cauterization of the left or right nare; regenerating the tissue permanently blocked the nostril. The anosmic olfactory bulb (ipsilateral to the blocked nare) and its contralateral counterpart, serving as the normal control, were compared for the following quantitative anatomic parameters: total number and distribution of mitral and tufted cells and olfactory glomeruli; average diameters of mitral cells and glomeruli; relative dimensions of the bulb and its layers. The effects on mitral cells and glomeruli were studied at the ages of 25 and 60 days and at 2 years; other studies were carried out in the 25-day-old animals only. In the normal mature bulb, the number of mitral cells, tufted cells and glomeruli was found to be about 70,000, 160,000 and 3000, respectively. It was found that the absence of early olfactory stimulation was invariably correlated with a significant and permanent loss of mitral cells, amounting to more than a quarter of the total number. This loss apparently occurred rapidly during the first 3 weeks, as it was already evident by day 25 and did not increase appreciably with prolongation of deprivation. Tufted cells were apparently even more susceptible, because their number decreased by about 45%. As evident from this distribution profiles, the loss of mitral and tufted cells occurred uniformly throughout the bulb. It is shown that these differences were due neither to inherent interbulbar differences, nor to a hyperplasia in the normal bulb. Early anosmia had no significant effects on the number or average diameter of the glomeruli. Morphometric studies revealed that the dimensions and thickness of layers (internal and external plexiform and granular) of the anosmic bulb were significantly reduced. It is suggested that early olfactory stimulation is necessary for survival of the developing mitral and tufted cells; thus the first 2-3 postnatal weeks, covering the final developmental stages of these cells, would constitute a vulnerable period in the development of the rat olfactory system.