Selective effects of thyroid hormonal deprivation on growth and development of olfactory receptor sheet during the early postnatal period: a morphometric and cell count study in the rat.

Light microscopic numerical and morphometric studies were conducted on the olfactory epithelium of postnatal normal and hypothyroid rats. The normal rat olfactory epithelium undergoes marked growth and development during the suckling period (days 1-25): thickness, 50%; area, x 8, total number of olfactory neurons, basal and supporting cells, x 10, x 11 and x 8, respectively. The effects of thyroid hormonal deprivation on these proliferative postnatal growth changes were studied by adding PTU (n-propylthiouracil, a reversible antithyroid goitrogen) to the litter's drinking water from birth to weaning (day 25). The general architecture of naso-olfactory cavities as well as the histology and thickness of the olfactory epithelium were unaffected in the hypothyroid pups. However, the surface area of the olfactory receptor sheet was reduced by 40%, the reduction occurring throughout the cavity, though not uniformly. The total number of olfactory neurons, supporting and basal cells were reduced by 33, 45 and 47%, respectively. These results indicate that the postnatal vertical accretion of olfactory neurons occurring across the epithelial thickness is unaffected in the hypothyroid pups, while the horizontal proliferation of neurons accompanying the expansion of the sheet's surface area is markedly reduced. The results suggest differential effects of thyroid hormones on these modes of proliferative growth and imply further that in addition to possible direct effects, the influence of thyroid hormones on developmental growth of the olfactory epithelial sheets may be secondary to effects on the underlying submucosal connective tissue.