Developmental influences of thyroid hormones on the rat olfactory receptor sheet

Abstract

Although much is known about the regulatory role of thyroid hormones on brain development, little is known about the effects of thyroid hormones on the development of the peripheral nervous system. The major aim of this thesis therefore, is to examine the role of thyroid hormones on the postnatal development of the olfactory receptor sheet (epithelium) of the rat. Quantitative, light microscopic morphometric and cell count studies were carried out on the olfactory epithelium of growing normal, hypothyroid, hypothyroid/recovery and hyperthyroid rats to measure olfactory neuronal proliferation and maturation.

Our results have found that hypothyroidism during early postnatal development results in partial suppression of olfactory neuronal proliferation and maturation, while prolonged hypothyroidism resulted in complete suppression of epithelial growth. Thus, hypothyroidism led to significant reductions in the normal accretion of olfactory neurons. Unlike thyroid deficiency, hyperthyroidism was found to have no significant effect on olfactory neuronal proliferation or maturation.

Animals rehabilitated from hypothyroidism at weaning showed a resumption of epithelial growth such that by the age of 90 days, the deficits in neuronal number were completely overcome, while the number of mature neurons showed substantial but incomplete recovery. These results show that unlike central neural structures, the olfactory epithelium of the rat retains the ability to recover from early hypothyroid retardation.