Cell formation in the motor nucleus and mesencephalic nucleus of the trigeminal nerve of rats made hypothyroid by propylthiouracil

Abstract

Neurogenesis in the motor and mesencephalic nuclei of the trigeminal nerve was examined using autoradiographic techniques. Two groups of pregnant rats (control and experimental) were injected with two successive daily doses of 3H thymidine in an overlapping series starting from day nine of gestation in order to label in their progeny, the dividing precursor of neurons of the motor nucleus and mesencephalic nucleus of the trigeminal nerve. Control group of rats was raised on a standard diet, while the experimental group was made hypothyroid by propylthiouracil (PTU). At postnatal ages ranging between 20-30 days in the pups of both the control group and experimental group, the percentage of cells labelled and the proportion of cells added during each embryonic day were determined quantitatively throughout the rostro-caudal extent for both motor and mesencephalic nuclei. The neurons of the mesencephalic nucleus undergo their final cell divisions between gestational days 9 and 10 (E9 and E10). More than 80% of the population is generated by E10. The neurons of the motor nucleus undergo their final cell divisions between E9 and E11, and nearly 88% of the cells is generated by E11. In the thyroid deficient rats, in both nuclear centers, only 61% of the cells is generated by E12, and labelled cells are observed even as late as E18 and E19. In the hypothyroid state, there is a significant lengthening of the proliferative period. On the basis of absolute datings and duration of neuron production, it is postulated that in normal development, thyroid hormone determines the duration of the proliferative period, and push cells into the differentiative phase by taking them out of the proliferative phase.

Pregnant rat uterus expresses high levels of the type 3 iodothyronine deiodinase

“…Thus, as early as day E9, the thyroid status of the fetus has been demonstrated to have important effects on the proliferation and differentiation of neurons in the motor and mesencephalic nuclei of the trigeminal nerve (46). These regions of the central nervous system are formed and undergo maturation early in development, with neuronal proliferation normally complete by day E12 (47).…”

Thyroidal dysfunction and environmental chemicals--potential impact on brain development.

“…Disorganization of the cellular layers of the cerebellar cortex has been shown to occur in postnatal hypothyroidism in rats (24). The cellular changes reported by Narayanan and Narayanan (23) are reflected * Progeny of control in behavioral effects. The trigeminal nerve is important for suckling and the PTU-treated pups had difficulty suckling.…”

“…These neurons normally form relatively early in gestation at a period that would correspond to the first half of gestation in humans (23). Eventually these neurons are produced, but the timing of neuronal birth, migration, and synaptogenesis is finely tuned and if neurons are not formed at the appropriate time, they may never reach the right cell layer of the brain and form normal neuronal synapses.…”

Thyroid hormones and the control of cell proliferation or cell differentiation: Paradox or duality?

“…in Zoeller and Rovet, 2004;Bernal, 2005;Bernal, 2007;Williams, 2008). The events controlled by THs comprise cellular proliferation, migration, and neuronal and glial cell differentiation (Narayanan and Narayanan, 1985;Berbel et al, 1993;Berbel et al, 1994;Lucio et al, 1997;Auso et al, 2004;Cuevas et al, 2005). Clinical and experimental studies have demonstrated that THs are essential for normal brain development.…”

“…These deficits depend on specific cellular impairments. Indeed, during the prenatal period, THs influence neurogenesis, neuronal proliferation and migration in the cerebral cortex, hippocampus and medial ganglionic eminence (Narayanan and Narayanan, 1985;Lucio et al, 1997;Auso et al, 2004;Cuevas et al, 2005). Axonal outgrowth, dendritic branching and synaptogenesis, together with the initiation of glial cell differentiation and migration are also affected by TH deficiency at prenatal stages (Portfield and Hendrich, 1993;Morreale de Escobar et al, 2000;Bernal et al, 2003).…”