Role of thyroid hormone in craniofacial and eye development using a rat model

Thyroid hormones (TH) are essential for somatic and neural development. Epidemiological studies have pointed to TH-dependent craniofacial features occurring during development. In an attempt to elucidate the precise role of TH in the developing eyes and adnexa (orbit, lids, nasolacrimal structures), we analysed the craniofacial and eyeball developmental characteristics in a rat model of congenital-neonatal hypothyroidism (HG), induced by combined chemical-surgical thyroidectomy. The heads and eyeballs from control and HG animals were obtained at key developmental stages and processed for scanning electron, light and transmission electron microscopy. On embryological day 13 (E13), significantly reduced values for head parameters (25% less), optic primordia area (0.053 +/- 0.0085 vs. 0.111 +/- 0.012 microm(2); p < 0.05) and volume (3.96 +/- 0.141 vs. 8.09 +/- 0.123 microm(3); p < 0.05) were found in the HG with respect to the controls. In addition, a delayed prenatal eye closure and postnatal eye opening took place in the treated rats. The photoreceptor and ganglion cell layer thickness displayed significantly lower values (p < 0.001) in HG, at each developmental time point. Postnatally, a delay in photoreceptor outer segment morphogenesis (in relation to retarded disc formation) and significantly lower values for ganglion cell nuclear volumes (p < 0.001) and nuclear pore density (p < 0.01) were observed in the TH-deficient animals. All data suggest that TH play a pivotal role in the development of the face and eye. Therefore, a series of defects due to a loss of TH actions involved in anterior-posterior development of the head and face and the loss of TH-dependent signals crucial for cell differentiation, migration, proliferation and establishment of definitive cell phenotypes in the eyes may appear. Gestational and neonatal screenings for thyroid functioning are suggested to paediatricians and ophthalmologists in order to prevent craniofacial malformations and visual abnormalities.