ABSTRACT
PURPOSE: It is well known that vertebrates Central Nervous System reacts to a wide variety of hormones, and growth factors. However the basic maturation mechanisms and remodelling processes remain in general unknown. Thyroid hormones tri-iodothyronine (T(3)) and thyroxine (T(4)) modulate this metabolism, playing a role in cell differentiation and proliferation and in gene expression during growth. The aim of the present study was to investigate whether or not low levels of thyroid hormone in blood, obtained with an experimental model of congenital neonatal hypothyroidism referred to herein, may induce changes in optic nerve development, mainly in processes of macroglial cell genesis and myelination. MATERIAL AND METHOD: We used an experimental model of materno-foetal hypothyroidism (MFHP) set up in a rat through chemical thyroidectomy. A solution of methyl-mercapto-imidazole and KClO(4) was administered with drinking water to pregnant rats and their offspring during gestation and lactancy. A group of rats was maintained in parallel as controls (C-G). Optic nerves were excised from both groups at key postnatal developmental stages (P) and these were prepared for light and transmission electron microscopy. RESULTS: Cross-sectional areas of optic nerves in the MFHP-G group appeared significantly smaller compared to C-G, this, during the perinatal phase as well as P25 (p<0.01). Macroglial cell nuclear cross-sectional areas showed increasing values in both groups. Data from MFHP-G, however, showed significantly lower than those of C-G (p<0.01). Between 4 to 6 days delayed myelination was at all times observed in the treated group. CONCLUSIONS: All results suggest that T(3) and T(4) regulate optic nerve development by stimulating glial cells function at cell nuclear level, probably through specific receptor binding sites, as suggested in earlier literature (Pinazo-Durán et al. Arch. Soc. Esp. Oftalmol., 1997).
Subject(s)
Fetal Diseases/embryology , Hypothyroidism/embryology , Optic Nerve/embryology , Pregnancy Complications , Animals , Female , Pregnancy , Rats , Rats, WistarABSTRACT
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.
