Synthesis of Carbamyl Phosphate Synthetase in Amphibian Liver in Vitro

Thyroid hormone modulates apolipoprotein B gene expression in HepG2 cells

We have investigated the modulation of apolipoprotein B gene expression in HepG2 cells by thyroid hormone. ApoB secretion rate in serum-free media was found to be significantly increased in the presence of the hormone in long-term cultures (48 h, 37%). This stimulatory effect was dose-dependent. The mechanisms underlying the stimulatory effect of triiodothyronine on apoB production were investigated. Triiodothyronine increased apoB mRNA levels by about 25-36% as determined by slot- and Northern-blot analysis of total RNA. ApoB synthesis rate was also found to be increased both in in vivo pulse-chase experiments (61%) and in in vitro translation studies (54.5%). Despite the 54.5-61% increase in apoB synthesis with triiodothyronine, only a 30% increase in apoB secretion was noted suggesting that part of the increase in the intracellular apoB pool may be lost by degradation. Overall, apoB gene expression appears to be modulated by thyroid hormone at both transcriptional and posttranscriptional levels.

Fetal thyroid function

Cordocentesis has permitted the study of fetal thyroid function in utero. In normal fetuses, fetal TSH, TBG, and thyroid hormone concentrations increase progressively throughout intrauterine life. Fetal TSH concentrations are always high compared to nonpregnant adult values. TBG concentrations reach adult levels at term. TT4 and FT4 concentrations reach adult levels at approximately 36 weeks gestation, but TT3 and FT3 are always below adult concentrations. There are no significant associations between fetal and maternal concentrations of TSH, TBG, or thyroid hormones. The maternal administration of TRH from at least 25 weeks gestation stimulates the fetal pituitary gland to produce TSH. The response is rapid, unrelated to gestational age, and much greater than that of the mother. These findings suggest that in intrauterine life there is independent and autonomous maturation of the pituitary, thyroid, and liver. The fetal pituitary is able to respond to the maternal administration of TRH and appears to be more sensitive than in the adult. In small-for-gestational-age fetuses, the concentrations of TSH are higher and the concentrations of TT4 and FT4 are lower than in appropriately grown fetuses. The degrees of elevation of TSH and fall in thyroid hormones are significantly related to the degree of fetal hypoxemia and acidemia, respectively. Although the low concentrations of thyroid hormones may have some beneficial effects by reducing oxygen requirements, they may adversely affect brain development.

A histological and dynamic study of the gastric region of Discoglossus pictus larvae, cultured with or without thyroxine

The organotypic culture of the gastric region is carried out on premetamorphic Discoglossus pictus larvae. Adding thyroxine to the culture medium provokes various transformations. On the cytological level, the reactions observed, which are variable depending on the cell category concerned, can be divided into two types of phenomena: histolytic and histogenetic. Autophagia linked to lysosome intervention is frequently found among the histolytic processes. Autophagic vacuoles and residual bodies are observed. The gastric lumen is filled with deteriorated cells that probably come from the degeneration of the tadpole epithelium (primary epithelium). The incorporation of tritiated thymidine makes it possible to study the evolution of cell proliferation in the control and in the thyroxinated cultures. After a 1-2 day latency period, possibly due to the adjustment of the tissue to the culture environment, the incorporation of the radioprecursor H3-thymidine into the epithelium and the tunica muscularis of thyroxine-treated gut tissue increased on day 3, reached a maximum on day 5, and then dropped slightly on day 7. In the control cultures H3-thymidine incorporation showed the same pattern but lower levels on the same days. The histolytic phenomena induced by thyroxine in vitro are comparable to those of natural metamorphosis. On the other hand, the histogenetic phenomena are incomplete. Proliferating and transitional phases occur but neoformated (or secondary) epithelium does not replace the degenerated primary epithelium, whatever the culture time.

Differentiation of red blood cells in vitro

Differentiation of red blood cells occurs in organ cultures of both livers and kidney tissue from tadpoles of the bullfrog Rana catesbeiana. Our evidence indicates that different red blood cell lines are produced by the two tissues and that these different cell lines contain different tadpole hemoglobins.