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Hepatocyte cultures derived from 15-day foetal rats produce the liver form of pyruvate kinase (EC 2.7.1.40) only after 3 days of culture. The appearance of the liver form of the enzyme can be blocked by the addition of 5-bromodeoxyuridine on day 2 of culture, but not by addition on day 3 of culture. The reversibility of the action of 5-bromodeoxyuridine was shown when the inhibitor was added on day 2 and removed on day 4. By day 6 of culture the liver form of pyruvate kinase was detectable. The specificity of the action of 5-bromodeoxyuridine was monitored by following changes in the closely related embryonic form of the enzyme as a control. This was unaltered by the inhibitor.

The glucocorticoid receptor activity that can be detected in the liver from 15-day foetal rats would appear to be associated with the haemopoietic cells. In hepatocytes, purified by culture for 1-2 days from 15-day foetal rats, the glucocorticoid receptor activity is low and dexamethasone does not induce the enzyme tyrosine aminotransferase. If culture is continued both receptor activity and steroid responsiveness are acquired. Cultured hepatocytes from 19-day foetal liver contain receptor from the first day of culture and, furthermore, the subsequent level of response to glucocorticoids is directly correlated with the actual receptor concentration. It would appear that the glucocorticoid receptor is not acquired by hepatocytes until after 18 days of gestation. Nevertheless, the fact that bromodeoxyuridine has no effect on the rate of accumulation of receptor in hepatocytes suggests that the differentiative event leading to the subsequent appearance of the receptor has already occurred before day 15 of gestation. However, the acquisition of the receptor would appear to be dependent on mitosis as cytosine arabinoside can inhibit the process.

1. The administration of dexamethasone to foetal rats in utero does not result in the appearance of specific tyrosine aminotransferase activity even after 24 h. 2. When foetal hepatocytes are cultured in vitro from animals treated in utero with dexamethasone, significantly higher activities of specific tyrosine aminotransferase are found than in untreated controls. 3. Dexamethasone in vitro induces specific tyrosine aminotransferase in cells cultured from control animals and the effect is maximal at 10 nM in the culture medium. 4. Actinomycin D at 0.2 microgram/ml in the culture medium completely prevents the induction of activity in vitro. 5. In cultures established from animals treated with dexamethasone in utero, the increase in specific tyrosine aminotransferase activity over the control cultures is only marginally decreased in the presence of actinomycin D. 6. The results can be interpreted to mean that dexamethasone in utero stimulates the transcription of enzyme-specific mRNA, which is not rranslated until a translational block in the foetal liver is removed by the conditions of culture in vitro.

During the acute phase response to inflammation the plasma concentration of some proteins, such as α(1-acid glycoprotein (AGP), increases dramatically. Since breakdown and remodeling of tissue is common to both nidation and inflammation we studied the tissue distribution and regulation of AGP mRNA levels during the embryonic development of the rat. High levels of mRNA coding for AGP were detected in the placenta during early fetal development. Expression of this mRNA was confined to the decidua and was first observed approximately 1 day after implantation when proliferation of the decidua is already well advanced. Maximum levels were attained about 5 days after implantation, after which the levels decreased rapidly. In contrast to the high levels of AGP mRNA in the decidua only very low levels were detected in fetal liver and visceral yolk sac, and there was only a small increase in the levels in maternal liver. Corticosteroid hormone responsiveness of AGP mRNA synthesis by hepatocytes appeared 3 days before birth. It is likely that the synthesis of AGP by the cells of the decidua is important in establishing the precisely controlled interaction between mother and embryo during nidation.

In this study, the incorporation of [14C]leucine into albumin and transferrin in early rat foetuses, vitelline plus amniotic membranes, chorioallantoic placenta and perinatal rat liver slices was measured and used to detect and compare the rates of synthesis of the two proteins. Albumin synthesis was detected in the body of foetuses from 13 days gestation onwards. Transferrin synthesis was detected only after day 15. Transferrin synthesis was demonstrable in the membranes but not in the chorioallantoic placenta of all the animals investigated, i.e. from 13 to 19 days gestation. Synthesis of albumin and transferrin by the liver of near-term and postnatal animals was shown to correlate with published data on the parenchymal cell number/unit wet wt. of liver. Near-term foetuses synthesized relatively more transferrin than albumin when compared with 10-day postnatal animals. The serum concentrations of the two plasma proteins were also determined. These increased before term whereas the rate of synthesis of albumin and transferrin declined. Postnatally, plasma albumin concentration increased but transferrin concentration decreased, yet the rates of synthesis of both proteins by the liver increased with age. This lack of correlation between the rates of synthesis of the two proteins and their respective plasma concentrations could be explained in part by their increased stability after birth. There was also evidence that the liver haemopoietic cells took up transferrin although they do not synthesize the protein. Thus the decrease in this population of cells during development could also contribute to the discrepancy between liver synthesis and serum concentrations of transferrin.