Synthesis of alpha-fetoprotein and albumin by fetal mouse liver cultured in chemically defined medium

Transferrin and tooth morphogenesis: retention of transferrin by mouse embryonic teeth in organ culture

Transferrin is the only serum protein that is required for the early morphogenesis of mouse embryonic teeth in organ culture. Transferrin is able to support tooth morphogenesis and dental cell differentiation by stimulating cell proliferation. Its role in this process is restricted exclusively to iron transport, which takes place by receptor-mediated endocytosis of iron-loaded transferrin. A lipophilic iron chelator, pyridoxal isonicotinoyl hydrazone (PIH), can replace transferrin and support tooth morphogenesis in organ culture. We studied the effects of these two iron transporters on cell proliferation in tooth germs during culture. We found that Fe-PIH and transferrin stimulate proliferation to a similar extent in early cap-stage teeth of 14-day mouse embryos, but have no effect on cell proliferation in bell-stage teeth of 16-day mouse embryos. Day-16 teeth undergo morphogenesis in unsupplemented chemically defined medium, whereas transferrin or Fe-PIH is needed for the morphogenesis of day-14 teeth. Although the need for exogenous iron-transport molecules is lost with advancing development, the level of mitotic activity is still fairly high in bell-stage teeth. The abundant binding of transferrin in areas of active cell proliferation in bell-stage teeth also suggests that transferrin is still needed and used for the transport of iron into proliferating cells. Transferrin is not degraded by the process of receptor-mediated endocytosis. After releasing iron into a cell, transferrin is returned to the extracellular space and is reused. We therefore studied whether the transferrin needed by bell-stage teeth could be adequately supplied by endogenous transferrin synthesized or stored in tissue explants.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of kidney differentiation by soluble factors secreted by the embryonic liver and the yolk sac

Since transferrin is necessary for the differentiation of the embryonic kidney in organ culture, we have suggested that the component is a growth factor for in vivo development as well. In the present study we demonstrate that transferrin is present in the serum of 11-day-old mouse embryos, at the time when kidney differentiation starts. We have also tested whether various embryonic tissues can replace transferrin as stimulators of the differentiation and proliferation of the metanephric mesenchyme. We used a transfilter model system where nephrogenic mesenchymes are cultured with spinal cord, a known inductor of kidney tubules. The embryonic liver could not replace the spinal cord as an inducer of tubular differentiation. However, when the kidney mesenchymes were cultured together with both the spinal cord and the liver, the mesenchymes proliferated and differentiated also in the absence of exogenous transferrin. In such cocultures the spinal cord had to be in close contact with the mesenchyme while the embryonic liver could be located several cell layers apart. The liver-mediated stimulation of proliferation of the induced mesenchyme could be inhibited by anti-transferrin antibodies. Immunoprecipitation and immunoblotting with these antibodies of the liver-conditioned medium demonstrated that the 11-day mouse liver produces transferrin. Other potential mitogens produced by liver cells, alpha-fetoprotein, or multiplication stimulating activity, did not in any way stimulate the proliferation of induced mesenchymes. These studies suggest that the mitogen in the liver medium is transferrin. This is supported by data which show that another embryonic transferring producer, the visceral yolk sac, can replace the effect of the liver, whereas a tissue not producing transferrin, the salivary mesenchyme, cannot. In conclusion, an essential function of the inducer is to make the mesenchyme responsive to transferrin. The liver and the yolk sac stimulate early kidney differentiation by producing the soluble factor, transferrin, but they are ineffective as inductors of the transferrin responsiveness.

Induction of tyrosine aminotransferase mRNA by glucocorticoids and cAMP in fetal rat liver

Tyrosine aminotransferase (L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) enzyme and mRNA activity were not detectable in day 20 fetal rat liver. Precocious induction of catalytic activity by in utero injection of dibutyryl cAMP was a direct consequence of the de novo appearance of translatable tyrosine aminotransferase mRNA. In contrast, in utero injection of hydrocortisone acetate failed to elicit fetal liver enzyme activity. This failure was due to the inability of the steroid hormone to induce the appearance of tyrosine aminotransferase mRNA activity. In fetal rat liver explants, either compound was capable of stimulating the synthesis of adult levels of enzyme and mRNA. However, catalytic and mRNA activity comparable with that seen in vivo 24 hr after birth required the concerted action of both inducers.