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Functional differentiation of mouse fetal liver in circumfusion system cultures
Abstract:
The authors investigated the in vitro functional differentiation of fetal mouse liver cultured in Rose's circumfusion system with the use of two biochemical markers: The analysis of the inductive response of key enzymes in carbohydrate metabolism to insulin, and the analysis of the liver-specific isozyme of pyruvate kinase [EC 2.7.1.40]. The glucokinase [EC 2.7.1.2] activity, which is only found in mammalian adult liver, emerged on cultivation of 2-3 days and reached a maximum level equivalent to one-half of the adult level. Two- or 3-fold increases in the glucokinase, pyruvate kinase, and glucose-6-phosphate dehydrogenase [EC 1.1.1.49] were induced by a single dose of insulin in fetal liver after 12 days of cultivaton. Hexokinase [EC 2.7.1.1] activity was barely influenced by insulin. These results suggested that fetal mouse liver cultured in this system for 2 weeks maintained the same response to insulin as in vivo adult mouse liver. The pyruvate kinase isozyme patterns of mouse livers in various developmental stages and of cultured fetal mouse liver in the present system were investigated by isoelectric fractionation. The pyruvate kinase isozymes having the highest relative activity were the pI-5.5 isozyme for the adult liver and the pI-6.5 isozyme for 13- to 14-day-old fetal liver. As development in vivo proceeded, a gradual change in isozyme pattern occurred; this consisted of a progressive decrease of the pI-6.5 pyruvate kinase isozyme, "fetal type," in favor of the pI-5.5 isozyme, "adult type." The pyruvate kinase isozyme pattern in 13- to 14-day fetal liver cultured in the system for 2 weeks was similar to that found in adult liver. Thus, it was shown that "fetal type" pyruvate kinase was also replaced by "adult type" pyruvate kinase in vitro. It can be concluded from these findings that fetal mouseliver cultured in the circumfusion system for 2 weeks maintains its functional and morphological identitites as it differentiates toward the adult liver.