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Vitamin carrier proteins during embryonic development in birds and mammals.

Egg maturation in oviparous vertebrates involves the hepatic synthesis, secretion, and deposition in the developing oocyte of several maternal proteins with specific nutrient carrier function. Thus, in the chicken, adequate yolk deposition of riboflavin, thiamin, etc. is obligatorily mediated by carrier proteins specific to each vitamin. Like vitellogenin, these are oestrogen-inducible specific gene products. Despite differences in patterns of embryonic development in mammals vis-à-vis oviparous species, immunologically and biochemically similar maternal vitamin carriers participate in the transplacental transport and fetal accumulation of these vitamins during gestation in the rat. The rodent riboflavin and thiamin carrier proteins are also oestrogen-induced maternal proteins of hepatic origin. Their functional importance in fetal development was established by in vivo passive immunoneutralization of the endogenous proteins, which precipitated fetal wastage leading to pregnancy termination, due to curtailment of the vitamin supply to the fetuses. Similarly, active immunization of female rats with the vitamin carrier proteins led to early fetal resorption without interference with maternal health, cyclicity and fecundity. The discovery of similar gestation-specific carrier proteins in higher mammals and humans suggests that carrier-mediated vitamin delivery mechanisms ensuring embryonic growth have been conserved during evolution.