ABSTRACT
Mustelus canis embryos rely on yolk in the external yolk sac during the first 12 weeks of development (TeWinkel 1963). Further, after 14 weeks the distal portion of the yolk sac, egg envelope and uterine attachment sites are intimately associated (Graham 1967) to form a functional placenta. Graham (1967) suggested that paraplacental transfer commenced at 12 weeks. He reported that 90 min after injection of tritiated glucose into a M. canis female harboring 12 week embryos, 3H (tritium) was present in the intracapsular fluid and embryos' stomach, umbilical cord and spiral intestine. However, tritium lability rendered Graham's results inconclusive as it was not clear if the tritium transferred to the embryo was still associated with injected glucose or its metabolites. Subsequently Graham
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and co-workers (1995) modified his original experiment and provided evidence for passage of maternally derived metabolites (enriched with stable non-radioactive 13C atoms) to periembryonic fluids. They injected females containing embryos with no placental connection after 12 weeks gestation with 3-13C-alanine. Periembryonic fluid was analyzed and found to contain 11C-enriched signals from alanine C-3 and lactate C-3 carbons. They interpreted the results as indicating that matrotrophy via the paraplacental route commences after 12 weeks gestation. However, 11Cenriched alanine was not detected in embryonic blood plasma and its uptake by embryos was not confirmed. ln a related experiment, 13C-lglucose was injected in animals with near term fetuses and no 13C-enriched glucose carbons were found in either periembryonic fluids or embryonic blood. This suggested to them that the mother utilizes glucose but does not transfer it to periembryonic fluid. These experiments demonstrate that the paraplacental matrotrophic nutrient route is established after 12 weeks of gestation in M. canis.
