Species-specificity of transferrin as investigated in the rat conceptus using an improved microinjection technique.

Transferrin, a widespread vertebrate protein, is responsible, along with its specific receptor, for all of the iron transported into the cell. Despite its ubiquity and structural similarity across species, heterologous molecules display different binding affinities for human cells. In general transferrin binds more avidly to the homologous receptor whatever the species. The impetus for the first part of this thesis came from these observations, coupled with research, which showed that rat conceptuses grown in human serum were anaemic and malformed, conditions rectified by supplementation with rat but not human transferrin. The second part of the thesis concerns the development of an intravitelline injection system to by-pass the rat visceral yolk sac which surrounds the embryo in its amniotic sac; in the human the yolk sac is situated externally to both. Due to this arrangement and also because of its known degradative abilities, the rat yolk sac presents toxicologists with a problem when using the rat model in teratological studies because they cannot be sure what quantity of a compound or even what metabolite the rat embryo is receiving. Thus if the extraembryonic membranes could be by-passed it would be possible to perform experiments which might provide more information regarding acute toxicity and help in the accurate assessment of teratological risk. This improved injection method allows such experiments to be carried out in a reproducible manner with greater accuracy and with more discrete quantities than was possible with the earlier system. The two parts have been drawn together in experiments which compare the uptake of human and rat transferrin in embryos cultured in serum containing, and injected with, these two molecules. Over all, the results indicate that the rat yolk sac has an affinity three times greater for rat transferrin than it does for human transferrin. Furthermore, it appears that the embryo cannot differentiate between the two molecules and that it probably does not possess a specific transferrin receptor. The practical aspects of this technique in relation to experimental embryology and acute reproductive toxicology have been explored in chapter 6 of this thesis. From the point of view of acute toxicity testing the results have been promising enough to elicit industrial sponsorship for developing the procedure further.