Spatial and temporal expression of transferrin gene in the rat mammary gland.

The distribution and concentration of transferrin mRNA in the rat mammary gland was analyzed using in situ hybridization, and the results were compared with those obtained for other milk protein mRNA. The [35S]RNA probes prepared from rat cDNA for transferrin and for alpha-, beta-, and gamma-caseins, alpha-lactalbumin, and whey acidic protein were used to probe mammary tissue from rats in late pregnancy and at different stages of lactation. The overall level of transferrin gene expression varied in a biphasic manner, decreasing after parturition to barely detectable levels at d 2 to 10 of lactation before increasing again markedly in late lactation. This temporal pattern contrasts sharply with that observed for the other genes, for which levels tended to rise or to remain relatively stable until late lactation. The spatial patterns of transferrin expression were also quite distinct, and, even during the period of low expression, some alveoli showed high concentrations of transferrin mRNA. In contrast, intramammary distribution of mRNA for the other genes was relatively uniform. Our results show that the patterns of transferrin gene expression differ both spatially and temporally from those of five other milk protein genes and suggest that transferrin gene expression is controlled by a regulatory system that turns individual alveoli either fully on or fully off.