Cytological Comparison of Milk Protein Synthesis of Rat Mammary Tissue In Vivo and In Vitro

Abstract

Cytological localization of incorporated sit leueine in the rat mammary gland during milk protein synthesis was compared autoradiographieally ha rive and in vitro. Preceding autoradiography, light and electron microscopy were employed to evaluate cellular integrity of tissues. Tissues were exposed to 3H leucine by one of two methods: a) cardiac puncture (in rive) ; or b) culture media containing the isotope (in vitro). Tissue was fixed for microscopy at 3, 5, 10, 15, 20, 30, 40, and 60 rain after the initial exposure to 3H leueine. Silver grains were categorized as being over the perivascular space, basal cytoplasm, Golgi region, or alveolar lumen of mammary tissue for each of the time intervals. Alveoli from both preparations were similar with regard to the rate of isotope passage and ultrastructure. Leucine entered eells in vivo from the perivaseular space, while entry from the alveolar lumen appeared most predominant in vitro. The isotope accumulated in the basal cytoplasm and moved as a pulse regardless of route of entry or the fact that the isotope was continually available in the media. Rate of isotope passage through the cell was synchronous within individual alveoli but variable between alveoli. This variation was most apparent at 40 and 60 min postexposure to the isotope. The percentage of label in the eytoplasm fell most rapidly from 3 to 15 rain with a concurrent increase in the percentage of label in the Golgi. Maximal labeling of the Golgi occurred from 15 to 30 min following exposure whereas labeling of the alveolar lumen was greatest from 30 to 60 rain. Thus, the rate of protein synthesis and the structural appearance of cells in vitro were similar to in rive.