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Cytological Comparison of Milk Protein Synthesis of Rat Mammary Tissue In Vivo and In Vitro¹

Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061

ABSTRACT

Cytological localization of incorporated ³H leucine in the rat mammary gland during milk protein synthesis was compared autoradiographically in vivo 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 vivo); or b) culture media containing the isotope (in vitro). Tissue was fixed for microscopy at 3, 5, 10, 15, 20, 30, 40, and 60 min after the initial exposure to 3H leucine. 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 cells in vivo from the perivascular 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 cytoplasm fell most rapidly from 3 to 15 min 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 min. Thus, the rate of protein synthesis and the structural appearance of cells in vitro were similar to in vivo.

¹ This work was supported by NIH Grant HD-01877.