Relative Distribution of Estrogen and Progesterone Receptors among the Epithelial, Adipose, and Connective Tissue Components of the Normal Mammary Gland*

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Abstract
The present studies were undertaken to determine the relative distribution of estrogen and progesterone receptors (PgR) in the epithelial, adipose, and connective tissues of normal mouse mammary gland and to determine in which tissue (s) the modulations in PgR concentration and mammary gland sensitivity to estrogenic stimulation of PgR occur. Mammary epithelium was separated from adipose and connective tissues surgically in vivo according to the cleared fat pad technique. Binding studies using [3H]R5020 or [3H]estradiol were carried out on cytoplasmic extracts of intact mammary gland or cleared fat pads which were devoid of epithelium and contained only mammary adipose and connective tissues. We found that mammary adipose and connective tissues contain separate, high affinity, steroid-specific, macromolecular binding sites for estrogen and progesterone, and the majority of PgR are present only in the glandular epithelium. Furthermore, the modulation in PgR concentration relative to mammary gland development as well as the estrogenic regulation of PgR concentration appear to be restricted to the epithelial component of mammary gland. Since the major mammary gland developmental changes that occur during pregnancy and lactation also occur in the epithelium, it is proposed that the receptor modulations may serve a regulatory function in mammary gland. (Endocrinology108: 825, 1981)