The progesterone receptor has been localized in the rabbit uterus by immunocytochemistry at the electron microscopic level, using monoclonal antibodies and the protein A-gold technique. The progesterone receptor in uterine stromal cells was mainly localized in the nucleus; however, a small fraction of antigen was present in the cytoplasm, where it was associated with the rough endoplasmic reticulum and with free ribosomes. The plasma membrane was not labeled. In the nucleus, the receptor was always associated with condensed chromatin or areas surrounding condensed chromatin, whereas the nuceolus was not labeled. In the chromatin, receptor distribution varied according to the hormonal state: in the absence of progesterone, the receptor was randomly scattered over the clumps of condensed chromatin; after administration of the progestin R5020, it was mainly detected in the border regions between condensed chromatin and nucleoplasm and, to a lesser extent, over dispersed chromatin in the nucleoplasm. These areas have been shown to be the most active sites of gene transcription.

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