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A mouse Leydig cell bioassay was established and validated to determine whether the bovine conceptus (age: 16-20 days after oestrus) synthesized and secreted a substance having luteinizing hormone (LH) activity. The response measured was testosterone production after incubating Leydig cells with various doses of LH. The assay was specific for LH as compared to other protein hormones, precise (an interassay CV of 21%) and sensitive (minimal detectable level 15.5 pg NIH-LH-B4/ml). The application of this assay to bovine conceptus homogenates or media in which similar conceptuses had been cultured for 0.5 to 20 h resulted in no LH activity being detected. Furthermore, concentrating the conceptus homogenate or culture media by freeze-drying and reconstitution in a minimum amount of buffer resulted in a similar finding. Because this bioassay is more specific and sensitive to the presence of LH-like activity than are other assays, and yet no activity was detected when conceptuses were manipulated in several ways, it is concluded that the bovine conceptus probably does not produce an LH-like substance at this stage of development.

Mouse Leydig cell bioassay shows lack of LH in the bovine preimplantation conceptus