Immunocytochemical evidence that endometrial stromal granulocytes are granulated lymphocytes

Endometrial stromal granulocytes (EGs) are prominent in late luteal phase human endometrium and in early pregnancy decidua. They have been believed to develop from endometrial stromal cells and to secrete relaxin. Recent immunohistochemical studies have suggested that EGs are derived from bone marrow but this has been difficult to prove, mainly because the characteristic cytoplasmic granules are not preserved in frozen tissues. Two separate approaches have now been employed to investigate the cellular lineage of EGs. Formalin-fixed paraffin-embedded sections of first trimester decidua were labelled by an immunoperoxidase method with four monoclonal antibodies (mAbs) reactive with routinely fixed and processed tissues. In addition, acetone-fixed smears of decidual cell suspensions were labelled with a panel of mAbs. Sections and smears were counterstained to demonstrate the characteristic cytoplasmic granules of EGs. Endometrial granulocytes were LCA+, CD2+, MT1+, and UCHL1+, which provides evidence that they are leucocytes. EGs are probably members of the large granular lymphocytes series and may have an essential role in normal implantation and placentation.