Optically clear nuclei. An alteration of endometrial epithelium in the presence of trophoblast.

Examples of focal clearing of endometrial epithelial nuclei associated with the presence of trophoblastic tissue have been observed recently in our surgical pathology laboratories. These nuclear alterations were initially observed in endometria from first- and second-trimester spontaneous abortions, term pregnancies, and a uterus harboring choriocarcinoma. Subsequently, an identical change was seen in small foci of endometriosis removed during postpartum tubal ligation. The prominent vacuolated appearance to the nuclei resembled, in some cases, the inclusions seen in herpesvirus infection, but electron-microscopic study showed that the nuclear clearing was due to replacement of normal chromatin by a network of fine filamentous material rather than herpesvirus DNA. A subsequent review of 200 consecutive first-trimester abortions found less-striking degrees of this change in 7% of cases. The endometrial nuclear clearing was focal; often the endometrium also contained cells which demonstrated the Arias-Stella reaction. The etiology of this type of nuclear clearing is not known, but the fibrils may represent a thread-like substructure of normal chromatin which forms as a result of hormonal stimulation of endometrial epithelial cells.