Pregnancy-associated ectopic decidua (deciduosis) of the greater omentum. An analysis of 60 biopsies with cases of fibrosing deciduosis and leiomyomatosis peritonealis disseminata.

Ectopic decidua (deciduosis) of the omentum was investigated in biopsies taken during cesarean sections and in tubal pregnancies in 60 cases. An ectopic decidua of various abdominal organs was present in 48 further cases. All omentum biopsies showed decidual reactions:focal deciduosis in 97% and diffuse deciduosis in 3%. With increasing duration of pregnancy, there was vacuolar degeneration and fragmentation of the decidua cells as a manifestation of regressive changes. Co-expression of antibodies to vimentin, desmin and actin can be detected immunohistochemically. Ectopic decidua is a physiological phenomenon of pregnancy and arises from a progesterone-induced metaplasia of the pluripotential cells of the "subcelomic mesenchyma". The involution of the decidua takes place in the fourth to sixth week post partum. A fibrosing deciduosis of the omentum with development of a collagenous connective tissue at its surface is described as metaplasia of the hormone-dependent "subcelomic" mesenchyma. Immunohistochemically, a second case with leiomyomatosis peritonealis disseminata from the same investigation material shows the transformation of desmin- and actin-positive decidual cells into myofibroblasts and leiomyocytes in the form of a direct metaplasia. As pregnancy-associated mesenchymal metaplasias, ectopic decidua, fibrosing deciduosis and leiomyomatosis peritonealis disseminata depend on hormone and may regress post partum.