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.