Decidualization and insulin-like growth factor (IGF) binding protein: implications for its role in stromal cell differentiation and the decidual cell in haemochorial placentation.

The somatomedins or insulin-like growth factors (IGFs) are polypeptides which are involved in proliferation of a number of cell types and have been implicated in fetal growth and development. Cellular responses to IGFs are mediated via binding to two classes of trans-membrane receptors. However, IGFs also bind with high affinity to binding proteins (IGF-BP) in plasma and other body fluids. In plasma the majority of IGF is associated with a GH-dependent 150 kd IGF-BP whereas a second small mol. wt (29-35 kd) IGF-BP is relatively unsaturated. It is this latter class of IGF-BP which has been demonstrated to be synthesized or secreted by a number of cell types and is detected in body fluids and secretions such as amniotic fluid, cerebrospinal fluid and milk. Although the exact relationship between these binding proteins and their role in IGF action has not been clarified, evidence suggests that the 29-35 kd form may function as an inhibitor and promoter of IGF action. During the menstrual cycle IGF-BP synthesis of this latter class appears associated with stromal fibroblast populations and it is proposed that this is involved in specifying proliferation of this cell type which differentiates into decidual cells. This IGF-BP also represents the major soluble secretory protein of the decidualized endometrium and its component decidual cell, during pregnancy and probably is the major source of the protein in pregnancy, contributing to the amniotic fluid and peripheral serum levels. In this article the implications of local decidual production of IGF-BP is discussed with reference to the thesis that decidualization, which is associated with species exhibiting haemochorial placentation, is involved in regulatory mechanisms of feto-placental development and growth.