Secretory proteins of the endometrium--potential markers for endometrial dysfunction

In contrast to our detailed knowledge of endometrial structure, the study of endometrial function is still in its infancy. It seems likely that most of the changes that occur in the endometrium during the cycle are directed towards the promotion of implantation, and that the proteins secreted by the tissue have a role to play in the maintenance of pregnancy. One approach to the study of these proteins has been the incubation of endometrium in vitro with labelled amino acid precursors, and this method has revealed that in early pregnancy two proteins are quantitatively more important than the others synthesized and secreted by the endometrium. One of these proteins is a homologue of beta-lactoglobulin, also known as alpha 2-PEG, PP14 or PEP. It is synthesized by endometrial glands and no other source has been identified. Its synthesis is induced in the luteal phase of the menstrual cycle, rises to a peak in the early weeks of pregnancy and then rapidly declines as the decidua spongiosa is replaced by decidua compacta. The direction of its secretion appears to be into the amniotic fluid, and its function may be to transport a vitamin or other essential substance to the early conceptus: another possible function is immunosuppression. The protein can be measured in peripheral blood and may be useful as a marker of the effect of progesterone-or its lack of effect, as in the inadequate luteal phase. The other quantitatively important endometrial protein is IGF binding protein, which is produced mainly by stromal fibroblasts in the late luteal phase of the cycle. During the cycle the contribution of the endometrium to serum levels of IGF-BP is slight, but in pregnancy, as the amount of decidua compacta increases, levels in peripheral blood rise, with peaks at 18-24 weeks and at 36-40 weeks. The function of the protein may be to protect the mother by inhibiting IGF, or to enable IGF to stimulate the growth of endometrial stroma. Links between this protein and disorders of fetal growth have been suggested. The diversity of the possible functions of these proteins, and the continuing uncertainty over their roles, underline the complexity of endometrial functions.(ABSTRACT TRUNCATED AT 400 WORDS)