Paracrine signaling in the endometrium: integrins and the establishment of uterine receptivity.

The importance of cell shape and polarity have long been recognized. In the endometrium, a complex association of different cell types undergo cyclic renewal, differentiation and eventually apoptosis and shedding, with the sole purpose of allowing implantation of the nascent embryo. Many of these physiologic processes depend on the timely expression of cell adhesion molecules which maintain tissue architecture by mediating cell-cell and cell-substratum attachments. One family of cell adhesion molecules are the integrins, cell surface glycoproteins composed of heterodimeric alpha and beta subunits that serve as receptors for the extracellular matrix. We have studied the endometrium as a unique site of integrin expression that appear to be under both endocrine and paracrine control. In addition, it is suspected that the engagement of integrins by extracellular matrix on the embryo results in signal transduction leading to the transcription and translation of genes critical for implantation. While the role of integrins in endometrial function is not yet clearly defined, these interesting molecules represent excellent markers of normal and abnormal states of receptivity and may provide clues to the regulation and mechanics of implantation in general.