THE PRIMATE ENDOMETRIUM - MORPHOLOGICAL AND SECRETORY CHANGES DURING EARLY PREGNANCY - IMPLICATIONS OF THE INSULIN-LIKE GROWTH-FACTOR AXIS

The uterus, at the appropriate phase of the menstrual cycle, provides a hospitable and privileged environment for implantation of the blastocyst. This specific period of time is known as the ''receptive window'' and appears to be primarily regulated by ovarian steroids. Embryo implantation is the natural culmination of this period, and successful nidation requires the precise preparation of both the blastocyst and the endometrium. Enders and Schlafke(1) defined the morphological changes associated with implantation into three distinct stages: (1) apposition of the trophoblast and uterine epithelium, (2) adhesion, which involves the formation of junctional complexes between the two cell types, and (3) epithelial penetration, which includes the spreading of trophoblast within the endometrium, regression of the uterine glands, and interruption of the integrity of the maternal vascular system. In the past decade there has been considerable interest in correlating the morphological events associated with implantation with biochemical markers, particularly cellular constituents, endometrial secretory proteins, and growth factors.(2-5) In spite of a large number of studies on the quantitative and qualitative characterization of endometrial peptides,(4) the question whether these proteins mediate essential functions associated with blastocyst/endometrial interactions remains to be resolved. Unfortunately, it is not possible to study the role of specific primate endometrial proteins and their potential autocrine/paracrine functions during implantation and early pregnancy in the human. Therefore, we have chosen to focus our studies on fetal/maternal interactions in the baboon, a phylogenetically related nonhuman primate. In this review, we will compare and contrast the morphological and biochemical changes associated with implantation and early pregnancy in the human, baboon, and rhesus macaque. The focus of the biochemical studies will be on the insulin-like growth factor (IGF), insulin-like growth factor binding proteins (IGFBP), and the Type IIGF receptor (IGF-Rc) in the baboon and their potential roles. We will also summarize comparative studies on endometrial peptides and steroid receptors. The reader is referred to other reviews that summarize the literature on cytokines,(6) growth factors,(5) and other endometrial peptides.(4)