Abstract
There is a dialogue between the developing conceptus (embryo-fetus and associated placental membranes) and maternal uterus which must be established during the peri-implantation period for pregnancy recognition signaling, implantation, regulation of gene expression by uterine epithelial and stromal cells, placentation and exchange of nutrients and gases. The uterus provide a microenvironment in which molecules secreted by uterine epithelia or transported into the uterine lumen represent histotroph required for growth and development of the conceptus and receptivity of the uterus to implantation. Pregnancy recognition signaling mechanisms sustain the functional lifespan of the corpora lutea (CL) which produce progesterone, the hormone of pregnancy essential for uterine functions that support implantation and placentation required for a successful outcome of pregnancy. It is within the peri-implantation period that most embryonic deaths occur due to deficiencies attributed to uterine functions or failure of the conceptus to develop appropriately, signal pregnancy recognition and/or undergo implantation and placentation. With proper placentation, the fetal fluids and fetal membranes each have unique functions to ensure hematotrophic and histotrophic nutrition in support of growth and development of the fetus. The endocrine status of the pregnant female and her nutritional status are critical for successful establishment and maintenance of pregnancy. This review addresses the complexity of key mechanisms that are characteristic of successful reproduction in sheep and pigs and gaps in knowledge that must be the subject of research in order to enhance fertility and reproductive health of livestock species.
Highlights
Molecules secreted by uterine epithelia or transported into the uterine lumen by uterine epithelia form histotroph required for conceptus development. [C] The ovine conceptus undergoes the adhesion cascade for implantation
Fibroblast growth factor 7, an estromedin in pigs Pig conceptuses secrete E2 between Days 10 and 15 for pregnancy recognition, and to increase expression of genes within the uterine LE, which act on conceptus trophectoderm and uterine LE to stimulate proliferation, migration, adhesion and gene expression that supports implantation and development of the conceptus [53]
Using SPP1-coated microspheres and pTr2 cells, it was determined that co-localization of ITGAV integrin subunit and talin were associated with assembly of focal adhesions at the apical domain of pTr2 cells. These results indicate that SPP1 stimulates migration and attachment of pig trophectoderm by stimulating forcedriven, integrin-mediated, focal adhesion assembly and haptotactic migration required for conceptus elongation and implantation [93]
Summary
Development of the conceptus (embryo/fetus and extraembryonic membranes) and implantation As indicated in Figure 1, uterine receptivity and implantation of blastocysts for ruminants and pigs includes: 1) hatching from zona pellucida; 2) precontact and orientation of the blastocyst with uterine LE; 3) apposition between conceptus trophectoderm and uterine LE; 4) adhesion of conceptus trophectoderm to uterine LE and 5) no endometrial invasion by the conceptus [1]. Pig conceptuses secrete E2 between Days 10 and 15 for pregnancy recognition and to increase expression of genes in uterine LE which stimulate proliferation, migration, and adhesion of trophectoderm, as well as implantation and conceptus development [4]. Fibroblast growth factor 7, an estromedin in pigs Pig conceptuses secrete E2 between Days 10 and 15 for pregnancy recognition, and to increase expression of genes within the uterine LE, which act on conceptus trophectoderm and uterine LE to stimulate proliferation, migration, adhesion and gene expression that supports implantation and development of the conceptus [53].
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