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The process of implantation is mediated by a complex network of signaling and adhesive factors. In the pig, latent and active transforming growth factor beta (TGFB), TGFB receptors (TGFBR), and integrins (ITGs) are present during the peri-implantation period. TGFB signals via TGFBR and activates downstream effector SMAD proteins 2 and 3 (p-SMAD2/3). Latency-associated peptide (LAP), part of the latent TGFB complex, is known to bind to ITG heterodimers and activate TGFB. We hypothesize that active TGFBs and TGFBRs along with LAP and ITGs functionally interact at the conceptus–maternal interface to mediate events essential for conceptus development and attachment in pigs. Uteri and conceptuses from days 10, 12, 16, 20, and 24 pregnant gilts were immunostained for TGFB, LAP, and ITG subunits (ITGAV, ITGB1, ITGB3, ITGB5, ITGB6, and ITGB8). Activation of TGFBRs was evaluated by the presence of phosphorylated downstream effector SMAD2/3. Binding of LAP to ITGs was also evaluated using porcine trophectoderm cells. Abundant active TGFB was detected at the apical surfaces of epithelia at the conceptus–maternal interface, and p-SMAD2/3 was detected at both conceptus attachment and nonattachment sites during implantation. Separate aggregates of LAP, ITGB1, ITGB5, and later ITGB3 were detected at the porcine conceptus–maternal interface, and binding of LAP to ITGs on apical surfaces was demonstrated. Results suggest that functional LAP–ITG adhesion complexes support conceptus attachment and promote TGFB activation leading to TGFB interaction with TGFBR supporting events of porcine implantation.
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In brief
Cattle are classified as having synepitheliochorial placentation in which the majority of the uterine luminal epithelial cells remain intact with some luminal epithelial cells fusing with binucleate trophoblast cells to form syncytial trinucleate cells. This study suggests the possibility that, for a limited and as yet undefined period of gestation, the majority of luminal epithelial cells are eliminated and replaced by trophoblast cells that express pregnancy-associated glycoproteins.
Abstract
What we understand about the early stages of placentation in cattle is based on an elegant series of electron microscopic images that provide exquisite detail but limited appreciation for the microanatomy across the uteroplacental interface. In order to achieve a global perspective on the histology of bovine placentation during critical early stages of gestation, i.e., days 21, 31, 40, and 67, we performed immunohistochemistry to detect cell-specific expression of pregnancy-associated glycoprotein (PAG), cytokeratin, epithelial (E)-cadherin, and serine hydroxymethyltransferase 2 (SHMT2) at the intact uteroplacental interface. Key findings from the immunohistochemical analyses are that there are: (i) PAG-positive cells with a single nucleus within the uterine luminal epithelial (LE) cells; (ii) PAG-positive cells with two nuclei in the LE; (iii) PAG-positive syncytial cells with more than three nuclei in the LE; (iv) LE cells that are dissociated from one another and from the basement membrane in regions of syncytialization within the LE layer; (v) replacement of the mononuclear LE with a multilayer thick population of PAG-positive cells invading into the uterine stroma of caruncles but not into the stroma of intercaruncular endometrium; and (vi) PAG-, E-cadherin-, and SHMT2-positive mononuclear cells at the leading edge of developing cotyledonary villi that eventually represent the majority of the epithelial surface separating caruncular stroma from cotyledonary stroma. Finally, the uteroplacental interface of ruminants is not always uniform across a single cross section of a site of placentation, which allows different conclusions to be made depending on the part of the uteroplacental interface being examined.
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In Brief
The trophectoderm of the elongating conceptuses of cattle, sheep, and pigs secrete high amounts of interferons that increase or induce the expression of interferon-stimulated genes (ISGs) in the endometrium. Research concerning ISGs, performed from 1995 through 2023, is reviewed in this manuscript.
Abstract
Expression of the classical interferon (IFN) stimulated genes (ISGs) increases in the endometrial stroma and glandular epithelium (GE) through activation of signal transducer and activator of transcription (STAT) signaling in response to the secretion of IFN tau (IFNT) and IFN gamma (IFNG) by the conceptuses of ruminants, including cattle and sheep, and pigs, respectively. The first of the classical ISGs to be characterized was ISG15 in cattle. Classical ISGs are not expressed by the endometrial luminal epithelium (LE) due to the expression of interferon regulatory factor 2 (IRF2) in the LE that prevents the expression of ISGs in the LE. Classical ISG expression in the endometrium serves as a reliable indicator of conceptus health and elongation in cattle. There are also nonclassical ISGs that are upregulated in endometrial LE in response to progesterone (P4) that are further stimulated by IFNT in sheep, the intracellular signaling pathway responsible for IFN effects on expression is unknown. ISGs are also upregulated in extrauterine tissues including CL and peripheral blood mononuclear cells (PBMCs). The expression of ISGs by the PBMCs of cattle serves as an early prognosticator of pregnancy. The physiological roles of ISGs remain obscure, but evidence suggests that they are at least in part involved in modifying the immune system to support endometrial remodeling necessary for the successful implantation of the conceptus. Our understanding of these ISGs is primarily the result of work from the laboratories of Drs Fuller Bazer, Thomas (Tod) Hansen, Gregory Johnson, Hakhyun Ka, Patrick Lonergan, Troy Ott, and Thomas Spencer.
