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Biology Articles » Developmental Biology » Animal Development » Implantation mechanisms: insights from the sheep » Figures

Figures
- Implantation mechanisms: insights from the sheep

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Figure 1 Early pregnancy events in sheep. This schematic summarizes the relative changes in embryo/blastocyst development after fertilization in relation to position in the female reproductive tract and circulating levels of ovarian steroid hormones. Fertilization occurs in the oviduct, and the morula stage embryo enters the uterus on day 4. The blastocyst is formed by day 6 and hatches from the zona pellucida on days 8–9. The blastocyst develops from a spherical to a tubular form by day 11 and then elongates to a filamentous conceptus between days 12 and 16. The elongation of the blastocyst marks the beginning of implantation, which involves apposition and transient attachment (days 12–15) and firm adhesion by day 16.

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Figure 2 Apposition and adhesion phases of blastocyst implantation in sheep. (A) Preattachment involving shedding of the zona pellucida (phase 1) and precontact and blastocyst orientation (phase 2). The first phase of implantation is the shedding of the zona pellucida on day 8 that exposes the trophoblast. The second phase of implantation is before contact and involves the blastocyst orientation which occurs on days 9–11 as the blastocyst grows from a spherical to a tubular conceptus and migrates to the middle region of the uterine horn ipsilateral to the corpus luteum. The antiadhesive mucin MUC-1 is present on the endometrial LE, thereby preventing contact of the trophoblast with adhesive receptors such as integrins. Histotroph, secreted from the endometrial LE and GE, nourishes the developing blastocyst. (B) Apposition and transient attachment (phase 3). After day 11, the tubular blastocyst elongates to form a filamentous conceptus. During this period, expression of MUC-1 declines on the LE, which exposes constitutively expressed integrins on the LE as well as trophoblast. Apposition occurs between the trophoblast and endometrial LE and between the trophoblast papillae and GE ducts. Elongation of the blastocyst probably requires apposition and transient attachment to the endometrial LE. (C) Adhesion (phase 4). Firm adhesion of the mononuclear cells of the trophoblast to the LE occurs between days 15 and 16. Available evidence indicates that several molecules (GlyCAM-1, galectin-15 and osteopontin) interact with receptors (integrins and glycoconjugates) on the apical surfaces of the trophoblast and LE to facilitate adhesion. The binucleate cells (BNC) differentiate from the mononuclear trophoblast and then migrate to and fuse with the LE to form a multinucleated syncytial plaque. Although the BNC are inherently invasive, they do not cross the basal lamina that supports the LE.

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Figure 3 Possible interactions between trophoblast and endometrial epithelial cells regulating adhesion during implantation. Evidence for these interactions are from in vivo and in vitro studies in sheep, mice and humans. (A) Homotypic binding of GlyCAM-1 by L-selectin. GlyCAM-1 functions as a carbohydrate ligand for the lectin domain of L-selectin. (B) Homotypic and heterotypic binding of glycans on proteins and/or lipids via the multifunctional, secreted bridging ligand galectin-15. There is evidence that galectin-15 can bind to a large number of proteins and lipids, including mucins and integrins, which contain ß-galactosides, using a carbohydrate recognition domain. (C) Homotypic and heterotypic integrin-mediated adhesion via the bifunctional, secreted bridging ligand OPN. There is evidence that OPN can homodimerize and bind integrins in a RGD-dependent and RGD-independent manner.

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Source: Reproduction (2004) 128 657-668. © 2004 Society for Reproduction and Fertility

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