Insulin-like growth factor II gene expression in the fetus and placenta of the horse during the first half of gestation
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Placentation in equids involves two types of trophoblast: a minor invasive component, the chorionic girdle, that gives rise to transient endocrine structures known as endometrial cups, and a major non-invasive component, the allantochorion, that forms the diffuse, microcotyledonary placenta. Growth factors are likely to be important in controlling these complex events at implantation and this study describes the use of in situ hybridization and northern blotting techniques to monitor expression of insulin-like growth factor II (IGF-II) in the fetus and placenta of the horse (Equus caballus), using 12 conceptuses recovered between 14 and 150 days of gestation (term is about 340 days). The anti-sense, but not the sense, ovine IGF-II oligonucleotide probe (45 mer) hybridized to a total of seven IGF-II mRNA transcripts (6.2–1.3 kb) in RNA extracted from horse fetal liver, demonstrating the specificity and validity of the probe for equine IGF-II mRNA. In situ hybridization demonstrated that the IGF-II gene was expressed intensely in the fetus at all stages examined, predominantly in tissues of mesodermal origin, but also in the endoderm-derived liver and epithelia of the gut and lung bronchioles, and the ectoderm-derived facial mesenchyme and choroid plexus. High concentrations of IGF-II mRNA were also detected in the extraembryonic mesoderm, invasive chorionic girdle and mature endometrial cup tissue. There was no signal in the maternal endometrium at any stage of pregnancy. Higher concentrations of IGF-II message in the invasive chorionic girdle than the non-invasive trophoblast at the time of invasion suggests that IGF-II promotes invasiveness of trophoblast, as proposed in humans. Furthermore, increased concentrations in the allantochorion after implantation suggest a possible role in the development of the epitheliochorial placenta.
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