INSULIN-LIKE GROWTH-FACTOR-II GENE-EXPRESSION IN THE FETUS AND PLACENTA OF THE HORSE DURING THE FIRST HALF OF GESTATION

Placentation in equids involves two types of trophoblast: a minor inva sive component, the chorionic girdle, that gives rise to transient end ocrine structures known as endometrial cups, and a major non-invasive component, the allantochorion, that forms the diffuse, microcotyledona ry 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 e xpression of insulin-like growth factor II (IGF-II) in the fetus and p lacenta of the horse (Equus caballus), using 12 conceptuses recovered between 14 and 150 days of gestation (term is about 340 days). The ant i-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 hybridizati on demonstrated that the IGF-II gene was expressed intensely in the fe tus at all stages examined, predominantly in tissues of mesodermal ori gin, but also in the endoderm-derived liver and epithelia of the gut a nd lung bronchioles, and the ectoderm-derived facial mesenchyme and ch oroid plexus. High concentrations of IGF-II mRNA were also detected in the extraembryonic mesoderm, invasive chorionic girdle and mature end ometrial cup tissue. There was no signal in the maternal endometrium a t any stage of pregnancy. Higher concentrations of IGF-II message in t he invasive chorionic girdle than the non-invasive trophoblast at the time of invasion suggests that IGF-II promotes invasiveness of trophob last, as proposed in humans. Furthermore, increased concentrations in the allantochorion after implantation suggest a possible role in the d evelopment of the epitheliochorial placenta.