Sn. Lennard et al., INSULIN-LIKE GROWTH-FACTOR-II GENE-EXPRESSION IN THE FETUS AND PLACENTA OF THE HORSE DURING THE FIRST HALF OF GESTATION, Journal of Reproduction and Fertility, 103(1), 1995, pp. 169-179
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.