The mammary gland is an example of a tissue of epidermal origin
that
depends for the development of its characteristic morphology on underlying
mesenchymal cells. The interaction between mesenchyme and epithelium appears
to
be mediated by polypetide growth factors. In situ hybridization
has been used to
study, in the mammary gland of female sheep fetuses, the distribution of
mRNA for
the mammary mitogens, insulin-like growth factor (IGF)-I and IGF-II, and
the IGF-I
receptor, from 10 to 20 weeks of intrauterine life (term is ∼22 weeks).
At 10 weeks,
secondary ducts had formed from the primary duct. By week 20, the gland
had
increased in volume and complexity, showing primitive lobules embedded
in
intralobular connective tissue disposed around main ducts. IGF-I and IGF-II
mRNA were expressed in cells of the intralobular connective tissue underlying
the
epithelium, while the IGF-I receptor was expressed in epithelium. Quantitation
by
absorbance measurements showed that mRNA expression increased with pregnancy
stage for IGF-I and IGF-II, but not significantly for the IGF-I receptor,
and that
IGF-II was more highly expressed than IGF-I. A role for the IGF system
in
mediating mesenchymal–epithelial interactions in mammary development
is indicated.