In the early stages of embryonic development, many growth-promoting
molecules must be provided by the
maternal system. The molecules involved in growth processes may be either
hormones or growth factors, or
molecules that interact with such factors. The pregnancy related hormone,
prolactin (PRL, MW 23 kDa) has
been implicated in the control of embryonic growth. The growth-promoting
potential of PRL and its
species-specificity was investigated by culturing 9.5 d rat embryos in
vitro
for 48 h in depleted serum in the
presence and absence of PRL from 3 different species. The growth-supporting
capacity of the serum was
reduced by removal of low molecular weight molecules by prolonged filtration
of the serum using filters with
a molecular weight exclusion of 30 kDa. This method provided a
‘semidefined’ medium (retenate) in which
embryonic growth and development was significantly reduced, demonstrating
that
the low molecular weight
fraction of serum may contain some growth-promoting factors. Addition of
PRL
(0.4–25.6 ng/ml) from
different species (human, sheep and rat) to retenate significantly improved
embryonic growth and
development, suggesting that the developing embryo may utilise PRL. Amongst
PRLs, rat PRL was found
to be active at much lower concentrations than either of the other molecules,
and human PRL had more
effect in low concentrations than sheep PRL suggesting a species-specificity
for this hormone. It may be that
the PRL receptors of the rat embryos have greater affinity for the rat
hormone as different responses for
hormones from different species have been shown. These findings suggest
that embryos may be able to
utilise maternally derived PRL during organogenesis.