[EN]Lactation is a physiological process associated
with hyperactivity of hypophyseal prolactin-producing
cells. It is known that the percentage of these cells is increased
during lactation, although there are discrepancies
in the reports regarding the mechanisms responsible for
increasing the number of prolactin cells. In order to analyse
whether this increase is a result of previous proliferation,
variations in the proliferation rate of prolactinpositive
cells were determined from late pregnancy to lactation
in adult female rats by means of observation of the
immunohistochemical expression of PCNA as a marker
of cellular proliferation. During late pregnancy, a very
significant increase in the percentage of proliferating prolactin
cells was observed in comparison to non-pregnant
females in the proestrus phase (p < 0.01). Although the
percentage of prolactin-positive cells after one week of
lactation was higher than in non-lactating or in pregnant
females (p < 0.01), the proliferation rate was lower than
in the other groups studied. In sum, our results suggest
that late pregnancy constitutes a preliminary proliferative
phase preparatory to the ensuing lactation phase and that
endocrine changes in late pregnancy involve the cellular
proliferation of hypophyseal prolactin cells in order to
prepare the gland for later demands and to prevent proliferative
changes from occurring during lactation