Immunohistochemical study of the cytogenesis of prolactin and growth hormone cells in the anterior pituitary gland of the fetal rat.

The objectives of this study were to determine the first developmental stage when immunoreactive prolactin (PRL) cells appear in the fetal rat anterior pituitary, and then to compare their cytogenesis with that of growth hormone (GH) cells. Immunoreactive PRL cells first appeared on Day 18.5 of gestation, the same stage when GH cells were found to differentiate in the pituitary. During the fetal period, PRL cells were concentrated mainly in the anterior half of the pituitary, whereas GH cells were found predominantly in its posterior half. When estimated as the total sum of immunoreactive areas, the proportion of PRL cells remained low during fetal life in contrast to a marked increase in the GH immunoreactive area. The flip-flop (mirror) section technique revealed that, in the fetal rat, adenohypophysis PRL and GH are contained in different cells. The present study thus indicates that GH and PRL accumulation occurs in independent cells at least in early developmental stages in the rat.