Pituitary lactotrophs and somatotrophs in pregnancy: a correlative in situ hybridization and immunocytochemical study.

Lactotroph hyperplasia is a prominent finding in the adenohypophyses of pregnant women. In order to elucidate the morphogenesis of this change, pituitaries from 16 women in various phases of pregnancy were collected at autopsy and studied by histology, immunocytochemistry and in situ hybridization. The results showed that the increase in the amount of prolactin (PRL) mRNA paralleled the progressive lactotroph hyperplasia. The presence of mitoses in PRL-immunoreactive cells provided evidence that proliferation of preexisting lactotrophs contribute to lactotroph accumulation. Growth hormone (GH) immunoreactive cells showed a marked reduction in GH mRNA indicating that GH synthesis was inhibited. In many GH-immunoreactive cells, PRL mRNA became apparent. These findings demonstrate that GH is stored following discontinuation of GH synthesis. It appears that, when PRL is secreted in excess during pregnancy, somatotrophs are recruited to produce PRL. These somatotrophs begin to express PRL mRNA, transform to bihormonal mammosomatotrophs and possibly later to lactotrophs, contributing to PRL production. Mature somatotrophs may be regarded as reserve cells in the adenohypophysis, having the potential to switch hormone synthesis and to convert to mammosomatotrophs and possibly lactotrophs.