Evidence that lactotrophs do not differentiate directly from somatotrophs during chick embryonic development

It is generally accepted that, in mammals, lactotrophs differentiate from somatotrophs through an intermediate cell type, the mammosomatotroph. However, little information exists about mammosomatotrophs and their relationship with lactotroph development in non-mammalian vertebrates. We reported previously that corticosterone (CORT) can induce both somatotroph and lactotroph differentiation in cultures of chicken embryonic pituitary cells. Our current objectives were to determine the abundance of mammosomatotrophs during chicken pituitary development, to identify mammosomatotrophs during CORT induction of lactotrophs, and to explore whether lactotrophs induced by CORT are derived from somatotrophs. Cells that produced prolactin (PRL) only, growth hormone (GH) only or both hormones simultaneously were detected by three approaches - dual immunofluorescence, a combination of immunofluorescence and immunocytochemistry (ICC), and by ICC using combinations of antibodies to GH and PRL. Mammosomatotrophs were not detected between embryonic day (E) 16 and E20, even though lactotrophs increased from nearly absent to greater than 10% of all pituitary cells during this period. CORT induced more than 10% of all E13 pituitary cells to produce PRL, while the percentage of mammosomatotrophs remained at less than 1% of all cells. When cells from the cephalic and caudal lobes of the anterior pituitary were treated separately, CORT increased GH cells in cultures from the caudal lobe. No PRL cells were found in the caudal lobe. In the cephalic lobe, CORT increased lactotrophs, while GH cells were barely detected. In summary, mammosomatotrophs are rare during chicken pituitary development, and CORT does not induce lactotrophs from somatotrophs. These findings indicate that, unlike in mammals, lactotrophs do not differentiate from somatotrophs during chicken embryonic development.