Description of two types of mammosomatotropes in mink (Mustela vison) adenohypophysis: changes in the population of mammosomatotropes under different physiological conditions.

The present study was undertaken to clarify the existence of mammosomatotropes (MS cells) in the mink adenohypophysis and their possible involvement in the interconversion of mammotrope and somatotrope cells under different physiological conditions: prepubertal, pubertal and adulthood. Electron microscope immunocytochemistry was used to detect growth hormone (GH) and prolactin (PRL) immunoreactivities in the anterior pituitary gland of mink. Primary antisera raised in rabbit (human anti-GH, NIDDK-AFP-1613102481; human anti-PRL, NIDDK-AFP-55781789) were localized with appropriate species-specific antisera coupled to colloidal gold particles of different sizes. MS cells were most frequently observed in adult mink. Double labeling for GH and PRL was presented in two types of MS cells. MS1 cells, observed only in adults, showed an irregular morphology, with many cytoplasmic processes. Within their cytoplasm there were numerous rounded secretory granules of approximately 135 nm mean diameter. In MS1 cells three types of secretory granules were identified. The most numerous contained only PRL. The least frequent contained only GH and the third type contained GH and PRL and appeared in an intermediate quantity. MS2 cells, observed in all age groups, presented a rounded morphology. These cells had a voluminous cytoplasm which showed little development of the organelles and contained a large number of rounded and very electron-dense secretory granules which measured about 160 nm mean diameter. Within MS2 cells two types of secretory granules, according to their immunolabeling, were observed. Monohormonal granules containing GH were the more abundant while bihormonal granules, containing GH and PRL, were present to a lesser degree. Data presented in this report suggest that MS cells could play a role in the interconversion of somatotropes into mammotropes in mink.