Hypophysial and meningeal melanocytes in the Zucker rat

Melanocytes have not been described in the pituitary of mammals or in the m eninges of the rat. In this paper, we report the presence of the cluster of melanocytes in the intermediate lobe of the pituitary and around the media n eminence of the hypothalamus forming an 'infundibulo-hypophysial circle', and also describe the characteristics of meningeal melanocytes in Zucker r ats, In the leptomeninges, numerous melanocytes were found on the ventrolat eral surface of cerebral hemisphere in the area of the middle cerebral arte ry. Pigment granules were also observed in the surrounding tissue outside t he melanocytes as well as incorporated in the cytoplasm of neural and epith elial cells. Electron microscopy revealed that melanosomes in hypophysial a nd meningeal melanocytes were in different (II-IV) stages of maturity. In t he leptomeninges of Zucker rats, HMB-45 immunoreactivity was found in round non-melanosome-containing cells, while no HMB-45 reaction was found in the leptomeninges of the albino rat. We conclude that both obese and lean Zuck er rats possess functionally active melanocytes in the meninges and the pit uitary and transfer pigment granules to neighboring cells. The distribution s of melanocytes in proximity to blood vessels in the leptomeninges and in the 'infundibulo-hypophysial circle' suggest an endocrine secretory functio n.