[An ultrastructural study on squamous cell nests of the human pituitary]

The squamous cell nests which appear to have developed in the base of pars tuberalis are studied electron microscopically. A fresh pituitary was obtained from a 59-year-old white woman, who underwent hypophysectomy to reduce the pain due to the breast cancer. Each squamous cell nest was completely enclosed by the basal lamina, consisted of squamous cells, and sometimes contained adenohypophyseal parenchymal cells. The squamous cells were classified into cells in the basal layer and those in the spinous layer with intercellular bridges. Most of the former cells (B cells) possessed a nuclear membrane of strong interdigitation, well-developed desmosomes and tonofibrils. Adjacent to the squamous cell nests, there were acini which consisted of adenohypophyseal parenchymal cells, a few agranulated stellate cells, follicular cells in rare cases. As no transitional types between the B cells and the adenohypophyseal parenchymal cells were observed, B cells may not be said that they have been formed through the direct metaplasia of existing adenohypophyseal parenchymal cells. In view of the morphological similarities of the earliest B cells to the stellate cells, of the close positional relationship between the squamous cell nests and the acini and between the adenohypophyseal parenchymal cells and the squamous cells, and of the past reports that squamous cell nests increase in number with aging, B cells appear to originate from the stellate cells.