Ultrastructure of the stellate cell in the pars intermedia of the frog, Rana pipiens.

Stellate cells in the pars intermedia of adult Rana pipiens black-and-white background adapted for 14 days were studied by electron microscopy. This cell type forms a capsule around the secretory cells at the surface of the gland; it also lines the perivascular spaces between the pars intermedia and pars nervosa. The cells are characterized by the lack of well-developed secretory organelles and by the presence of long procesees which extend into the intercellular spaces of the secretory cells and reach the surface of the gland and the perivascular spaces. Occasional desmosomes link the stellate cells to adjacent cells; bundles of microfilaments occur frequently. Microtubules run parallel to the long axis of the cell processes. In light of the possibility that the stellate cells might be involved in transfer of substances to and from the melanophore-stimulating hormone (MSH) secretory cells, they were studied for indications of membrane activity. The stellate cell plasma membranes abutting the pericapillary spaces showed intense micropinocytotic vesicle activity. Also at the gland surface the electron-lucent vesicles were common at the plasma membranes of the stellate cells. Along the plasma membranes of the stellate cell processes extending between the secretory cells were vesicles which were somewhat electron-dense. This membrane activity would suggest an active role for these cells in addition to a sustentacular role.