Electron-microscopic investigation of the innervation of the pars distalis of the hypophysis in adult Rana temporaria L.

In the pars distalis of the hypophysis of adult Rana temporaria, three types of nerve-fiber profiles were found at two distinct sites, in both lateral parts of the bordering regions of the anterior lobe with the intermediate lobe of the hypophysis. The first type of nerve-fiber profile consists of bundles of very fine axonal elements (diameter: less than 0.7 micron). The second type is formed by larger nerve fibers (diameter up to 4 microns) containing a few neurosecretory granules of approximately 100 nm. The third type of nerve-fiber profile resembles the second type but these nerve fibers make synaptoid contacts on at least two different types of glandular cells. The possible functional significance of these nerve fibers in the pars distalis is discussed. No nerve fibers were found in the central part of the bordering region of the pars distalis with the intermediate lobe, at the bordering region with the median eminence and with the neurohypophysial stalk, and in all other parts of the pars distalis.