PHOTORECEPTORS IN THE AMPHIBIAN FRONTAL ORGAN

Abstract

The frontal organ (stirnorgan or Stieda's organ) of frogs and toads has long been a physiological enigma, as has likewise its reptilian homologue, the parietal eye. Some investigators have suggested that the stirnorgan is sensory,l others that it is secretory;2^ 3 still others4 5 have found no evidence of either function. This study of the frontal organ in the tadpole of the Pacific Treefrog, Hyla regilla, indicates that it is photoreceptive. Ultrastructure.-Electron microscopy of the stirnorgan of young larvae of H. regilla by Eakin and Westfall6 reveals processes, extending into its tiny lumen, which are structurally similar to those of rods and cones in the retina of the lateral eye and to those recently discovered in the parietal eye of lizards.7-' Figure 1 shows a longitudinal section of one of these processes. The outer segment (os) contains an array of about 135 discs (d) or laminae formed apparently by infoldings of the surface membrane (see arrows). In this feature the outer segment resembles that of cones in the lateral eye of the anuran Xenopus laevis.ll Many of the processes, such as the one figured, are short, stout, and bent in contradistinction to the long, straight filamentous outer segments of rods and cones of the vertebrate lateral eye and of receptors in the parietal eye. The inner segment (is), of which the distal part only is shown in Figure 1, contains many large elongate mitochondria (m) densely packed near the distal end of the segment as they are in the comparable region, the ellipsoid, of known photoreceptors; lipid droplets which are sometimes very large; osmiophilic granules (g, Fig. 2); and vesicles (v) of diverse sizes, some (see Fig. 2) reaching a diameter of 1A and containing material (secretion?) slightly more electron dense than the surrounding cytoplasm. The connecting piece, a short, narrow part of the process joining inner and outer segments, is not shown in Figure 1 because the plane of section is to one side of it. Figure 2, however, illustrates the connecting piece (cp) of another process. A fibrillar apparatus, presumably derived from that of a cilium of an ependymal cell, arises from a basal granule or axial centriole (cl) situated at the tip of the inner segment, passes through the connecting piece, and fans out into the clear matrix (s) of the outer segment. The apparatus is composed of nine peripheral fibrils (count based upon other micrographs), parts of which may be observed (f, Fig. 2). Adjacent to the axial centriole lies the oblique centriole, a short cylinder which appears in cross section (c,, Fig. 2) to consist of nine peripheral fibers. Stirnorganectomy.-Although the fine structure of the processes of the frontal organ just described is remarkably similar to that of rods and cones, evidence of light sensitivity is needed before the cells which bear them are designated photoreceptors. Bagnaral2 recently observed that the destruction of the epiphysis in tadpoles of Xenopus laevis eliminates the blanching reaction of the skin (concentration of pigment in the dermal melanophores of the body) commonly exhibited by amphibian larvae when placed in the dark. The frontal organ is known to be connected to the epiphysis by a nerve or pineal tract. -4 '3 I have traced it in larvae of H. regilla