No abstract

Previously, immunoreactive rod-opsin and S-antigen (arrestin), two highly characteristic markers of retinal photoreceptors and pinealocytes, were shown to be present in certain medulloblastoma cells. It, thus, has been suggested that such cells differentiate along the photoreceptor lineage. This is corroborated in the present immunocytochemical investigation using antibodies against another photoreceptor-cell marker, the interphotoreceptor retinoid-binding protein (IRBP). As shown in preparations of human retina and pineal organ, IRBP can be successfully demonstrated in formalin-fixed and paraffin-embedded tissue: the IRBP immunoreaction is located to the outer and inner segments of retinal photoreceptor cells and to perikarya of certain pinealocytes. Examination of formalin-fixed, paraffin-embedded biopsy specimens of 66 cerebellar medullo-blastomas revealed varying numbers of IRBP-immuno-reactive tumor cells in 19 cases that were formerly shown to contain rod-opsin and S-antigen immunoreaction. IRBP-immunoreactive tumor cells were also found in a retinoblastoma and a pineocytoma, but not in neuroblastoma, ganglioneuroblastoma, glioblastoma, oligodendroglioma and astrocytoma. The results indicate: (1) cerebellar medulloblastomas are heterogeneous in their differentiation potential; (2) one type of medulloblastoma displays photoreceptor characteristics; (3) this type appears to be closely related to retinoblastoma and pineal cell tumors; and (4) all three types of tumors may display additional common features to be explored in future studies.

In Xenopus laevis Daud., the ontogenetic occurrence of two photoreceptor-specific proteins, S-antigen and rod-opsin, was investigated and correlated to the maturation of the neurohormonal effector system involved in melatonin-dependent color-change mechanisms. Tadpoles ranging from stage 12 to 57 (Nieuwkoop and Faber 1956) were fixed in Zamboni's or Bouin's solution. Frozen or paraffin sections of either total heads or dissected brains and eyes were prepared and treated with highly specific antisera against S-antigen and rod-opsin. In the retina, immunoreactive S-antigen and rod-opsin were first demonstrated in a few centrally located photoreceptors at stage 37/38. Photoreceptors of the peripheral (iridical) portions of the retina gradually became immunoreactive during further development. As in the retina, the first S-antigen-immunoreactive photoreceptors in the pineal complex appeared at stage 37/38. At this and all later stages investigated rod-opsin immunoreactivity was restricted to a few dot-like structures resembling developing pineal outer and inner segments. In most animals rod-opsin immunoreactivity was completely absent from the pineal complex. The analysis of retinal proteins with the immunoblotting technique (Western blot) revealed that the S-antigen antibody bound to a 48-kDa protein and the rod-opsin antibody to a 38-kDa protein. The body lightening reaction was determined with the aid of the melanophore index in larvae fixed in light or darkness, respectively. Aggregation of melanophore melanosomes in darkness (the melatonin-dependent primary chromatic response) first occurred at stage 37/38 when melanophores started to differentiate and became pigmented. These results indicate that in Xenopus laevis (i) the molecular mechanisms of photoreception develop simultaneously in retina and pineal complex; (ii) most pineal photoreceptors differ from retinal rods in that they contain immunoreactive S-antigen but essentially no immunoreactive rod-opsin; and (iii) the differentiation of phototransduction processes coincides with the onset of melatonin-dependent photoneuroendocrine regulation of color-change mechanisms.