Neurotrophin receptors (Trk A, Trk B, and Trk C) in the developing and adult human retina.

In this study, the ontogeny and distribution patterns of three neurotrophin receptors (Trk A, Trk B, and Trk C) were examined in the human retinas. Immunohistochemistry was performed on sections of retina and optic nerve from fetuses (11-24 weeks of gestation, wg), one infant (4-month-old) and two adult (35- and 65-years-old) subjects. At 11 wg, Trk A was expressed in the nerve fiber and inner plexiform layers, while Trk B and Trk C were expressed in many neuroblastic cells. By 16-17 wg, the photoreceptors showed immunoreactivity for all three receptors. The ganglion cell layer and amacrine cells were conspicuously immunoreactive for Trk A and Trk C, but labeled diffusely for Trk B. The horizontal cells were labeled for Trk A and Trk B. The pattern was same in the retinas at midgestation (20-21 wg). Shortly after this period, there was an apparent decrease in receptor immunoreactivity in the fetal retinas. In the infant retina, Trk A immunoreactivity was absent from horizontal cells. The photoreceptors were immunopositive for Trk B and Trk C, in infant and adult retinas. In the adults, few cells of the ganglion cell layer and inner nuclear layer were clearly labeled for Trk A and Trk C, and diffusely for Trk B. The glial cells of the retina and optic nerve immunoreacted for Trk A only, right from fetal 16 wg. The early expression of Trk B and Trk C on neuroblastic cells suggests that both play a role in cell proliferation. The developmental distribution pattern of Trk A, on the other hand, provides evidence for its involvement in differentiation of the inner plexiform layer, horizontal cells and neuroglia. The results strongly suggest that photoreceptor development is mediated by Trk receptors. The novel localization of Trk B and Trk C on adult photoreceptors points to a possible therapeutic potential for BDNF and NT-3, respectively, in photoreceptor diseases.