Differentiation of both rod and cone types of photoreceptors in the in vivo and in vitro developing pineal glands of the quail

Abstract

The avian pineal is a photo-endocrinal organ and is considered to synthesize and secrete melatonin in an intrapineal rhythm which can be modified by direct light stimulation of the pineal photoreceptors. Since the avian retina contains numerous different types of photoreceptors, at least 6 types in the quail retina, it is interesting to ask how many types of photoreceptors are present in the avian pineal. In the present study, we have identified two types of photoreceptors in the quail pineal organ, one appears rod-like and the other cone-like, using an immunohistochemical method with highly specific anti-chicken rhodopsin and anti-iodopsin monoclonal antibodies. Rhodopsin-immunoreactive (Rho-I) cells were much larger in number than iodopsin-immunoreactive (Iodo-I) cells. During pineal development, Rho-I cells were first observed at embryonic day 13 (E13: 13 days of incubation), whereas Iodo-I cells were found at day E15. Rho-I cells showed numerous neurite-like processes, but Iodo-I cells had few, if any, processes. We developed a new culture system for avian pineal cell differentiation by seeding cells on nitrocellulose membrane filters. By this method both types of pineal photoreceptors differentiated in vitro: Rho-I cells were much larger in number and had much more fine processes than Iodo-I cells, similar to those seen in the intact developing pineal. With the new culture system the relation between pineal photoreceptor differentiation and sympathetic innervation was examined in vitro. Norepinephrine (NE) significantly suppressed in vitro differentiation of Rho-I cells, when pineals removed from E8 embryos were cultured in the presence of NE, but other transmitters, dopamine, acetylcholine and γ-aminobutyric acid, were not effective. As development proceeded, NE gradually became less effective. Since the sympathetic innervation to the quail pineal is initiated later than day E13, we suggest that most Rho-I cells are not disturbed by NE in their in vivo differentiation. This view differs substantially from rat pineals, which, as shown in our previous study, do not contain Rho-I cells in vivo but show many Rho-I cells in vitro when pineals removed from newborn rats are transferred to the culture condition.