Types Of Pinealocytes

According to structural and ultrastructural criteria vertebrate pinealocytes have been subdivided into three major categories: true pineal photoreceptors, modified pineal photoreceptors and pinealocytes sensu stricto (11,59; cf. 43). True pineal pho-toreceptors are restricted to anamniotes, modified pineal photoreceptors are predominantly found in lacertilian and avian species and pinealocytes sensu stricto form the parenchyma of the mammalian and also the ophidian pineal gland.

2.1. True Pineal Photoreceptors

True pineal photoreceptors bear an outer segment that protrudes into the pineal lumen and consists of numerous disks produced by successive basoapical invaginations of the plasma membrane (Figure 1). Depending on the species, the number of outer segment disks varies between 10 and 300. The outer segment is connected to the inner segment via a cilium of the 9 x 2 + 0 type. Opposite to the outer segment, the pineal photoreceptor gives rise to a basal process originating from the perikaryon and contributing to prominent intrapineal neuropil formations. Its enlarged terminals contain numerous electron lucent synaptic vesicles intermingled with synaptic ribbons and scattered dense core vesicles (Fig. 1). Via these terminals, the true pineal photore-ceptors establish synapses with intrapineal second-order neurons. Adjacent photore-ceptor cells are connected via gap junctions, suggesting that they are electrically coupled (cf. 19).

By means of immunocytochemical and biochemical investigations it was shown that pineal photoreceptors contain molecules of the phototransduction cascade which are very closely related to or even identical with those expressed by retinal photore-ceptors. Thus, immunoreaction for rod-opsin, the proteinous component of the rod visual pigment rhodopsin has been found in the outer segments of many pineal pho-toreceptors in lamprey (77), teleosts (85), frogs (84) and certain reptiles (81). In the pineal of the clawed toad and the agamid lizard (Uromastix hardwicki) very few or no rod-opsin immunoreactive pineal outer segments were found (26,32). These species, however, have pineal photoreceptors that react with antibodies raised against chicken cone-opsin. Cone-opsin immunoreactive outer segments are also found in ranid frogs, but they are less frequent than the rod-opsin immunoreactive outer segments. To date




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