Kivonat:
The breeding of birds is expected to solve problems of nourishment for
the growing human population. The function of the pineal organ
synchronizing sexual activity and environmental light periods is
important for successful reproduction. Comparative morphology of the
avian pineal completes data furnished by experiments on some frequently
used laboratory animals about the functional organization of the organ.
According to comparative histological data, the pineal of vertebrates
is originally a double organ (the "third" and the "fourth eye"). One of
them often lies extracranially, perceiving direct solar radiation, and
the other, located intracranially, is supposed to measure diffuse
brightness of the environment. Birds have only a single pineal,
presumably originating from the intracranial pineal of lower
vertebrates. Developing from the epithalamus, the avian pineal organ
histologically seems not to be a simple gland ("pineal gland") but a
complex part of the brain composed of various pinealocytes and neurons
that are embedded in an ependymal/glial network. In contrast to organs
of "directional view" that develop large photoreceptor outer segments
(retina, parietal pineal eye of reptiles) in order to decode
two-dimensional images of the environment, the "densitometer"-like
pineal organ seems to increase their photoreceptor membrane content by
multiplying the number of photoreceptor perikarya and developing
follicle-like foldings of its wall during evolution ("folded retina").
Photoreceptor membranes of avian pinealocytes can be stained by
antibodies against various photoreceptor-specific compounds, among
others, opsins, including pineal opsins. Photoreceptors immunoreacting
with antibodies to chicken pinopsin were also found in the reptilian
pineal organ. Similar to cones and rods representing the first neurons
of the retina in the lateral eye, pinealocytes of birds posses an
axonal effector process which terminates on the vascular surface of the
organ as a neurohormonal ending, or forms ribbon-containing synapses on
pineal neurons. Serotonin is detectable immunocytochemically on the
granular vesicles accumulated in neurohormonal terminals. Pinealocytic
perikarya and axon terminals also bind immunocytochemically
recognizable excitatory amino acids. Peripheral autonomic fibers
entering the pineal organ through its meningeal cover terminate near
blood vessels. Being vasomotor fibers, they presumably regulate the
blood supply of the pineal tissue according to the different levels of
light-dependent pineal cell activity. Microsc. Res. Tech. 53:12-24,
2001. (C) 2001 Wiley-Liss, Inc.
