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HOWARD A. BERN
thalamic system. Although there is no doubt of the importance of the
synthetic and packaging organelles of the perikaryon in the production of
neurosecretory granules, there is increasing morphological evidence for
the fabrication of granules in the axon itself. Tubular and reticular systems, not identical with neurofibrillar microtubules, have been encountered, even in the axon terminals (Fridberg et aZ., 1966a).
Once the granules reach the terminals, the processes involved in the
release of neurosecretory product remain uncertain. Slow release of contents from the membrane-limited granule is suggested by electron microscope evidence. Fragmentation of the granule envelope into small vesicles, synapticlike in appearance, is also a possibility. These vesicles may
then fuse with the axon membrane to release their contents by an exocytic
process. The fusion of the large electron-dense granules with the axon
membrane has also been visualized, with ejection of their contents into
the basement membrane region. It has also been proposed (Oota, 1963)
that the small vesicles are in fact synaptic (i.e., cholinergic). This is a
recurrent debate in the field of neurosecretion; however, in this instance
there appeared to be a pharmacological basis for the contention (Kobayashi et al., 1963).
What role impulse conduction plays in secretion discharge can only
be conjectured at present (Fridberg et aZ., 1966b). However, depolarization of the terminal membrane should certainly facilitate release of axonal contents. It is also possible that electrical activity of the membrane
has "electrophoretic" influences on the granules and vesicles themselves,
increasing the likelihood of their contact with the membrane.
IV. PHYSIOLOGY
The obvious structural analogy between the caudal neurosecretory
system in the posterior spinal cord of teIeosts and the cranial neurosecretory system in the hypothalamus has biased investigations oriented toward
functional analysis of the urophysis since Enami's first observations. In
view of the antidiuretic action of many neurohypophysial octapeptides,
an attempt has been made to ascribe an osmoregulatory action to the
urophysis. In fact, the majority of the data supporting such a role is at
best equivocal and in part negative. Nevertheless, the general area of
water and ion metabolism remains a major focus of what little active investigation is now being pursued, and it may well emerge that the caudal
system is an auxiliary osmoregulatory device, of significance under special
circumstances. It has been suggested that it may provide a means for
HOWARD A. BERN
thalamic system. Although there is no doubt of the importance of the
synthetic and packaging organelles of the perikaryon in the production of
neurosecretory granules, there is increasing morphological evidence for
the fabrication of granules in the axon itself. Tubular and reticular systems, not identical with neurofibrillar microtubules, have been encountered, even in the axon terminals (Fridberg et aZ., 1966a).
Once the granules reach the terminals, the processes involved in the
release of neurosecretory product remain uncertain. Slow release of contents from the membrane-limited granule is suggested by electron microscope evidence. Fragmentation of the granule envelope into small vesicles, synapticlike in appearance, is also a possibility. These vesicles may
then fuse with the axon membrane to release their contents by an exocytic
process. The fusion of the large electron-dense granules with the axon
membrane has also been visualized, with ejection of their contents into
the basement membrane region. It has also been proposed (Oota, 1963)
that the small vesicles are in fact synaptic (i.e., cholinergic). This is a
recurrent debate in the field of neurosecretion; however, in this instance
there appeared to be a pharmacological basis for the contention (Kobayashi et al., 1963).
What role impulse conduction plays in secretion discharge can only
be conjectured at present (Fridberg et aZ., 1966b). However, depolarization of the terminal membrane should certainly facilitate release of axonal contents. It is also possible that electrical activity of the membrane
has "electrophoretic" influences on the granules and vesicles themselves,
increasing the likelihood of their contact with the membrane.
IV. PHYSIOLOGY
The obvious structural analogy between the caudal neurosecretory
system in the posterior spinal cord of teIeosts and the cranial neurosecretory system in the hypothalamus has biased investigations oriented toward
functional analysis of the urophysis since Enami's first observations. In
view of the antidiuretic action of many neurohypophysial octapeptides,
an attempt has been made to ascribe an osmoregulatory action to the
urophysis. In fact, the majority of the data supporting such a role is at
best equivocal and in part negative. Nevertheless, the general area of
water and ion metabolism remains a major focus of what little active investigation is now being pursued, and it may well emerge that the caudal
system is an auxiliary osmoregulatory device, of significance under special
circumstances. It has been suggested that it may provide a means for