64
J. N. BALL AND BRIDGET I. BAKER
between the neurohypophysis and the pars intermedia, these two regions
being so intimately associated in fishes. In the eel, Knowles and Vollrath
( 1965a,b,c, 1966a) observed that terminations of neurosecretory fibers are
associated with extravascular channels bordering the pars intermedia,
these channels probably draining into blood capillaries. The pars intermedia cells appear to release their products into these extravascular
channels, and the authors suggested that the Type A fibers might also discharge into the channels and influence the activity of the pars intermedia
cells by this route. The same Type A fibers were also observed to make
synaptic contact with neighboring pituicytes, and these synapses increased
in eels briefly exposed to a white background. The pituicytes surround
the tubelike extensions of the third ventricle, and in these white-exposed
eels there was an increase in the PAS + ve material in these tubes. Because of these findings, it has been suggested that this PAS + ve material
may be secreted by the pituicytes to act as a feedback link between the
neurohypophysis and the NPO by way of the third ventricle (Knowles
and Vollrath, 1965c, 1966a). This is an interesting idea, but it requires
more evidence before it can be considered as more than a working
hypothesis; some support comes from observations in higher vertebrates,
in which an association between neurons and ependymal cells, and secretion by the ependymal cells into the third ventricle, have been observed
(Nishioka et al., 1964). At any rate, it is now becoming clear that pituicytes (ependymal and glial elements) may play a more important role
in neurosecretory control than was hitherto supposed (Bern and Knowles,
1966).
Sulmo and Perca exhibit an anatomical relationship between Type A
fibers and pars intermedia cells similar to that in the eel, but other teleosts
( Poecilia reticulatu, Xiphophorus, Phoxinus, and Tilupia ) show nerve
fibers ending directly on the pars intermedia cells, with no basement
membrane or extravascular channel in between (Follenius, 196313, l M a ,
b; Nishioka and Bern, 1967).
Apart from the eel, changes in background color or in illumination
have been found to alter the neurohypophysial structure in other teleosts.
In Zources, continuous illumination in April resulted in depletion of
AF + ve material and of ultrastructural granules from the cells of the
NPO, together with an increase in nuclear size, compared with animals
kept in the dark (Oztan, 1966b), although a similar experiment on
September fish gave different results, with no alterations in the nuclei
and accumulation of material in the cells. In Porichthys also, constant
illumination led to a depletion of A F + ve material from the NPO ( Sathyanesan, 1965a). Whether these changes related to alterations in pars intermedia activity and pigmentary conditions is not known.
J. N. BALL AND BRIDGET I. BAKER
between the neurohypophysis and the pars intermedia, these two regions
being so intimately associated in fishes. In the eel, Knowles and Vollrath
( 1965a,b,c, 1966a) observed that terminations of neurosecretory fibers are
associated with extravascular channels bordering the pars intermedia,
these channels probably draining into blood capillaries. The pars intermedia cells appear to release their products into these extravascular
channels, and the authors suggested that the Type A fibers might also discharge into the channels and influence the activity of the pars intermedia
cells by this route. The same Type A fibers were also observed to make
synaptic contact with neighboring pituicytes, and these synapses increased
in eels briefly exposed to a white background. The pituicytes surround
the tubelike extensions of the third ventricle, and in these white-exposed
eels there was an increase in the PAS + ve material in these tubes. Because of these findings, it has been suggested that this PAS + ve material
may be secreted by the pituicytes to act as a feedback link between the
neurohypophysis and the NPO by way of the third ventricle (Knowles
and Vollrath, 1965c, 1966a). This is an interesting idea, but it requires
more evidence before it can be considered as more than a working
hypothesis; some support comes from observations in higher vertebrates,
in which an association between neurons and ependymal cells, and secretion by the ependymal cells into the third ventricle, have been observed
(Nishioka et al., 1964). At any rate, it is now becoming clear that pituicytes (ependymal and glial elements) may play a more important role
in neurosecretory control than was hitherto supposed (Bern and Knowles,
1966).
Sulmo and Perca exhibit an anatomical relationship between Type A
fibers and pars intermedia cells similar to that in the eel, but other teleosts
( Poecilia reticulatu, Xiphophorus, Phoxinus, and Tilupia ) show nerve
fibers ending directly on the pars intermedia cells, with no basement
membrane or extravascular channel in between (Follenius, 196313, l M a ,
b; Nishioka and Bern, 1967).
Apart from the eel, changes in background color or in illumination
have been found to alter the neurohypophysial structure in other teleosts.
In Zources, continuous illumination in April resulted in depletion of
AF + ve material and of ultrastructural granules from the cells of the
NPO, together with an increase in nuclear size, compared with animals
kept in the dark (Oztan, 1966b), although a similar experiment on
September fish gave different results, with no alterations in the nuclei
and accumulation of material in the cells. In Porichthys also, constant
illumination led to a depletion of A F + ve material from the NPO ( Sathyanesan, 1965a). Whether these changes related to alterations in pars intermedia activity and pigmentary conditions is not known.