2. THE NEUROHYPOPHYSIS
141
principles have been so frequently doubtful or negative, even when extraordinarily high doses of principles have been used, it is reasonable to
consider the possibility that they have only local effects within the pituitary. The histological evidence given previously suggests that it would be
possible for neurosecretory materials to pass from the median eminence
through the portal system to the adenohypophysis, where they might
perform a regulatory function. Since each of the species examined by
Meurling (1967a) possessed either a component of the portal system
which reached the neurointermediate lobe or a neural lobe sinus which
could facilitate the passage of substances from the pars nervosa to the
pars intermedia, it appears possible that the neurohypophysis could influence the pars intermedia by a vascular route. Besides this, nerve axons
pass directly to the pars intermedia, where they form terminals on its cells
( e.g., Mellinger, 1963b; Polenov and Belenki, 1965). Mellinger ( 1963a)
has shown that damage to the preoptico-hypophysial tract of Scylwrhinus
caniculus is always followed by melanophore dispersion and a consequent
darkening of the skin. This is explained by the removal of an inhibitory
action on the intermedia cells. With the removal of this inhibition, the
pars intermedia cells liberate melanocyte stimulating hormone ( MSH )
into the circulation, and this hormone causes dispersion of pigment
throughout the melanocytes of the skin. From Mellinger’s experiments, it
seems possible that the inhibitory influence is mediated by the neurohypophysis, and it is worth remembering that Knowles (1963, 1965a) has
distinguished individual types of neurosecretory fibers which pass either
to the “synthetic” or to the “release” poles of the intermedig cells. Moreover, Knowles (1965b) has noted that secretomotor junctions are formed
between the neurosecretory fibers and areas of the pars intermedia cells
which are distinguished by a sharply delimited endoplasmic reticulum.
There was some degree of correlation between the state of the reticulum
and signs of hormone liberation from the neurosecretory fibers. Knowles
suggests that this would be compatable with the view that a neurohypophysial principle directly affects MSH synthesis in the cytoplasm of the
intermedia cell, in the region of the endoplasmic reticulum. Despite these
histological indications, it cannot be certain that the inhibitory effect
results from neurosecretory fibers or their peptides, since nonneurosecretory axons are also present in the preoptico-hypophysial tract. Indeed, in
recent work, Meurling et al. (1969) have succeeded in preferentially
cutting the neurosecretory fibers of the infundibular stem of Raiu radiuta,
a species in which the neurosecretory axons are localized in a medial position. They found that the release of MSH was little affected. However,
section of the lateral, possibly adrenergic nonneurosecretory axons caused
profound effects in which color responses to a white background were
141
principles have been so frequently doubtful or negative, even when extraordinarily high doses of principles have been used, it is reasonable to
consider the possibility that they have only local effects within the pituitary. The histological evidence given previously suggests that it would be
possible for neurosecretory materials to pass from the median eminence
through the portal system to the adenohypophysis, where they might
perform a regulatory function. Since each of the species examined by
Meurling (1967a) possessed either a component of the portal system
which reached the neurointermediate lobe or a neural lobe sinus which
could facilitate the passage of substances from the pars nervosa to the
pars intermedia, it appears possible that the neurohypophysis could influence the pars intermedia by a vascular route. Besides this, nerve axons
pass directly to the pars intermedia, where they form terminals on its cells
( e.g., Mellinger, 1963b; Polenov and Belenki, 1965). Mellinger ( 1963a)
has shown that damage to the preoptico-hypophysial tract of Scylwrhinus
caniculus is always followed by melanophore dispersion and a consequent
darkening of the skin. This is explained by the removal of an inhibitory
action on the intermedia cells. With the removal of this inhibition, the
pars intermedia cells liberate melanocyte stimulating hormone ( MSH )
into the circulation, and this hormone causes dispersion of pigment
throughout the melanocytes of the skin. From Mellinger’s experiments, it
seems possible that the inhibitory influence is mediated by the neurohypophysis, and it is worth remembering that Knowles (1963, 1965a) has
distinguished individual types of neurosecretory fibers which pass either
to the “synthetic” or to the “release” poles of the intermedig cells. Moreover, Knowles (1965b) has noted that secretomotor junctions are formed
between the neurosecretory fibers and areas of the pars intermedia cells
which are distinguished by a sharply delimited endoplasmic reticulum.
There was some degree of correlation between the state of the reticulum
and signs of hormone liberation from the neurosecretory fibers. Knowles
suggests that this would be compatable with the view that a neurohypophysial principle directly affects MSH synthesis in the cytoplasm of the
intermedia cell, in the region of the endoplasmic reticulum. Despite these
histological indications, it cannot be certain that the inhibitory effect
results from neurosecretory fibers or their peptides, since nonneurosecretory axons are also present in the preoptico-hypophysial tract. Indeed, in
recent work, Meurling et al. (1969) have succeeded in preferentially
cutting the neurosecretory fibers of the infundibular stem of Raiu radiuta,
a species in which the neurosecretory axons are localized in a medial position. They found that the release of MSH was little affected. However,
section of the lateral, possibly adrenergic nonneurosecretory axons caused
profound effects in which color responses to a white background were