54
J. N. BALL AND BRIDGET I. BAKER
quite different from pigmentation control. The conditions under which
the pars intermedia cells show cytological evidence of functional alterations are, therefore, of quite unusual interest.
Attempts have been made to locate the cellular origin of MSH by
observing cytological changes in the pars intermedia in response to adaptation to a black or white background. In Cichlmom and Bknnius
adapted to a black background for several weeks, the PAS + ve cells
became larger and better granulated, with larger nuclei and nucleoli
than in fish maintained on a white background, in which these cells regressed (Baker, 1963b). While these observations point to the secretion
of MSH by the PAS + ve cells, other work produced conflicting data.
Thus, Knowles and Vollrath (1966a) kept eels for several weeks in complete darkness, and then transferred them to an illuminated black or
white background for 4 hr before sacrifice. There were many PAS + ve
cells in the pars intermedia of animals transferred to the black background, and in these eels the melanophores were in a dispersed state,
indicating high levels of MSH secretion. In contrast, the eels transferred
to a white background displayed concentration of melanophores ( i.e.,
low levels of MSH secreted), and in the pars intermedia few PAS + ve
cells could be found. Since in such a short-term experiment the scarcity
of PAS + ve cells in the white-background fish must have been owing to
discharge of secretory granules, we would interpret these observations as
indicating that MSH is not secreted by these cells. Unfortunately, in this
work there is no detailed information about the state of the pars intermedia in the eels in total darkness, but it seems that they displayed
PAS + ve cells in the pars intermedia, which lends support to our interpretation. In other teleosts ( Ameiurus, Carassius, and Spinachia), adaptation to black or white backgrounds did not lead to cytological alterations in the pars intermedia (Baker, 1963b). Chavin (1059), however,
reported an increase in MSH content and hypertrophy of the amphiphils
in the pars intermedia of black-adapted Carassius, which would indicate
that these cells, PAS - ve and PbH + ve, secrete MSH. However, this
interpretation is not entirely secure since Chavin stated that the gonadal
state of his fish was variable, and there does exist some evidence of a
correlation between the number and size of these PbH + ve cells in
Carassius and the degree of gonadal maturation (Scruggs, 1951; Baker,
1962). Nevertheless, the different MSH contents of the pituitary of blackadapted and white-adapted fish is interesting and highly suggestive, and
similar experiments should be repeated to avoid the complication of
gonadal variations.
Melanin dispersion in fishes may occur independently of background
adaptation. It should perhaps be emphasized that teleostean melano-
J. N. BALL AND BRIDGET I. BAKER
quite different from pigmentation control. The conditions under which
the pars intermedia cells show cytological evidence of functional alterations are, therefore, of quite unusual interest.
Attempts have been made to locate the cellular origin of MSH by
observing cytological changes in the pars intermedia in response to adaptation to a black or white background. In Cichlmom and Bknnius
adapted to a black background for several weeks, the PAS + ve cells
became larger and better granulated, with larger nuclei and nucleoli
than in fish maintained on a white background, in which these cells regressed (Baker, 1963b). While these observations point to the secretion
of MSH by the PAS + ve cells, other work produced conflicting data.
Thus, Knowles and Vollrath (1966a) kept eels for several weeks in complete darkness, and then transferred them to an illuminated black or
white background for 4 hr before sacrifice. There were many PAS + ve
cells in the pars intermedia of animals transferred to the black background, and in these eels the melanophores were in a dispersed state,
indicating high levels of MSH secretion. In contrast, the eels transferred
to a white background displayed concentration of melanophores ( i.e.,
low levels of MSH secreted), and in the pars intermedia few PAS + ve
cells could be found. Since in such a short-term experiment the scarcity
of PAS + ve cells in the white-background fish must have been owing to
discharge of secretory granules, we would interpret these observations as
indicating that MSH is not secreted by these cells. Unfortunately, in this
work there is no detailed information about the state of the pars intermedia in the eels in total darkness, but it seems that they displayed
PAS + ve cells in the pars intermedia, which lends support to our interpretation. In other teleosts ( Ameiurus, Carassius, and Spinachia), adaptation to black or white backgrounds did not lead to cytological alterations in the pars intermedia (Baker, 1963b). Chavin (1059), however,
reported an increase in MSH content and hypertrophy of the amphiphils
in the pars intermedia of black-adapted Carassius, which would indicate
that these cells, PAS - ve and PbH + ve, secrete MSH. However, this
interpretation is not entirely secure since Chavin stated that the gonadal
state of his fish was variable, and there does exist some evidence of a
correlation between the number and size of these PbH + ve cells in
Carassius and the degree of gonadal maturation (Scruggs, 1951; Baker,
1962). Nevertheless, the different MSH contents of the pituitary of blackadapted and white-adapted fish is interesting and highly suggestive, and
similar experiments should be repeated to avoid the complication of
gonadal variations.
Melanin dispersion in fishes may occur independently of background
adaptation. It should perhaps be emphasized that teleostean melano-