3. FISH PROLACTIN AND GROWTH HORMONE
219
It is not certain that stimulation of melanogenesis is a physiological
function of the native fish prolactin. An observation suggesting that this
may be so is that the prolactin ( 7 ) cells were extremely active in hybrid
Xiphophrus-bearing melanomes ( Schreibman, 1964 ) . One postulate that
should be checked is that if fish prolactin is indeed a stimulator of melanogenesis, the skin of Poecilb or F. heteroclitus in freshwater might be
expected to contain more melanin than the skin of seawater fish, since
fish prolactin is probably secreted at a higher rate in the former medium
( Section I, B ) ,
D. Prolactin and Epidermal Mucous Cells in Teleosts
Evidence is accumulating that suggests the possibility that fish prolactin might maintain the epidermal mucous cells in at least some teleosts.
Burden (1956) showed that the activity and number of mucous cells in
the gills of F. heteroclitus were reduced by hypophysectomy, a change
which he suggested might be correlated with the inability of the operated
fish to live in freshwater (Section I, B). Removal of the pituitary was
subsequently shown to reduce the mucous cells in the skin of Bettu splendens, again associated with impaired ability to tolerate freshwater
(Schreibman and Kallman, 1965), and prolactin was shown to cause
proliferation of mucous cells in the skin of certain intact cichlids (Bliim
and Fiedler, 1965). Mucous cells are reduced in number and size in the
goldfish skin following hypophysectomy and are maintained by ectopic
pituitary transplants (Ogawa and Johansen, 1967), possibly by fish prolactin secreted by the transplants (cf. Ball et d., 1965). A special case is
the discus fish, a cichlid Symphysodon discus, in which the skin of
brooding fish hypertrophies and secretes mucus (“discus milk“) which is
eaten by the fry (Hildeman, 1959). It has been claimed that skin proliferation and skin claminess ( = excessive mucus secretion) was induced
in young nonbreeding Symphysodon by repeated injections of prolactin
on alternate days for 10 days (Egami and Ishii, 1962). The dose of prolactin was extremely high (each injection, 1 mg of prolactin, probably
about u) IU in fish weighing about 4 7 g). However, much lower doses
(0.03-2.0 IU) caused proliferation of mucous cells in Symphysodon when
given over a period of 5 weeks (Bliim and Fiedler, 1965). Prolactin has
also been shown to increase the density of mucous cells in the gills of
winter Gmterosteus transferred to freshwater ( Leatherland and Lam,
The above works suggest an influence of prolactin on mucous cells,
but not all workers have been able to obtain this effect. It should be noted
1968).
219
It is not certain that stimulation of melanogenesis is a physiological
function of the native fish prolactin. An observation suggesting that this
may be so is that the prolactin ( 7 ) cells were extremely active in hybrid
Xiphophrus-bearing melanomes ( Schreibman, 1964 ) . One postulate that
should be checked is that if fish prolactin is indeed a stimulator of melanogenesis, the skin of Poecilb or F. heteroclitus in freshwater might be
expected to contain more melanin than the skin of seawater fish, since
fish prolactin is probably secreted at a higher rate in the former medium
( Section I, B ) ,
D. Prolactin and Epidermal Mucous Cells in Teleosts
Evidence is accumulating that suggests the possibility that fish prolactin might maintain the epidermal mucous cells in at least some teleosts.
Burden (1956) showed that the activity and number of mucous cells in
the gills of F. heteroclitus were reduced by hypophysectomy, a change
which he suggested might be correlated with the inability of the operated
fish to live in freshwater (Section I, B). Removal of the pituitary was
subsequently shown to reduce the mucous cells in the skin of Bettu splendens, again associated with impaired ability to tolerate freshwater
(Schreibman and Kallman, 1965), and prolactin was shown to cause
proliferation of mucous cells in the skin of certain intact cichlids (Bliim
and Fiedler, 1965). Mucous cells are reduced in number and size in the
goldfish skin following hypophysectomy and are maintained by ectopic
pituitary transplants (Ogawa and Johansen, 1967), possibly by fish prolactin secreted by the transplants (cf. Ball et d., 1965). A special case is
the discus fish, a cichlid Symphysodon discus, in which the skin of
brooding fish hypertrophies and secretes mucus (“discus milk“) which is
eaten by the fry (Hildeman, 1959). It has been claimed that skin proliferation and skin claminess ( = excessive mucus secretion) was induced
in young nonbreeding Symphysodon by repeated injections of prolactin
on alternate days for 10 days (Egami and Ishii, 1962). The dose of prolactin was extremely high (each injection, 1 mg of prolactin, probably
about u) IU in fish weighing about 4 7 g). However, much lower doses
(0.03-2.0 IU) caused proliferation of mucous cells in Symphysodon when
given over a period of 5 weeks (Bliim and Fiedler, 1965). Prolactin has
also been shown to increase the density of mucous cells in the gills of
winter Gmterosteus transferred to freshwater ( Leatherland and Lam,
The above works suggest an influence of prolactin on mucous cells,
but not all workers have been able to obtain this effect. It should be noted
1968).