244
AUBREY GORBMAN
and Gorbman (1960) by repeated injections of thiouracil into late
embryos of Squalus suckleyi produced a slight histological stimulation of
the thyroid, suggestive of an activated TSH secretion. The entire question
of possible hypothalamic control over TSH secretion in cartilaginous fish
obviously requires attention. Although there is no conclusive evidence
for it, the possibility that it exists in some form is reasonable since several
workers have described annual histology cycles in elasmobranch thyroids
that are closely linked with season (Pickford and Atz, 1957; Woodhead,
Evidence for the existence and the character of hypothalamic control
over teleostean TSH is much fuller than for cyclostomes or elasmobranchs,
but it is still relatively incomplete. Many descriptions have appeared
characterizing cyclic structural changes in teleost thyroids correlated with
annual environmental cycles (partially reviewed by Pickford and Atz,
1957, and by Matty, 1960). To supplement this there have been studies of
the specific influence of temperature and photoperiod upon thyroid
structure and function (partially reviewed by Fontaine and Leloup, 1964;
also by Singh, 1967, and Jorgensen and Larsen, 1967). Thus, there
would appear to be sensory-evoked nervous afFerents to the hypothalamus
of teleosts that can eventually alter TSH secretion by the pars distalis.
Lesioning experiments to show the positions of these afferents, or of the
putative TRF-forming loci in the hypothalamus have not yet been done.
Some very interesting experiments bearing on the question were done by
Ball et al. (1963) by transplanting the pituitary gland of Poecilia fomosa
to nonhypophysial loci in hypophysectomized specimens. Away from any
hypothalamic innervation these pituitaries secreted as much TSH as the
normal pituitary, or more, to judge from thyroid histology. Gonadotropic
or adrenotropic function in such transplants was reduced. Thus, it would
appear that in this fish, the hypothalamic influence in regulating TSH
secretion is normally inhibitory, not stimulatory as described in higher
vertebrates.
Further evidence comes from experiments with goitrogens and with
low-iodine diets or low-iodine environmental waters. Limited availability
of exogenous iodine would be expected to lower circulating thyroxine
levels and, through negative feedback, to evoke hypothalamic (or direct
pituitary) activation for eventual increased TSH secretion. The classical
work of Marine and Lenhart (1910) showed that low-iodine waters promote overwhelming thyroid hyperplasia in trout, and that this thyroid
condition can be reduced or prevented by addition of iodine to the water.
Since then many similar observations have been made in other species of
fish (see, for example, Berg and Gorbman, 1954; Gorbman and Gordon,
1951; La Roche, 1952; Schlumberger and Luck&, 1948; Robertson and
1966).
AUBREY GORBMAN
and Gorbman (1960) by repeated injections of thiouracil into late
embryos of Squalus suckleyi produced a slight histological stimulation of
the thyroid, suggestive of an activated TSH secretion. The entire question
of possible hypothalamic control over TSH secretion in cartilaginous fish
obviously requires attention. Although there is no conclusive evidence
for it, the possibility that it exists in some form is reasonable since several
workers have described annual histology cycles in elasmobranch thyroids
that are closely linked with season (Pickford and Atz, 1957; Woodhead,
Evidence for the existence and the character of hypothalamic control
over teleostean TSH is much fuller than for cyclostomes or elasmobranchs,
but it is still relatively incomplete. Many descriptions have appeared
characterizing cyclic structural changes in teleost thyroids correlated with
annual environmental cycles (partially reviewed by Pickford and Atz,
1957, and by Matty, 1960). To supplement this there have been studies of
the specific influence of temperature and photoperiod upon thyroid
structure and function (partially reviewed by Fontaine and Leloup, 1964;
also by Singh, 1967, and Jorgensen and Larsen, 1967). Thus, there
would appear to be sensory-evoked nervous afFerents to the hypothalamus
of teleosts that can eventually alter TSH secretion by the pars distalis.
Lesioning experiments to show the positions of these afferents, or of the
putative TRF-forming loci in the hypothalamus have not yet been done.
Some very interesting experiments bearing on the question were done by
Ball et al. (1963) by transplanting the pituitary gland of Poecilia fomosa
to nonhypophysial loci in hypophysectomized specimens. Away from any
hypothalamic innervation these pituitaries secreted as much TSH as the
normal pituitary, or more, to judge from thyroid histology. Gonadotropic
or adrenotropic function in such transplants was reduced. Thus, it would
appear that in this fish, the hypothalamic influence in regulating TSH
secretion is normally inhibitory, not stimulatory as described in higher
vertebrates.
Further evidence comes from experiments with goitrogens and with
low-iodine diets or low-iodine environmental waters. Limited availability
of exogenous iodine would be expected to lower circulating thyroxine
levels and, through negative feedback, to evoke hypothalamic (or direct
pituitary) activation for eventual increased TSH secretion. The classical
work of Marine and Lenhart (1910) showed that low-iodine waters promote overwhelming thyroid hyperplasia in trout, and that this thyroid
condition can be reduced or prevented by addition of iodine to the water.
Since then many similar observations have been made in other species of
fish (see, for example, Berg and Gorbman, 1954; Gorbman and Gordon,
1951; La Roche, 1952; Schlumberger and Luck&, 1948; Robertson and
1966).