hypophysial peptides, but if there are separate stimuli for their release, these are
unknown.
Neurohypophysial peptides are rapidly removed from the blood in mammals,
sustained circulatory levels presumably being maintained as a result of continuous
secretion. Injected vasopressin or oxytocin have half- lives of less than 5 minutes
in rats, dogs and rabbits. This is largel y the result of their enzymatic inactivation
in the liver and kidneys, accompanied by a small (about 10%) excretion in the urine.
The only observations on such clearances in non-mammalian species are those of
HASAN and HELLER (1968) in chickens and toads, Bufo marinus, in which injected
vasopressin, oxytocin and vasotocin were found to disappear three to twenty times
as slowl y as in the mammals. Such variations could provide a basis for differences
in the role and function of the neurohypophysis in different groups of vertebrates.
Sustained circulating hormone levels may be better suited to mediate functions that
have a prolonged hormonal requirement.
3. The Thyroid Gland
Despite exhaustive studies extending over the last 30 and more years, the th yroid
gland has not been shown to have a direct role in the integration of osmotic homeostasis in vertebrates. It does , however, influence water and salt metabolism indirectly in man y ways and it may potentiate the actions of hormones that are
directly involved in osmoregulation.
a) Structure and Secretions
Embryologically the th yroid gland is formed as a median downgrowth from the
floor of the pharynx, and this reflects its anatomical site in the region of the 'neck,'
and its affinities to the pharyngeal endostyle in lower chordates. Ph yletically, the
th yroid gland has the longest continuous history of any vert ebrate endocrine organ,
as homologous tissue and th yroid hormone have been identified in the endostyle
of the ammocoete larva of the lamprey, and in protochordates like Amphioxus and
the ascidians . In vertebrates the th yroid gland contains hollow follicles surrounded
be secretory cells; colloidal material containing the stored hormones is accumulated
within these follicles. The tissue is surrounded by a discrete capsule in the tetrapods,
but in the fishes it is more diffuse and often extends along the region of the ventral
aorta.
The thyroid secretions are unique among the vertebrate hormones, in that they
contain an inorganic element, iodine, as an essential part of their structure. Synthetic analogues containing bromine and isopropyl moieties, instead of iodine, may
have considerable biological activity (TAYLOR, Tu, BARKER, and JORGENSEN, 1967)
but these do not appear to exist in nature. The iodine occurs in the aromatic ring
of a tyrosine molecule (Table 2.3), and four such compounds have been identified
in th yroid tissue; monoiodotyrosine, diiodotyrosine, tetraiodothyronine (th yroxine) and triiodothyronine. The latter two substances are the secreted hormones.
Thyroxine is the predominant secretion, and differs from triiodothyronine in th e
59
unknown.
Neurohypophysial peptides are rapidly removed from the blood in mammals,
sustained circulatory levels presumably being maintained as a result of continuous
secretion. Injected vasopressin or oxytocin have half- lives of less than 5 minutes
in rats, dogs and rabbits. This is largel y the result of their enzymatic inactivation
in the liver and kidneys, accompanied by a small (about 10%) excretion in the urine.
The only observations on such clearances in non-mammalian species are those of
HASAN and HELLER (1968) in chickens and toads, Bufo marinus, in which injected
vasopressin, oxytocin and vasotocin were found to disappear three to twenty times
as slowl y as in the mammals. Such variations could provide a basis for differences
in the role and function of the neurohypophysis in different groups of vertebrates.
Sustained circulating hormone levels may be better suited to mediate functions that
have a prolonged hormonal requirement.
3. The Thyroid Gland
Despite exhaustive studies extending over the last 30 and more years, the th yroid
gland has not been shown to have a direct role in the integration of osmotic homeostasis in vertebrates. It does , however, influence water and salt metabolism indirectly in man y ways and it may potentiate the actions of hormones that are
directly involved in osmoregulation.
a) Structure and Secretions
Embryologically the th yroid gland is formed as a median downgrowth from the
floor of the pharynx, and this reflects its anatomical site in the region of the 'neck,'
and its affinities to the pharyngeal endostyle in lower chordates. Ph yletically, the
th yroid gland has the longest continuous history of any vert ebrate endocrine organ,
as homologous tissue and th yroid hormone have been identified in the endostyle
of the ammocoete larva of the lamprey, and in protochordates like Amphioxus and
the ascidians . In vertebrates the th yroid gland contains hollow follicles surrounded
be secretory cells; colloidal material containing the stored hormones is accumulated
within these follicles. The tissue is surrounded by a discrete capsule in the tetrapods,
but in the fishes it is more diffuse and often extends along the region of the ventral
aorta.
The thyroid secretions are unique among the vertebrate hormones, in that they
contain an inorganic element, iodine, as an essential part of their structure. Synthetic analogues containing bromine and isopropyl moieties, instead of iodine, may
have considerable biological activity (TAYLOR, Tu, BARKER, and JORGENSEN, 1967)
but these do not appear to exist in nature. The iodine occurs in the aromatic ring
of a tyrosine molecule (Table 2.3), and four such compounds have been identified
in th yroid tissue; monoiodotyrosine, diiodotyrosine, tetraiodothyronine (th yroxine) and triiodothyronine. The latter two substances are the secreted hormones.
Thyroxine is the predominant secretion, and differs from triiodothyronine in th e
59