250
AUBREY GORBMAN
means of active transport of the ion. The next phase involves oxidation
of the iodide and iodination of tyrosine in a specific thyroidal globulin.
Synthesis of the thyroglobulin itself has been subjected to recent study
and has been found to proceed by formation of smaller protein units
which are then used as monomers in buildup of a polymeric molecule.
The iodinated thyroglobulin must be hydrolyzed within the follicle, or
within the follicular epithelium, to free the iodinated tyrosines (monoand diiodotyrosine ) and their simple condensation products, thyroxine
( tetraiodothyronine ) and triiodothyronine. Most of the freed iodotyrosines are destroyed by enzymic dehalogenation, but the iodothyronines
&e
into the blood where, in loose combination with certain serum
proteins, they are distributed to the tissues.
This brief synopsis has been given to serve as a background against
which the general discussion of thyroxinogenesis in various fishes can be
conducted. Although the process has been found generally similar in most
vertebrates, there are some features that are peculiar to the fishes. Comparative descriptions of the process of thyroxinogenesis in vertebrates may
be found in reviews by Berg et ul. ( 1959) , Dodd and Matty ( 1964) , and
Gorbman ( 1959).
An anatomical feature must be considered at the outset since it confers
a unique characteristic to the fish thyroid. In the hagfish, and in a large
majority of the teleosts, the thyroid follicles are not assembled into a
single gland, but instead they are scattered generally in connective tissue
throughout the subpharyngeal and parapharyngeal area. K. F. Baker et al.
(1955; K. F. Baker, 1958) discovered that in several species of small
teleosts thyroid follicles migrate extensively from the pharyngeal region
forming a large concentration in the head kidney, but occurring also at
many other places, including even the eye (Fig. 2; see also Baker-Cohen,
1959). Chavin (1956) confirmed this phenomenon for the goldfish and
showed further that head kidney thyroid is more active than pharyngeal
thyroid in the quantitative sense and responds differentially to goitrogens.
However, Frisbn and Frisbn (1967) believe that the two kinds of thyroid
are equivalent in function, and quantitative differences can be explained
on the basis of differences in total numbers of follicles in the pharynx and
kidney.
The cyclostome thyroid gland has additional special morphological features. In addition to widespread scattering of thyroid follicles (Gorbman,
1963; Waterman and Gorbman, 1963), some of the largest known thyroid
follicles, about 2 mm in diameter, occur in Mydine glutinosu, and they are
almost completely avascular. The poor blood supply of the hagfish thyroid
may indicate an extremely low physiological value of thyroid secretion to
the adult hagfish. The adult lamprey thyroid resembles that of teleosts in
AUBREY GORBMAN
means of active transport of the ion. The next phase involves oxidation
of the iodide and iodination of tyrosine in a specific thyroidal globulin.
Synthesis of the thyroglobulin itself has been subjected to recent study
and has been found to proceed by formation of smaller protein units
which are then used as monomers in buildup of a polymeric molecule.
The iodinated thyroglobulin must be hydrolyzed within the follicle, or
within the follicular epithelium, to free the iodinated tyrosines (monoand diiodotyrosine ) and their simple condensation products, thyroxine
( tetraiodothyronine ) and triiodothyronine. Most of the freed iodotyrosines are destroyed by enzymic dehalogenation, but the iodothyronines
&e
into the blood where, in loose combination with certain serum
proteins, they are distributed to the tissues.
This brief synopsis has been given to serve as a background against
which the general discussion of thyroxinogenesis in various fishes can be
conducted. Although the process has been found generally similar in most
vertebrates, there are some features that are peculiar to the fishes. Comparative descriptions of the process of thyroxinogenesis in vertebrates may
be found in reviews by Berg et ul. ( 1959) , Dodd and Matty ( 1964) , and
Gorbman ( 1959).
An anatomical feature must be considered at the outset since it confers
a unique characteristic to the fish thyroid. In the hagfish, and in a large
majority of the teleosts, the thyroid follicles are not assembled into a
single gland, but instead they are scattered generally in connective tissue
throughout the subpharyngeal and parapharyngeal area. K. F. Baker et al.
(1955; K. F. Baker, 1958) discovered that in several species of small
teleosts thyroid follicles migrate extensively from the pharyngeal region
forming a large concentration in the head kidney, but occurring also at
many other places, including even the eye (Fig. 2; see also Baker-Cohen,
1959). Chavin (1956) confirmed this phenomenon for the goldfish and
showed further that head kidney thyroid is more active than pharyngeal
thyroid in the quantitative sense and responds differentially to goitrogens.
However, Frisbn and Frisbn (1967) believe that the two kinds of thyroid
are equivalent in function, and quantitative differences can be explained
on the basis of differences in total numbers of follicles in the pharynx and
kidney.
The cyclostome thyroid gland has additional special morphological features. In addition to widespread scattering of thyroid follicles (Gorbman,
1963; Waterman and Gorbman, 1963), some of the largest known thyroid
follicles, about 2 mm in diameter, occur in Mydine glutinosu, and they are
almost completely avascular. The poor blood supply of the hagfish thyroid
may indicate an extremely low physiological value of thyroid secretion to
the adult hagfish. The adult lamprey thyroid resembles that of teleosts in