4. THYROID FUNCTION AND ITS CONTROL IN FISHES
251
Fig. 2. Diagram of the anterior part of a platyfish showing the distribution of
thyroid follicles: (0) Normal distribution and ( @ ) heterotopic distribution of
thyroid follicles. In the eye the follicles are in the chorioid layer but concentrated in
the chorioid gland. In the heart and kidney the follicles are both deep and supedicial.
Fish with thyroid tumors in the kidneys have thyroid follicles in all the areas shown
and, in addition, in the spleen (not shown here). Some nontumorous fish have thyroid
follicles in the head kidneys and in the pharyngeal thyroid area but nowhere else.
From K. F. Baker et d. ( 1955).
that the follicles are concentrated in the ventral pharyngeal region and
they are well vascularized. However, much interest attaches to the iodine
metabolism in the embryonic precursor of the thyroid, the endostyle or
subpharyngeal gland of the larval lamprey. Such studies have served as
the basis for much speculation concerning evolution of thyroid function.
The endostyle is a complex tubular organ which opens by a duct to the
mid-ventral pharynx, and certain cells of the lining epithelium of this
organ function in an adult thyroidlike manner (see below).
Functionally, the thyroid follicles of h a d s h appear typically vertebrate, but relatively slow. An injected dose of 1311, as iodide, is accumulated by the pharyngeal thyroid tissue to a maximum of 2.14: of the in-
251
Fig. 2. Diagram of the anterior part of a platyfish showing the distribution of
thyroid follicles: (0) Normal distribution and ( @ ) heterotopic distribution of
thyroid follicles. In the eye the follicles are in the chorioid layer but concentrated in
the chorioid gland. In the heart and kidney the follicles are both deep and supedicial.
Fish with thyroid tumors in the kidneys have thyroid follicles in all the areas shown
and, in addition, in the spleen (not shown here). Some nontumorous fish have thyroid
follicles in the head kidneys and in the pharyngeal thyroid area but nowhere else.
From K. F. Baker et d. ( 1955).
that the follicles are concentrated in the ventral pharyngeal region and
they are well vascularized. However, much interest attaches to the iodine
metabolism in the embryonic precursor of the thyroid, the endostyle or
subpharyngeal gland of the larval lamprey. Such studies have served as
the basis for much speculation concerning evolution of thyroid function.
The endostyle is a complex tubular organ which opens by a duct to the
mid-ventral pharynx, and certain cells of the lining epithelium of this
organ function in an adult thyroidlike manner (see below).
Functionally, the thyroid follicles of h a d s h appear typically vertebrate, but relatively slow. An injected dose of 1311, as iodide, is accumulated by the pharyngeal thyroid tissue to a maximum of 2.14: of the in-