The vertebrate pharynx is a region of much evolutionary modification. One of the oldest structures found there is the thyroid gland, foreshadowed in the endostyle of tunicates and amphioxus, where it is an open groove in the floor of the pharynx, lined with mucus-producing cells and supplied with long cilia.
In other vertebrates there is a similar evagination of endodermal tissue from the floor of the pharynx between the first pair of gill pouches (Fig. 413) that corresponds to the endostyle. It begins to develop very early in man, when the embryo is about 1/2 mm. in length, and does not fail to put in its appearance in every vertebrate. It soon loses its connection with the pharynx, however, and becomes a ductless gland, made up of a mass of closed follicles, surrounded by connective tissue.
The duct of this embryonic thyroid gland still remains open during the “ammocoetes” stage of the lamprey eel but is obliterated in other vertebrates. In man it disappears by the eighth week of fetal life, although its place of former opening is marked by the foramen caecum at the base of the tongue.
The thyroid grows slowly but steadily throughout the prenatal period, until at birth its weight is about 0.125 per cent of the total weight of the body. After birth, while its actual weight still increases, its relative weight decreases, so that eventually in adult life it normally has a weight of about 40 grams, being somewhat heavier in the human female than in the male.
The blood supply of this gland, which comes from both the carotid and the subclavian arteries, is very generous. Wiedersheim states that it may equal or even exceed that of the brain.
In most fishes the thyroid gland is composed of several groups of loose follicles scattered along the ventral aorta, but in amphibians, reptiles, and birds, although double embryonically, it becomes increasingly compact, and in mammals finally assumes a bilobed shape, with flattened lateral portions joined together by a connecting isthmus (Fig. 414).
The shifting of the thyroid gland from its original anterior position in the floor of the pharynx to a more posterior situation near the upper part of the trachea is made possible because it is in no way anchored at the point of origin by a hampering duct. Every time the act of swallowing takes place it moves up and down with the larynx.
Obviously there has been phylogenetic change of function in this organ. From a device, partially glandular, useful for the mechanical manipulation of food particles taken in with water, it has become an endocrine structure, producing a hormone affecting the oxidation processes, and consequently the rate of living. Davenport aptly says of the endocrine glands generally, “They are made up mostly of odds and ends of more or less discarded organs, the gonads of course excepted.”
The hormone produced by the thyroid gland is thyroxin, a compound rich in iodine, which has been not only chemically isolated and crystallized, but also artificially synthesized. Kendall of the Mayo clinic had to use 6550 pounds of fresh thyroid tissue to get one ounce of pure thyroxin.
When introduced into an animal either with food or by subcutaneous or intravenous injection, it may produce marked results. Tadpoles, for example, fed with thyroid gland, change into tiny frogs the size of a fly without “growing up.” If instead they are thyroidectomized, they grow into large tadpoles without metamorphosis.
Human beings with a deficient thyroid apparatus while still young children may develop abnormally into unfortunate defectives called cretins, having retarded or imperfect physical and mental development. Thyroid-deficient adults may suffer from myxedema, a condition characterized by over-growth of connective tissue or fat, loss of hair, weak muscular development, oedematous skin, impoverished blood, deficient sexuality, lowered metabolism, nervous depression, and frequently impaired mentality. Fortunately this condition may be relieved or removed entirely, when taken in time, by the proper administration of thyroid extract.
An excessive development of the thyroid gland results in abnormally increased oxidation and manifestations of disease such as exophthalmic goitre, or Graves’ disease, in which the patient becomes extremely nervous and thin, exhibiting characteristically protruding eyeballs. This is a serious progressive disturbance not to be confused with common goitre, an enlargement of the thyroid due to deficiency of iodine, an essential constituent of thyroxin. Common goitre usually results from living in a region like Switzerland or the Midwest in the United States where the soil, and consequently food materials derived therefrom, lack iodine.
The periodic administration of a small quantity of sodium iodide to the school children of the Middle States far removed from the sea shore, has resulted in the reduction of the occurrence of common goitre in those regions.
The thyroid gland has been aptly called the “pace setter,” or metabolic regulator of the body.
Budding off embryonically from the walls of the gill pouches are various epithelial structures that later become glandular. Among these are the epithelial bodies, thymus glands, ultimobranchial bodies, and parathyroids.
In cyclostomes there are seven separate pairs of these structures which form the epithelial bodies, situated in the ventral region of the gill pouches. They are lacking in teleost fishes, but from the amphibians on there are usually present at least two pairs of glandular structures that are probably homologous with epithelial bodies, since they arise like them from the dorsal region of the gill pouches, particularly from the third and fourth pairs. Epithelial bodies, which have been identified in lizards, some birds, and many mammals, are known as parathyroids (Fig. 413), because they eventually assume a position either in direct contact with, or close to, the thyroid gland (Fig. 415).
In man they are small yellow encapsuled bodies, one pair of which lies embedded in the median dorsal surface of the thyroid gland on either side, while another pair is just above the thyroid, and still others may be distributed farther down the sides of the neck.
Although in man their combined weight is less than half a gram and their massed size a few millimeters in diameter, they are indispensable to life, since their complete removal always results in death. Children’s convulsions are found to be correlated with parathyroid deficiency, and the fatal tetanic fits, which almost invariably followed thyroidectomy in the earlier days, are now known to be due, not to the removal of the thyroid gland itself, but to the accidental extirpation of the parathyroids during thyroid operations. The hormones produced by the parathyroid glands play an important role in the vital calcium metabolism of the body.
The thymus glands, like the parathyroids, have a multiple origin, but come from the ventral region of the gill pouches instead of the dorsal region (Fig. 414). Although present in all vertebrates, they remain epithelial in character in some fishes, while in higher forms they become lymphoid and highly vascular. The different embryonic elements of the thymus frequently fuse together to form continuous masses of tissue down either side of the neck.
In mammals including man, the thymus gland is principally derived from the posterior epithelium of the third gill pouch. It reaches its actual maximum in size at about the time of puberty, although its greatest relative size is attained much earlier, in infancy.
The total removal of the thymus is not fatal. Although its function is not known, Gudernatch has shown that in tadpoles fed upon thymus growth is accelerated and metamorphosis is delayed, an effect just opposite to that obtained when thyroid is employed as food.
Recent researches have by no means sustained the hypothesis that the thymus is an endocrine gland. Hoskins summarizes the present state of our knowledge by saying, “In all probability the organ is of significance in the physiological and pathological processes, merely by virtue of its lymphoid character.”
The ultimobranchials (Fig. 413), as their name indicates, are the most posterior of the series of pharyngeal derivatives which may be suspected of endocrine activities. They arise behind the fifth pair of gill pouches. Only the left ultimobranchial body develops in lizards (Fig. 416), and their occurrence in birds and mammals is doubtful. The ultimobranchial bodies in function may be connected with other lymphoid structures throughout the body, like the tonsils, which are also pharyngeal derivatives.