The thyroid gland of the Atlantic Salmon
Hoar, William Stewart
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The anlage of the thyroid gland of the Atlantic salmon, Salmo salar L., appears during the eleventh week or incubation when the embryo is approximately 8.5 mm. long. The pharyngeal region is partially differentiated at this time. Five gill arches are mapped out as lateral concentrations of mesoderm, the epithelial pouches and grooves, with the exception of the first true gill cleft, being solid between. There is no indication of cartilage formntion, nor have the copulae of the arches appeared ventral to the solid pharynx. Two blood vessels, the hyoids, extend cranially from the S-shaped tubular heart into the hyoid arch, but the aortic arches proper are as yet undeveloped. The thyroid anlage arises as a solid knob-like proliferation from the entoderm of the mid-ventral floor of the pharynx at the caudal margin of the hyomandibular junction. The nuclei of the cells soon become arranged around the periphery of the mass, leaving a clear cytoplasmic area within. No evidence of a lumen, however, is seen at any time. The anlage increases in size and its unattached extremity comes to lie in the bifurcation of the truncus arteriosus. The development of the basihyoid cartilage and the rapid cranial growth of the lower jaw during the thirteenth and fourteenth week (9 mm. embryos) tend to sever the anlage from the floor of the pharynx. Since the thyroid, heart and associated vessels do not grow forward with the jaw their position becomes relatively more caudal. The result of this positional change is that, by the end of the seventeenth week, the thyroid although still in the bifurcation of the ventral aorta is entirely caudal to the basihyoid complex. At this time the cranial portion of the anlage lies dorsal and rostral to the bifurcation, while the caudal and lateral portions lie beneath the border of the hyoid vessels at their point of origin. When the embryo is between 10 and 11 mm. long the cells of the thyroid anlage become arranged into solid spherules in which colloidal material collects and the structure shows evidence of breaking up and scattering. At this same time the ventral aorta is developing into an elongated vessel from a sac-like expansion of the cranial end of the heart. The actively proliferating thyroid tissue simply spreads out around this elongating vessel in whatever space is available. There is no evidence of an active migration of the follicles. Tje process of primary follicle formation is seen to involve a proliferation of cells, the rearrangement of these into solid spherules, cavity formation within the spherules and the collection ot chromophobic and finally chromophilic material within the cavities. Secondary follicles arise from the primary follicles through a process of budding. By the time the embryo is 11 mm. long three more or less distinct, but not entirely separate masses, of the thyroid tissue are evident. These consist of areas of undifferentiated thyroid tissu together with primary follicles containing typical colloid. Two of these masses lie dorsal and one lies ventral to the aorta. The anterior dorsal mass is found at the cranial border ot the ventral aorta and the posterior dorsal mass at the level of the second aortic arch. The differentiating thyroid tissue seems to have been forced into these positions by the growth of surrounding structures. The same general arrangement of the thyroid is evident in the construction or the glands of salmon of all ages. Although the proliferation of thyroid tissue continues during the entire period of growth the gland has attained its final form by the time the fish is 28 mm. long. The bulk of the oxygen increases but the same general boundaries are always maintained after this time. The thyroid of the adult salmon is a diffuse organ consisting of independent follicles scattered in the loose connective tissue of the throat region. Only in the very large salmon do the acini become so closely related as to form a compact and nodular gland. The thyroid tissue extends form the caudal border of the first basibranchial to a short distance behind the common origin of the third and fourth aortic arches. It is not normally found cranial to the hyoid complex, but in a few cases anomalies occur in the region of the primary anlage, i.e. the hyomandibular junction. The largest mass of the adult gland is found around the second aortic arches. There is little thyroid tissue lateral to the ventral aorta, and the number of follicles ventral to this blood vessel is always less than the number dorsal to it. The characteristics of the thyroid acini do not differ from those of other vertebrates, In salmon the follicles are supported by a fatty connective tissue of a reticular nature. The adipose cells have a peculiar vacuolated appearance when treated with osmic acid. Elastic fibers and smooth muscle cells identified by some investigators could not be found. Lymphatics seem to play a predominant role in the vascularization of the salmon thryoid. Although the hypobrachial artery was studied, branches from this to the thyroid tissue could not be found, nor could the thyroid veins be identified. On the other hand a complicated system of lymphatics forms a large sinus about the thyroid tissue. This sinus consists of a large dorsal channel and a smaller ventral vessel with many lateral connections between the two. The system communicates with the inferior jugular veins in the cranial direction and drains into the pericardial lymph sinuses caudally. Intimate contact is formed between the epithelium of the follicles and the endothelium of the lymphatics. In the more massive glands lymph capillaries extend from the main sinus between the acini. Although it can be said that blood capillaries are unimportant in the physiology of the salmon thyroid, it seems evident that the lymphatics play an important role in the removal of the colloid . Histophysiological studies have shown a correlation between the functional condition of the thyroid gland and the activities of the Atlantic salmon. Seasonal changes in thyroid activity are very definite. Like all cold blooded animals the salmon shows an increase in thyroid activity during the spring and summer, with a marked decline in the fall and winter. As evidence of this there is a complete withdrawal of colloid material from the follicles during the early part of June. This coincides with the period of most rapid growth of the fish. Throughout the summer a lightly acidophilic colloid with numerous chromophobic peripheral vacuoles indicates a mild degree of activity. Dense acidophilic colloid within the follicles during the fall and winter is characteristic of a passive gland. However, seasonal activity of the salmon thyroid is not great enough to cause a pronounced increase in the height of the epithelium or in the size of the follicle. In general it is found that the glands of the young salmon are more active than those of the older fish. An active hyperplasia of the thyroid occurs during the smolt stage, that is when the young fresh water fish undergoes changes preparatory to its migration to the sea. The changes taking place in the salmon at this time seem to be of sufficient magnitude to warrant the classing of this period as a "metamorphosis" and comparing it with metamorphosis as seen in other forms. Different degrees of thyroid activity are found in the smolt, but the follicles always show a change in the epithelium from cuboidal to columnar, together with an increase in the size of the follicle and in the number of follicles. The colloidal material becomes much less acidophilic and may be entirely withdrawn from the acini and appear in the neighboring lymphatics. This activity occurs in the early spring and is followed by a partial involution of the gland during the summer of the same year. Involution becomes complete as winter approaches and the thyroid assumes the appearance of a gland of "colloid goiter". In other words the thyroids of the large sea salmon consist of very large macroscopic follicles associated with numerous small microscopic ones. This condition is seen to be the result of the collection of colloid in the hyperplastic follicles of the smolt. The gland has become so enlarged in these adult salmon that it often forces the neighboring connective tissue into the semblance of a capsule. The male salmon parr is precocious in its sexual development. Significant changes in the structure of the thyroid gland are not found during the process of sexual maturation either in the parr or in the large adult fish.
Thesis (Ph.D.)--Boston University