Physiology of Finfish and Shellfish (2+1)

5.2.6. The thyroid gland

The thyroid gland in teleosts is present in a diffused condition (diffuse means to spread from another place that can be changed, ignored, or mixed with another idea) around the ventral aorta and the afferent branchial arteries of the gills (Fig.). In general, located below the gills and is not always divided into distinct lobes. However, in some teleosts, patches of thyroid tissue are found elsewhere in the body associated with the kidneys, spleen, heart or eyes.

Under a microscope, large number of follicles (thyroid follicles- discrete, cystlike units of the thyroid gland that are lined with cuboidal epithelium and are filled with a colloid substance, about 30 to each lobule) is seen in the thyroid tissue. The thyroid follicles are very similar to mammalian thyroid tissue. Thyroid follicles vary in shape and size and are bound together by connective tissue and are richly supplied with blood capillaries. Thyroid follicles are distributed throughout the connective tissue of the pharyngeal area and may be observed around the eye, ventral aorta, hepatic veins and anterior kidney. It is important to realize that thyroid tissue can be widely distributed. Temperature and photoperiod have been reported to exert specific influence on the structure and function of the thyroid in teleosts.

(The first chamber in the heart is called the sinus venosus; it is the preliminary collecting chamber. In teleosts it is filled from two major veins called the hepatic veins and the left and right branches of the Curvierian ducts which in turn collect blood from the paired (left and right) lateral veins the inferior jugulars, the anterior cardinals and the posterior cardinals. However in the elasmobranchs only one hepatic vein leads into it).