Fish possess a complex endocrine system that is less well understood than that of mammals. Wendelaar Bonga has reviewed the major fish hormones with fairly well defined functions.

Thyroid

The thyroid follicles arise from the pharyngeal endoderm. As with the pancreas, there is not a distinct thyroid gland as seen in higher vertebrates,. Instead, the zebrafish thyroid is composed of loosely scattered follicles present in soft tissue,. They are best observed in the ventral pharynx in the vicinity of the ventral aorta (Figure 9.1). The histology is similar to that of other species,. The follicles are composed of cuboidal to flat follicular cells surrounding an eosinophilic colloid material composed mostly of thyroglobulin (Figure 9.2). The size of the thyroid follicles ranges from 14 to 140 µm in diameter. As in higher vertebrates, the follicular cells produce thyroglobulin, which is then released into the follicle. In the thyroid follicle, thyroglobulin undergoes iodination of its tyrosine residues to produce thyroxine (T4) and tri-iodothyronine (T3), which are stored in the thyroid follicle. These hormones are important regulators of growth, metabolism, and development.

Ultimobranchial gland

The ultimobranchial gland is derived from the last pharyngeal pouch. It is homologous to the medullary C cells of the mammalian thyroid. In mammals, the C cells fuse with the thyroid glandandbecome known as the parafollicular cells. They produce calcitonin, a hypocalcemic hormone important in regulating serum calcium levels. In zebrafish, fusion with the C cells does not take place and these cells remain as a separate gland; the ultimobranchial gland. The ultimobranchial gland produces calcitonin. The gland is best observed lying ventral to the esophagus (Figure 9.3). It has follicles arranged around a central lumen.