How does the endostyle of a lamprey larva change during metamorphosis?

During the metamorphosis of a lamprey larva (ammocoete) into an adult, the endostyle undergoes a significant transformation. Initially, the endostyle functions as a mucus-secreting organ located in the floor of the pharynx, which the larva uses to filter food particles from the water.12

As the lamprey undergoes its radical metamorphosis, the endostyle breaks into fragments. These resulting clumps of cells then differentiate to form the separate follicles of the thyroid gland. This process reflects the evolutionary development of the thyroid gland from the endostyle, a structure also found in protochordates. Notably, thyroid hormones are not just a product of this change; they are also responsible for directing the metamorphic process itself in lampreys.3124