Endostyle

The endostyle is an organ found in invertebrate chordate species of tunicates, lancelets, and in the larval stage of vertebrate lampreys.

Since the endostyle is found in all three chordate lineages, it is presumed to have arisen in the common ancestor of these taxa, along with a shift to internal feeding for extracting suspended food particles from the water.

[2] When feeding, food particles suspended in the water adhere to the mucus the endostyle produces.

[3] The filtered water is then expelled through the gill slits, while the food and mucus are swept into the esophagus by movements of the cilia that coat the endostyle.

However, during this research, it was found that ammocoete endostyles include the capability and functionality to accumulate radioactive iodine isotopes.

Zone 1 resides in the bottom portion of the endostyle, in which is easily recognizable by tall cilia and droplets of acid mucopolysaccharides.

When using Hale's dialysed iron method, Zone 2 of the endostyle is the darkest definitive region.

Zone 2 contains streaky and prominent pyroninephilia from the apical pole to the nuclei on the cell surface.

There has been no definitive proof of secretory ability within this zone, despite the existence of an acidic mucosal material that accumulates on the surface of the cells.

The ventral glandular region secretes a protein and mucopolysaccharide combination mixed with either mucoproteins or glycoproteins.

Throughout a multitude of studies, it has been believed that the ventral and dorsal gland cylinders do not form follicles of lamprey thyroid.

Thus, the products derived from the endostyle's glandular region are associated with the traditional function of food collection through the secretory mucus.

Future research is needed to determine if the material in the mucosal substance has any metabolic purpose more than trapping food.

Previous research has concluded the presence of an iodination center in some Zone 5 cells, which contain a multitude of substances.

The presence of iodine collection and breakdown promoted the function of the endostyle as a precursor to the thyroid.