The endostyle is an anatomical feature found in invertebrate

chordate A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...

s and larval

lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...

s. It is an organ which assists chordates in

filter-feeding Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

. It is found in adult

urochordate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time c ...

s and

cephalochordate A cephalochordate (from Greek: κεφαλή ''kephalé'', "head" and χορδή ''khordé'', "chord") is an animal in the chordate subphylum, Cephalochordata. They are commonly called lancelets. Cephalochordates possess 5 synapomorphies, or pri ...

s, as well as in the larvae of the

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

lampreys, but has been transformed through evolutionary time into the

thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thy ...

in all other

s. Since the endostyle is found in all three branches of

s, it is presumed to have arisen in the common ancestor of these

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

, along with a shift to internal feeding for extracting suspended food from the water. The endostyle is found in the

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

. The food particles suspended in the water adhere to the

mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...

it produces. The filtered water is expelled through the gill slits, while the food and mucus is then passed, by the sweeping movement of cilia that coats the endostyle, through the

of the organism and into the

esophagus The esophagus ( American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to ...

. The endostyle in larval lampreys (ammocoetes)

metamorphoses The ''Metamorphoses'' ( la, Metamorphōsēs, from grc, μεταμορφώσεις: "Transformations") is a Latin narrative poem from 8 CE by the Roman poet Ovid. It is considered his ''magnum opus''. The poem chronicles the history of the ...

into the

gland in adults, and is regarded as being homologous to the thyroid gland in

s due to its iodine-concentrating activity. Early beliefs of the endostyle included the idea by Muller in 1873 stating that the endostyle that reside in the ammocoete has extremely similar functions as the protochordate hypobranchial groove. Thus, numerous investigations into the endostyle ensued, only for the theory to be denied by future researchers. However, during this research, it was found that ammocoete endostyles include the capability and functionality to accumulate radioactive iodine isotopes. This once again made the endostyle a very pertinent topic of research by a multitude of scientists. Recent research has concluded that cephalochordate and tunicate endostyles have the ability to capture iodine, thus further perpetuating new research.

Zones of the endostyle

The endostyle can be recognized in transverse section by a multitude of differing zones. Zone 1 resides in the bottom portion of the endostyle, in which is easily recognizable by tall cilia and droplets of acid mucopolysaccharides. Investigations into Zone 1 have concluded that the cells contain glycogen, however, do not include mucins. Zone 2 is classified with ventral glandular tracts. The cells identified in Zone 2 contain an acid mucous material. 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. Zone 3 is classified by narrow ciliated bands nestled between glandular zones. Zone 3 contains a granulated apical border. Zone 4 has a positively reacting granulation noticeably rougher than previous zones. The cytoplasm in Zone 4 contains large amounts of pyroninophile material. This is similar to the consistency and appearance of the ventral glandular tracts. Zone 4 is nearly the same as the dorsal glandular tract. Zone 5 has extremely large and abnormal amounts of granulated material at cell surfaces. This makes Zone 5 identifiable, while also drawing a comparison to the lateral edge of Zone 1. The cytoplasm of this zone contained PA/S-positive droplets which were neutralized with diastase. The surface material did not neutralize and digest PA/S-positive droplets. Does not contain pyroninophilia. Zone 6 forms the lateral edge of the endostylar groove and is very distinct from the bands of previous zones, as it contains small cytoplasmic granulation after undergoing paraldehyde fuchsin procedures. Zone 7 is of particular interest because of the reported ability of this zone to trap radioactive iodine. It has additionally been proposed that there may also be an indication of RNA. Granules of a variety of sizes exist in this zone. Thus, the granules can contribute to a variety of different functions within the organism. There is no evidence of secretory activity within this zone. Zone 8 contains an enormous amount of cilia that are present within the zone. Zone 8 exists on the lateral edge of the endostylar groove. There has been no definitive proof of secretory ability within this zone, despite the existence of an acidic mucosal material that has accumulated within the surface of the cells. Lancelet's endostyle1

Zone 11 is used to describe the lateral granulations on the endostyle. The band of cells in Zone 11 mimic but are vastly different from Zone 1 cells. Zone 11 cells more closely resemble the characteristics and makeup of Zone 2. There are no cilia in Zone 11 and have a unique cytoplasm. Zone 11 contains a combination sulfur and protein.

Cellular makeup

Type 1 cells make up the cells of the gland cylinders. Type 1 cells in the endostyle contain enlarged, circular nuclei and a large nucleolus. They reside on the inner area of gland cylinders. There are noticeable functional and structural differences between dorsal and ventral cylinders. Ventral cylinders show a stronger cyanophole reaction, while dorsal cylinders give a weak PA/S-positive reaction. Dorsal cylinder cells are identified by irregular strands between nuclei and the middle of the gland cylinder. Dorsal cylinder cells are basophile, as well as pyroninophile. Ventral cylinder cells are in contrast to dorsal cylinder cells and are less characterized by basophile strands and are never phyroninophile. Cylinder opening cells are identified as type 2 cells. Type 2 cells are subdivided in the endostyle into 2a, 2b, and 2c regions. There are no histochemical reactions regarding type 2a and 2b cells in recent studies. 2c cells, are richly granulated and very easily recognizable. It is also characterized by the pyroninophile zone close to the nucleus at the basal pole. The gland cells of the endostyle handles all secretory activity. Because of the large number of ribonucleoproteins in the gland cells, the synthesis of protein and a protein material rich in -SS and -SH bridges. It is also releases disease-resistant carbohydrates. The two zones in which secretory activity occurs include Zone 2 and 4. Materials for secretion is accumulated in Zone 1 and 2. Secretion accumulation and secretion may also accumulate in Zone 5. Zones 1,3, and 6 do not directly contribute to secretion, but contain ciliated cells. The secretory regions in which the endostyle are known for include the ventral glandular region and the dorsal glandular region. The ventral glandular region secretes a protein and mucopolysaccharide combination mixed with either mucoproteins or glycoproteins. The dorsal glandular region is rich in either cystin, cysteine, or lipid solutions. Neither of the above glandular regions are involved in iodine metabolism. 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. The ability of the endostyle to trap radioiodine has a linear path to becoming the thyroid. Previous research has concluded the presence of an iodination center in some Zone 5 cells, which contain a multitude of substances. Contradictory to that study, other studies have instead pointed toward Zone 7 as the iodination center, whereas other studies have leaned toward the iodine center in Zone 8. The reason for the multitude of studies garnering different results could be the trapping of iodine in the more ciliated zones, instead of the trapping and breakdown of iodine materials. The presence of iodine collection and breakdown promoted the function of the endostyle as a precursor to the thyroid.

References

External links

*Marine, David. 1913
The Metamorphosis of the Endostyle (Thyroid Gland) of ''Ammocoetes branchialis'' (Larval Land-Locked ''Petromyzon marinus'' (Jordan) or ''Petromyzon dorsatus'' (Wilder)
''The Journal of Experimental Medicine'', 17:379-395. Chordate anatomy {{animal-anatomy-stub