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Endostyle
The endostyle is an organ found in invertebrate chordate species of tunicates and lancelets, and in the larval stage of vertebrate lampreys. It assists in filter-feeding. It has evolved into the thyroid in vertebrate chordates.
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. When feeding, food particles suspended in the water adhere to the mucus the endostyle produces. 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.
The endostyle of larval lampreys (ammocoetes) metamorphoses into the thyroid gland in adults, and is regarded as being homologous to the thyroid in other vertebrates due to its iodine-concentrating activity.
One early hypothesis for the function of the endostyle, developed in 1873 by Muller, proposed that the ammocoete endostyle has extremely similar functions as the tunicate hypobranchial groove. 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 can accumulate radioactive iodine isotopes. This revived academic interest in the endostyle.[citation needed] Already in 1963, research had concluded that cephalochordate and tunicate endostyles have the ability to capture iodine, thus further perpetuating new research. A half century later, the homology between the thyroid in vertebrates and the endostyle in amphioxi and ascidian larvae was further supported by showing that their development involved fairly homologous transcription factors. Similar genetic studies on a hemichordate tentatively indicate that the endostyle also might share an origin with the stomochord.
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 pyroninophilia 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 pyroninophilic 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.
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Endostyle
The endostyle is an organ found in invertebrate chordate species of tunicates and lancelets, and in the larval stage of vertebrate lampreys. It assists in filter-feeding. It has evolved into the thyroid in vertebrate chordates.
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. When feeding, food particles suspended in the water adhere to the mucus the endostyle produces. 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.
The endostyle of larval lampreys (ammocoetes) metamorphoses into the thyroid gland in adults, and is regarded as being homologous to the thyroid in other vertebrates due to its iodine-concentrating activity.
One early hypothesis for the function of the endostyle, developed in 1873 by Muller, proposed that the ammocoete endostyle has extremely similar functions as the tunicate hypobranchial groove. 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 can accumulate radioactive iodine isotopes. This revived academic interest in the endostyle.[citation needed] Already in 1963, research had concluded that cephalochordate and tunicate endostyles have the ability to capture iodine, thus further perpetuating new research. A half century later, the homology between the thyroid in vertebrates and the endostyle in amphioxi and ascidian larvae was further supported by showing that their development involved fairly homologous transcription factors. Similar genetic studies on a hemichordate tentatively indicate that the endostyle also might share an origin with the stomochord.
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 pyroninophilia 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 pyroninophilic 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.