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Anatomy of a larval tunicate showing the placement of the 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.[1][2][3] 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.[4] When feeding, food particles suspended in the water adhere to the mucus the endostyle produces.[5] 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.[5][6]

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.[7]

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.[6] 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.[8] Similar genetic studies on a hemichordate[9] tentatively indicate that the endostyle also might share an origin with the stomochord.[9]

Zones of the endostyle

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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.[6]

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.[6]

Zone 3 is classified by narrow ciliated bands nestled between glandular zones. Zone 3 contains a granulated apical border.[6]

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.[6]

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 contains 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.[6]

Zone 6 forms the lateral edge of the endostyle groove and is very distinct from the bands of previous zones, after undergoing the paraldehyde fuchsin procedure, small cytoplasmic granulation is seen.[6]

Zone 7 is of particular interest because of the ability of this zone to trap radioactive iodine, which is a primary function of the thyroid. Granules of a variety of sizes exist in this zone. Thus, the granules can contribute to a variety of different functions. There is no evidence of secretory activity within this zone.[6]

Zone 8 contains an enormous number of cilia. Zone 8 exists on the lateral edge of the endostyle groove. 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.[6]

Microscopic view of the endostyle

Zone 11 has the lateral granulations of 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 the cells have a unique cytoplasm.[6]

Cellular makeup

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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 cyanophile reaction, while dorsal cylinders give a weak PA/S-positive reaction.[6]

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.[6]

Ventral cylinder cells are in contrast to dorsal cylinder cells and are less characterized by basophile strands and are never phyroninophile.[6]

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.[6]

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.[6]

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.[6]

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.[6]

Zones 1, 3, and 6 do not directly contribute to secretion, but contain ciliated cells.[6]

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.[6]

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.[6]

References

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Further reading

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Endostyle

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The endostyle is a specialized organ found exclusively in non-vertebrate chordates, such as amphioxus and , and in the ammocoete larvae of lampreys, where it serves as an epithelial for filter-feeding and protein iodination. Structurally, it appears as a longitudinal, ciliated groove along the ventral wall of the , divided into multiple zones that include supporting epithelial cells, glandular cells secreting mucoproteins, and regions homologous to tissue. Functionally, it traps suspended food particles in propelled by ciliary action through the pharyngeal slits, while also concentrating iodine and synthesizing iodinated compounds akin to . In terms of development, the originates from the pharyngeal during embryogenesis and undergoes positional shifts to integrate with the alimentary canal. In larvae, it transforms into discrete follicles during , marking a key transitional stage in organ evolution. Single-cell analyses in ascidians reveal a diverse cellular composition, including ciliated cells, hemolymphoid regions with immune functions, and thyroid-like cells expressing genes such as TPO and DUOX1, highlighting its multifaceted roles beyond nutrition. Evolutionarily, the endostyle represents an ancestral innovation, bridging non- and lineages through shared dorsoventral patterning genes like Pax2/5/8 and Otx, which regionalize its anterior-posterior axis. Recent studies suggest that the acquisition of cells in early facilitated the evolution of the from the endostyle, enhancing its endocrine functions. Its iodine-binding capacity and activity underscore its homology to the , which derives directly from pharyngeal in jawed and , suggesting the endostyle's traits may have been secondarily emphasized in certain lineages like lampreys. This organ also hints at broader pharyngeal diversification, potentially contributing to the origins of endocrine, immune, and sensory structures in .

Overview

Definition

The endostyle is a longitudinal, ciliated, mucus-secreting fold located in the ventral wall of the in chordates, serving primarily as a key component in filter-feeding mechanisms by trapping food particles in and directing them toward the digestive tract. This structure is an epithelial composed of glandular cells that produce mucoproteins and ciliated cells that facilitate particle transport. It originates from the during embryonic development, highlighting its role as a pharyngeal organ derived from the primitive gut lining. The term "endostyle" derives from the Greek roots "endon" (ἔνδον, meaning "within" or "inside") and "stylos" (στυλος, meaning "pillar" or "column"), reflecting its appearance as an internal, groove-like or pillar-shaped glandular feature when viewed in cross-section. For example, in certain colonial ascidians like Botryllus schlosseri, it measures approximately 0.7 mm in length, scaling with the overall size of the pharynx in larger chordates. In evolutionary terms, the endostyle is regarded as homologous to the gland, based on shared molecular and functional traits related to iodine uptake and precursor production.

Occurrence

The endostyle is present in all non- chordates, including urochordates () and cephalochordates (lancelets). In urochordates, such as the ascidian , the endostyle occurs in both larval and adult stages, functioning as a organ involved in filter-feeding. In cephalochordates, exemplified by species, the endostyle is retained throughout the organism's life, lining the ventral and contributing to for particle capture. Among vertebrates, the endostyle is restricted to the larval stages of cyclostomes, specifically in ammocoete larvae of lampreys like Petromyzon marinus, where it serves a similar glandular role before undergoing regression and transformation during into the adult form. In contrast, , the other cyclostome group, lack an endostyle entirely, with their developing directly from pharyngeal . The endostyle is absent in jawed vertebrates (gnathostomes), where it has been evolutionarily replaced by the thyroid gland, which arises from a distinct endodermal thickening near the first pharyngeal pouch; vestigial traces are not reported in these groups. The organ was first described in ascidians by Thomas Huxley in 1851, highlighting its role in anatomy.

Anatomy

Location and Morphology

The endostyle is positioned on the ventral floor of the in non-vertebrate chordates and larvae, forming a longitudinal structure that extends from the anterior to the posterior region of the . This placement integrates it into the overall architecture of the pharyngeal region as part of the filter-feeding apparatus, forming the endostylar groove. In gross morphology, the endostyle manifests as a U- or V-shaped trough lined with ciliated , typically measuring around 100–200 μm in width across , and it opens laterally toward the pharyngeal slits. The ciliated surface contributes to its distinctive appearance, with the trough-like form varying slightly by but consistently oriented along the ventral midline. Morphological variations occur across chordate groups; in lancelets such as , the endostyle is elongated, spanning much of the pharyngeal length (up to several millimeters in adults), whereas it is comparatively shorter in larvae, reflecting the smaller overall body size of these stages. In larvae (ammocoetes), it forms a bilobed, trough-like organ extending from the second to fourth pharyngeal arches. Associated structures include connections to the dorsal strand or remnants in cephalochordates, anchoring it within the axial framework. The endostyle further displays internal zonation along its length, consisting of distinct epithelial regions that vary by (e.g., 6–7 zones in cephalochordates, 8–9 in urochordates).

Zonation

In ascidians such as the colonial species , the endostyle exhibits a distinct zonation consisting of eight parallel longitudinal zones along its length, arranged side-by-side and visible in cross-section, with abrupt transitions marked by changes in cell types and secretory activities. These zones allow for specialized regional functions within the organ, and in B. schlosseri, the anterior zones (1–4) comprise roughly half the structure. Zone 1, located at the anterior end, is characterized by ciliated epithelial cells without secretory granules, serving primarily as a non-secretory structural component that facilitates and particle movement. Zone 2 follows as a mucus-secreting region, featuring large cuboidal cells rich in rough (RER) and Golgi apparatus, which produce mucoproteins essential for filter-feeding; these secretions are highlighted by periodic acid-Schiff () staining due to their high content. Zone 3 comprises enzyme-producing glandular cells with well-developed RER and electron-dense granules, contributing proteolytic and other to the matrix. Zone 4 acts as a ciliated transitional , with multiciliated cells aiding in the propulsion and mixing of anterior secretions toward posterior zones. Zone 5, present in certain chordates such as ascidians, includes cells capable of iodine accumulation, though this varies by species and developmental stage. Zone 6 is glandular, similar to zone 2, with PAS-positive secretions from tall columnar cells that further enrich the endostylar . The posterior zones include zone 7, the primary site for iodine trapping, where cuboidal cells synthesize thyroglobulin-like proteins and incorporate iodine into residues, as demonstrated by autoradiographic studies showing silver grain localization. Zone 8 forms a posterior ciliated cap, dominated by densely ciliated cells that propel the completed mucus sheet forward and laterally out of the endostyle groove. Detailed cellular compositions within these zones, such as specific epithelial subtypes, are further elaborated in studies of endostylar . Zonation patterns differ in other chordates, such as lampreys, where distinct glandular and ciliated regions are present but not identically structured.

Cellular Composition

The endostyle comprises a diverse array of epithelial cell types specialized for and , primarily observed in urochordates, cephalochordates, and larval lampreys. Mucocytes, also referred to as goblet cells in ascidians like Styela plicata, are columnar cells containing granules of that stain positively with periodic acid-Schiff (PAS) for neutral mucins and Alcian blue at pH 2.5 for acidic mucins, appearing magenta or in combined stains, respectively. These cells predominate in specific zones and feature an expanded apical region filled with secretory vesicles. Ciliated cells form a critical component, characterized by numerous apical cilia exhibiting the canonical 9+2 axonemal structure, as revealed by electron microscopy in ascidians such as . These cells often intersperse with mucocytes and display microvilli adjacent to cilia on their apical surfaces, facilitating coordinated movement of endostylar secretions; they attach basally to a thin lamina underlying the . Glandular cylinder cells, prominent in the (Petromyzon marinus) endostyle, consist of two subtypes: Type I cells, which are tall, basophilic, and protein-secreting with abundant visible ultrastructurally, and Type II cells, which are vacuolated, open directly to the lumen via narrow ducts, and contain electron-dense granules. These columnar cells line the ventral regions and contribute to the organ's secretory profile. Additional specialized cells include enzyme-secreting types in zone 3, which demonstrate activity through histochemical assays, appearing as cuboidal cells with lysosomal-like bodies under electron microscopy. Iodinating cells in zone 7, found in ascidians like Styela clava, are non-ciliated cuboidal epithelial cells expressing homologs of (TPO) and dual oxidase 1 (DUOX1), confirmed via single-cell RNA sequencing and ; these cells show immunoreactivity for precursors using . Electron across endostyles consistently reveals apical microvilli on secretory and ciliated cells for enhanced surface area, along with basal attachment to a continuous lamina separating the from underlying . Cell densities vary zonally, interspersed with supportive epithelial cells. Staining techniques such as Alcian blue highlight acidic mucins in mucocytes, while targets neutral glycoproteins, and immunohistochemical markers like anti-TPO localize iodinating activity. The zonal arrangement of these cell types is described in the Zonation section.

Function

Filter-Feeding Mechanism

The endostyle plays a central role in the filter-feeding process of chordates such as lancelets and ascidians by secreting a glycoprotein-rich sheet that traps planktonic particles ranging from 1 to 50 μm in size. This secretion primarily occurs from glandular zones 2 and 6 within the endostyle's epithelial structure, where specialized mucin-producing cells release the to form a continuous sheet along the ventral pharyngeal groove. The net effectively captures suspended food particles, including and , through rather than strict sieving, enabling efficient collection even of sub-micron particles in some cases. Ciliary action drives the transport of the mucus-food complex posteriorly toward the . Metachronal waves generated by densely packed cilia, particularly along the endostylar groove and lateral branchial surfaces, beat at frequencies of approximately 7 Hz, propelling the mucus sheet at speeds of 0.3 mm/s. These coordinated waves ensure continuous movement of the particle-laden without disruption, integrating the endostyle's output with the pharyngeal pumping mechanism. The sheet integrates seamlessly with the pharyngeal architecture, adhering to the branchial bars as water currents pass through the slits, where filtered water is expelled and the compacted bolus is directed to the digestive tract. This process is particularly efficient in low-nutrient marine environments, allowing chordates to sustain feeding on sparse populations. In lancelets like , the mechanism processes 30-138 ml of water per hour per individual, with capture efficiencies reaching 100% for particles ≥4 μm. Overall retention rates for suitable planktonic particles are 80-100%, underscoring the endostyle's adaptation for high-throughput filtration in dilute suspensions.

Iodine Metabolism

The endostyle in chordates, such as amphioxus and larval lampreys, actively transports iodide ions from the surrounding seawater primarily through homologs of the sodium-iodide symporter (NIS), a membrane glycoprotein that facilitates iodide entry across the basolateral membrane of specialized cells. This uptake occurs predominantly in zones 5 and 6 of the amphioxus endostyle (noting species-specific variations, e.g., 9 zones in ascidians), where ciliated and glandular cells significantly concentrate iodide relative to seawater levels, enabling efficient iodine acquisition in iodine-scarce environments. In amphioxus (Branchiostoma floridae), NIS homologs have been identified genomically, supporting active transport. Similarly, in larval lampreys (Petromyzon marinus), the endostyle exhibits robust iodide accumulation, mirroring vertebrate thyroid function. Once internalized, iodide undergoes organification in the endostyle through the action of (TPO), an that oxidizes and catalyzes its covalent attachment to residues within secreted mucoproteins. This process, requiring generated by dual oxidase (DUOX), forms monoiodotyrosine (MIT) and diiodotyrosine (DIT) as initial precursors to . In amphioxus, TPO expression overlaps with the transcription factor (NKX2-1 homolog) in zones 5 and 6, localizing iodination to the dorsal region of the endostyle. These zones show pronounced peroxidase activity essential for this coupling reaction. The endostyle secretes thyroglobulin-like iodoproteins, large glycoproteins that serve as scaffolds for iodine storage and hormone synthesis, as demonstrated by radioiodine labeling studies using ¹³¹I. In larval s, these iodoproteins incorporate iodine into MIT, DIT, and low levels of thyroxine (T4) and (T3), with autoradiography revealing selective accumulation in iodinating cells. These proteins are released from the endostyle into the bloodstream of lamprey larvae, providing circulating hormonal precursors that support larval development prior to . Experimental evidence from ¹³¹I incorporation confirms the endostyle's role in producing biologically active iodinated compounds, bridging filter-feeding and endocrine functions in protochordates.

Evolutionary Aspects

Homology to Thyroid Gland

The endostyle and the gland exhibit striking structural and molecular homologies, positioning the endostyle as the evolutionary precursor to the . Both organs share the ability to concentrate iodine, a critical process for synthesis, with the endostyle's iodine-binding cells demonstrating activity analogous to that in follicular cells. Molecularly, expression of key genes such as (TPO), sodium-iodide symporter (NIS, encoded by Slc5a5), and (TG) is conserved in the endostyle of non-vertebrate chordates and larvae, mirroring their roles in production. Additionally, dorsoventral patterning in the endostyle is regulated by Pax2/5/8 and FoxE genes, which establish zonal expression domains similar to those in the , underscoring a shared developmental genetic toolkit. In lampreys, a basal vertebrate, the homology is vividly illustrated by the direct transformation of the endostyle into thyroid follicles during metamorphosis. Larval endostyle cells, particularly types II and III, migrate and reorganize into follicular structures post-metamorphosis, with TPO, NIS, and TG expression persisting and intensifying in these nascent follicles to support hormone synthesis. This process, observed from Tahara's stages 23 to 26, involves the breakdown of the glandular epithelium and formation of colloid-filled follicles, providing a transitional model for thyroid evolution. Further molecular evidence bolsters this precursor relationship through conserved anterior-posterior regionalization mediated by codes. In the urochordate Oikopleura dioica, Hox1 expression demarcates the posterior endostyle, paralleling Hoxa1 and Hoxb1 patterns in the mouse pharyngeal that gives rise to the , indicating an ancient Hox-dependent patterning mechanism. Recent 2025 studies highlight the role of cells in this evolution, showing that -derived influences endostyle differentiation in lampreys via genes like Tfap2a, FoxD3, and SoxE, a feature absent in chordates but essential for follicle maturation. Comparative genomic analyses provide indirect support, as direct fossils of soft-tissue organs like the endostyle are unavailable. Genomes of amphioxus (Branchiostoma) and tunicates reveal orthologs of thyroid genes, including Nkx2.1, Pax2/5/8, FoxE4, TPO, TG, and Duox, expressed specifically in the endostyle, confirming its proto-thyroid function across chordates. These orthologs enable thyroid hormone production in amphioxus, bridging the gap from ancestral filter-feeding structures to the endocrine thyroid.

Presence Across Chordates

The endostyle is a defining feature across the phylum, serving as a synapomorphy that unites Urochordata, Cephalochordata, and Vertebrata in their shared ancestry. This glandular structure, specialized for mucus secretion in filter-feeding, persists in varying forms depending on the lineage's ecological adaptations. Its evolutionary conservation highlights the chordate transition to pharyngeal-based feeding mechanisms, while its selective loss underscores shifts in trophic strategies. In Urochordata, the endostyle is present in both filter-feeding larvae and adults, playing a crucial role in the pharyngeal that captures particulate . This organ is particularly vital for the sessile lifestyle of ascidian adults, where ciliary action and facilitate passive suspension feeding in marine environments. Although some urochordate life stages, such as post-metamorphic non-feeding phases in certain species, may reduce reliance on the endostyle, it remains a persistent feature in feeding forms across and . Cephalochordates, exemplified by amphioxus ( species), retain the endostyle as a lifelong structure without undergoing , integrating it into their benthic burrowing . The organ continuously secretes iodinated mucoproteins that trap sub-micron food particles in the pharyngeal basket, enabling efficient filter-feeding in burrows. This persistent presence reflects the subphylum's primitive morphology and stable . Within Vertebrata, the endostyle is retained only in the larval stages of cyclostomes, specifically the ammocoete larvae of lampreys (Petromyzontida), where it functions in filter-feeding before metamorphosing into tissue. It is absent in gnathostomes (jawed vertebrates), which develop glands directly from pharyngeal without an endostyle intermediate. Recent from 2022 to 2025 on (Myxini), the to lampreys, confirms the absence of an endostyle throughout their life cycle, with follicles forming directly, challenging prior assumptions of a uniform cyclostome endostyle. This pattern suggests the endostyle's evolutionary loss in jawed vertebrates correlates with the rise of active predation and jaw-mediated feeding, marking a key divergence in vertebrate trophic .

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