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Choana
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| Choana | |
|---|---|
 Lateral wall of nasal cavity. | |
 Dissection of the muscles of the palate from behind. (Choanae visible at center top.) | |
| Details | |
| Identifiers | |
| Latin | choana, apertura nasalis posterior |
| TA98 | A02.1.00.096 |
| TA2 | 501 |
| FMA | 76585 |
| Anatomical terminology | |
The choanae (sg.: choana), posterior nasal apertures or internal nostrils are two openings found at the back of the nasal passage between the nasal cavity and the pharynx, in humans and other mammals (as well as crocodilians and most skinks). They are considered one of the most important synapomorphies of tetrapodomorphs, that allowed the passage from water to land.[1]
In animals with secondary palates, they allow breathing when the mouth is closed.[2] In tetrapods without secondary palates their function relates primarily to olfaction (sense of smell).
The choanae are separated in two by the vomer.
Boundaries
[edit]A choana is the opening between the nasal cavity and the nasopharynx.
It is therefore not a structure but a space bounded as follows:
- anteriorly and inferiorly by the horizontal plate of palatine bone,
- superiorly and posteriorly by the sphenoid bone
- laterally by the medial pterygoid plates.
- medially by the Vomer
Etymology
[edit]The term is a latinization from the Greek χοάνη, "choanē" meaning funnel.
Choanae in different animals
[edit]Early bony fishes (~420 mya) had two pairs of nostrils, one pair for incoming water (known as the anterior or incurrent nostrils), and a second pair for outgoing water (the posterior or excurrent nostrils), with the olfactory apparatus (for sense of smell) in between. In the first tetrapodomorphs (~415 mya) the excurrent nostrils migrated to the edge of the mouth, occupying a position between the maxillary and premaxillary bones, directly below the lateral rostral (a bone that vanished in early tetrapods).[1]
In all but the most basal (primitive) tetrapodomorphs (or "choanates"), the excurrent nostrils have migrated from the edge of the mouth to the interior of the mouth. In tetrapods that lack a secondary palate (basal tetrapods and amphibians), the choanae are located forward in the roof of the mouth, just inside the upper jaw. These internal nasal passages evolved while the vertebrates still lived in water.[3] In animals with complete secondary palates (mammals, crocodilians, most skinks) the space between the primary and secondary palates contain the nasal passages, with the choanae located above the posterior end of the secondary palate.
In animals with partial secondary palates (most birds and reptiles), the median choanal slit separates the two halves of the posterior half of the palate, connecting the nasal cavity with the buccal cavity (mouth) and the pharynx (throat).[4]
Fish
[edit]Most fish do not have choanae, instead they have two pairs of external nostrils: each with two tubes whose frontal openings lie close to the upper jaw, and the posterior openings further behind near the eyes. A 395-million-year-old fossil lobe-finned fish called Kenichthys campbelli has something between a choana and the external nostrils seen on other fish. The posterior opening of the external nostrils has migrated into the mouth.[2]
In lungfish, the inner nostrils are regarded as an example of parallel evolution. The fossil lungfish Diabolepis shows an intermediate stage between posterior and interior nostril and supports the independent origin of internal nostrils in the lungfish.[2]
Hagfishes have a single internal nostril that opens inside the mouth cavity, while chimaeras have open canals that leads water from their external nostrils into their mouths and through their gills.
References
[edit]- ^ a b Clack, Jennifer (2012). Gaining Ground: The Origin and Evolution of Tetrapods. Indiana University Press. p. 74. ISBN 978-0-253-35675-8. Retrieved 5 August 2015.
- ^ a b c Janvier, Philippe (2004) "Wandering nostrils". Nature, 432 (7013): 23–24. doi:10.1038/432023a
- ^ Linzey, Donald W. (2012). Vertebrate Biology. JHU Press. pp. 150–1. ISBN 978-1-4214-0040-2. Retrieved 8 August 2015.
- ^ Farner, Donald S. (22 October 2013). A. J. Marshall (ed.). Biology and Comparative Physiology of Birds. Elsevier Science. p. 412. ISBN 978-1-4832-6379-3. Retrieved 8 August 2015.
Choana
View on Grokipediafrom Grokipedia
Overview and etymology
Definition
The choanae, also known as the posterior nasal apertures or internal nostrils, are paired openings that connect the nasal cavity to the nasopharynx in tetrapods.[6][5] The singular form is choana, referring to each individual aperture.[6] In tetrapods, the choanae serve as a fundamental anatomical feature, enabling the separation of olfactory and respiratory pathways while permitting continuous airflow through the nasal passages during oral activities such as feeding or vocalization.[5] This configuration represents a key evolutionary adaptation in land vertebrates, facilitating efficient respiration on land without interruption.[5]Etymology
The term choana derives from New Latin choana, which is a borrowing from the Ancient Greek χοάνη (khoánē), meaning "funnel".[1] This Greek root itself originates from the verb χέω (khéō), "to pour," combined with the suffix -άνη (-ánē), used for denoting utensils or instruments, thereby evoking the image of a funnel-shaped passage. The anatomical application of the term highlights its funnel-like form, facilitating the posterior opening of the nasal cavity.[9] The term entered anatomical nomenclature through the work of Andreas Vesalius, who introduced it in his seminal 1543 publication De Humani Corporis Fabrica, describing the posterior nasal apertures as funnel-shaped foramina.[9] Vesalius, working at the University of Padua, drew on classical Greek terminology to standardize anatomical descriptions, marking an early adoption in Renaissance anatomy.[10]Anatomy in humans
Structure and location
The choanae are paired, oval-shaped openings located at the posterior end of the nasal cavity in humans, serving as the direct passage to the nasopharynx.[2] In adults, these apertures measure approximately 2.5 cm in height and 1.5 cm in width on average, though slight variations exist between males and females.[11] They are separated by the vomer bone and positioned posterior to the nasal septum, ensuring bilateral symmetry within the midline of the skull.[6] Structurally, each choana constitutes a bony aperture lined with respiratory mucosa, formed entirely by osseous margins that provide rigidity.[2][6] This composition marks a transitional zone from the more rigid bony framework of the nasal cavity anteriorly to the softer, muscular nasopharynx posteriorly.[2] The choanae's integration with these regions facilitates their role in connecting the upper respiratory tract.[6]Boundaries
The choanae, or posterior nasal apertures, are defined by distinct osseous boundaries that delineate their edges within the human skull base, separating the nasal cavity from the nasopharynx. These bony limits provide the structural framework for the apertures, ensuring precise anatomical demarcation.[6] The medial boundary of each choana is formed by the posterior free edge of the vomer bone, which serves as the midline partition between the left and right nasal cavities and extends posteriorly to frame this inner edge.[6][12] Laterally, the choana is bordered by the medial plate of the pterygoid process of the sphenoid bone, also known as the medial pterygoid lamina, which projects downward and contributes to the lateral wall of the nasal cavity.[6][12] The superior boundary consists of the body of the sphenoid bone and the ala of the vomer, where the sphenoid's undersurface forms the primary roof, augmented by the wing-like alae projecting from the vomer's superior border to articulate with the sphenoid rostrum.[6][12][2] Inferiorly, the choana is delimited by the horizontal plate of the palatine bone, which extends posteriorly from the hard palate to form the floor of the aperture.[6][12] These boundaries collectively contribute to the structural stability of the nasal cavity, supporting the integrity of the posterior nasal passages.[6]Function
Respiratory pathway
The choanae function as the primary conduit for inhaled air, transitioning it directly from the nasal cavity into the nasopharynx during normal respiration. This posterior nasal aperture allows air to flow posteriorly after passing through the nasal passages, where it is conditioned by warming to approximately 37°C and humidification to near 100% relative humidity, preparing it for descent into the lower respiratory tract.[6][13] By providing this unobstructed pathway, the choanae ensure efficient nasal breathing as the default mode of respiration in humans, supporting continuous airflow without interruption under typical physiological conditions.[14] The anatomical positioning of the choanae, separated from the oral cavity by the hard and soft palates, maintains distinct nasal and oral airways. This separation permits exclusive use of the nasal route for air intake, with the oral pathway serving as an alternative only during temporary nasal obstruction, such as from inflammation or allergens.[6][15] In newborns, who are obligate nasal breathers, the choanae's role in this separation is particularly critical for survival, as it prevents reliance on underdeveloped oral breathing mechanisms.[8] Within the respiratory pathway, the choanae contribute to optimized airflow dynamics by facilitating a transition that promotes relatively laminar flow patterns during quiet nasal breathing, in contrast to the higher-velocity, more turbulent conditions often associated with oral inhalation. Computational models indicate that nasal airflow, including through the choanae, remains predominantly laminar at rest with Reynolds numbers below 2000, minimizing energy loss and resistance compared to oral routes.[16][17] This configuration also aids in subtle support for mucus clearance by directing conditioned air toward the nasopharynx, enhancing mucociliary transport without primary reliance on turbulent eddies.[14]Drainage and olfaction support
The choanae, as the posterior openings of the nasal cavity, provide the essential drainage pathway for nasal mucus and secretions into the pharynx. The ciliated pseudostratified columnar epithelium lining the nasal cavity and paranasal sinuses propels mucus posteriorly through mucociliary clearance, at rates typically ranging from 3 to 25 mm/min, trapping pathogens and particulates along the way. Gravity aids this process, particularly in an upright position, facilitating the flow of secretions through the choanae into the nasopharynx, where they are swallowed or expelled, thereby maintaining nasal patency and preventing accumulation that could lead to infections.[6][13] Beyond direct drainage, the choanae offer indirect support for olfaction by enabling the posterior movement of air currents that carry odorants to the olfactory epithelium in the superior nasal cavity before exiting. This airflow pattern ensures prolonged exposure of odor molecules to the moist mucosal surface, enhancing their solubility and binding to olfactory receptors without relying on anterior sniffing alone. The maintenance of a hydrated nasal epithelium via ongoing mucus production and clearance is crucial, as it preserves the integrity of the olfactory region and prevents desiccation that could impair sensory function.[13] The choanae also play a role in the auxiliary humidification and warming of inhaled air, conditioning it to near-body temperature (approximately 37°C) and 100% relative humidity as it traverses the nasal cavity prior to posterior exit. This process, supported by the extensive vascular plexus and seromucinous glands in the nasal mucosa, protects the lower airways from drying out and reduces irritation, with the choanae serving as the final conduit for this preconditioned air into the nasopharynx.[6][13]Comparative anatomy
Evolutionary origin
The choana, or internal nostril, represents a pivotal innovation in the evolution of tetrapods, emerging as primitive internal nares in early sarcopterygian fishes during the Devonian period. Fossil evidence from Kenichthys campbelli, a basal tetrapodomorph discovered in the Early Devonian (Emsian stage, approximately 395 million years ago) deposits of the Chuandong Formation in Yunnan, China, documents the initial stages of this structure. In Kenichthys, the posterior external nostril is positioned in the upper lip between the maxilla and premaxilla, marking a transitional form that precedes the fully internalized choana seen in later tetrapodomorphs such as Eusthenopteron. This configuration indicates that the choana originated through the gradual migration and internalization of the posterior nostril, rather than a sudden evolutionary shift, and was fully developed by the Middle Devonian in more derived tetrapodomorphs.[5] The adaptive significance of the choana lies in its role in enhancing respiratory efficiency during the transition from aquatic to terrestrial environments. By connecting the nasal sacs directly to the roof of the mouth, the choana separates olfaction from the gill or oral water flow, allowing sensory detection of odors in water or air without requiring constant mouth opening or water passage through the oral cavity. This decoupling enabled early sarcopterygians to maintain olfactory function independently of feeding or gill ventilation, a crucial advantage in oxygen-poor habitats prevalent in the Late Devonian.[5][18] As a key evolutionary step in the fish-to-tetrapod transition, the choana facilitated the shift toward aerial breathing by permitting continuous nasal airflow to the lungs with the mouth closed, independent of feeding activities. This innovation, predating the evolution of limbs, allowed for more effective air inhalation using modified buccal pumping mechanisms inherited from sarcopterygian ancestors, thereby supporting survival and diversification in shallow-water and marginal aquatic settings. In modern tetrapods, variations in choanal position and structure reflect this ancient adaptation, though the core function persists across the clade.[5][18]In fish
In most fish, true choanae are absent, with nasal openings consisting of external pits or tubes dedicated exclusively to olfaction. These nostrils, typically paired anterior (incurrent) and posterior (excurrent) openings on each side of the snout, allow water to flow into and out of the nasal sacs without any connection to the pharynx or oral cavity. This configuration supports chemosensory detection by facilitating the passage of odorants over olfactory epithelium lined with sensory neurons acting as chemoreceptors, enabling fish to perceive chemical cues in their aquatic environment for tasks such as locating food or mates.[19][20] Unlike tetrapods, fish nasal sacs lack integration with the respiratory system, as breathing occurs via gills that extract oxygen directly from water passing over them, rendering internal nasal-pharyngeal openings unnecessary. The olfactory function remains isolated, with water circulation driven by ciliary action or swimming-induced flow rather than respiratory airflow.[19] In primitive bony fish, such as lungfish, rudimentary internal connections between the nasal passages and the mouth exist, serving as potential precursors to tetrapod choanae, though these do not form a persistent posterior opening to the pharynx and differ significantly in anatomical relations to surrounding structures. These features highlight an evolutionary transition in nasal function adapted to semi-aquatic lifestyles, but olfaction in lungfish remains primarily sensory without respiratory coupling.[19]In tetrapods
In amphibians, the choanae are simple openings that connect the nasal sacs directly to the roof of the buccal cavity, facilitating both olfaction by allowing air to reach the mouth and respiration in species that rely on buccal pumping for aquatic breathing.[21] Without a secondary palate, these choanae remain anteriorly positioned and unobstructed, enabling efficient sensory and ventilatory functions in moist environments.[22] In reptiles, the choanae often feature elongated choanal tubes that extend from the nasal cavity to the oral cavity, reflecting adaptations to terrestrial life while maintaining connections for olfaction and air passage.[23] Birds exhibit a derived structure with a single median choana forming a longitudinal slit in the hard palate, which connects the nasal and oral cavities and supports efficient unidirectional airflow essential for continuous respiration during flight.[24] This choanal slit, lined with ciliated epithelium and glands, also aids in mucosal protection and olfaction across diverse avian species.[25] Mammals possess paired choanae located posteriorly to a fully developed secondary palate, directing airflow into the nasopharynx and isolating the nasal passages from the oral cavity for enhanced breathing efficiency.[21] In humans, as a representative mammalian variant, these choanae open at the junction of the nasal cavity and pharynx, supporting continuous respiration independent of mouth closure.[21] Crocodilians display a specialized fully separated secondary palate that positions the choanae at the rear of the pharynx, allowing submerged breathing through the nostrils while the mouth remains open for predation.[26] This posterior shift, enclosed by pterygoid bones, evolved convergently and enhances semi-aquatic adaptations.[27] Across tetrapods, choanal folds represent a key adaptation, forming fleshy, valve-like structures anchored by bony choanal crests to temporarily seal the nasal and oral cavities during swallowing, thereby preventing aspiration and maintaining airway patency.[21] These folds, prominent in mammal-like reptiles and modern mammals, project medially and contribute to coordinated feeding and respiration in diverse lifestyles.[28]Development and anomalies
Embryological formation
The embryological development of the choanae begins during the fourth week of gestation with the formation of nasal placodes, which are ectodermal thickenings on the ventrolateral aspects of the frontonasal prominence under the influence of forebrain signals.[29] These placodes, derived from surface ectoderm, undergo invagination around the fifth week to form nasal pits, which deepen into nasal sacs as mesenchymal proliferations from neural crest cells separate the ectoderm from underlying mesoderm.[30] This process establishes the primordia of the nasal cavities, with the posterior portions of the sacs extending toward the primitive oral cavity.[8] By the seventh to eighth weeks, the posterior extensions of the nasal sacs approach the bucconasal membrane (also known as the nasobuccal or oronasal membrane), a thin epithelial barrier composed of ectoderm and endoderm that initially separates the developing nasal and oral regions.[8] Perforation of this membrane occurs through programmed resorption and apoptosis, creating the primitive choanae that initially connect the nasal sacs directly to the stomodeum (primitive mouth).[30] This breakthrough is facilitated by the elevation and medial growth of the lateral nasal and maxillary processes, which help position the openings posteriorly.[29] Concurrent with these events, between the sixth and ninth weeks, the secondary palate develops from shelf-like outgrowths of the maxillary processes, which elevate and fuse with the primary palate (formed earlier from the intermaxillary segment) and the nasal septum.[29] This fusion process, involving epithelial breakdown and mesenchymal continuity, separates the nasal and oral cavities while relocating the primitive choanae further posteriorly into the nasopharynx, establishing the definitive secondary choanae bounded by the vomer anteriorly and the palatine bones laterally.[30] Neural crest cells play a critical role throughout this development, migrating from the dorsal neural tube during the third to fourth weeks to populate the frontonasal and maxillary prominences, providing the mesenchyme that differentiates into the skeletal elements framing the choanae, such as the vomer, palatine, and pterygoid bones.[31] These cells, guided by signaling pathways like BMP and FGF, ensure the structural integrity and precise positioning of the choanal openings.[29]Congenital anomalies
Congenital anomalies of the choanae primarily involve choanal atresia and choanal stenosis, which result from developmental disruptions in the posterior nasal passages.[8] These conditions arise from the persistence of embryological structures, such as the buccopharyngeal membrane, leading to incomplete canalization during fetal development.[8] Choanal atresia represents a complete obstruction of the posterior nasal aperture, occurring in approximately 1 in 5,000 to 8,000 live births.[32] It is approximately twice as common in females as in males.[8] Historically classified as bony in about 90% of cases or membranous in the remaining 10%, most choanal atresias are now recognized as mixed, featuring both bony and membranous elements, as determined by endoscopic evaluation and imaging.[8] Unilateral atresia, which accounts for roughly two-thirds of cases, often presents later in infancy with unilateral nasal discharge or recurrent infections, allowing affected infants to breathe through the unobstructed side or mouth.[33] In contrast, bilateral atresia constitutes a neonatal respiratory emergency, as newborns are obligate nasal breathers, manifesting with cyanosis that improves with crying due to oral airflow.[8] The prognosis for unilateral cases is generally favorable with timely intervention, while bilateral cases require immediate stabilization to prevent asphyxia.[34] Choanal atresia is frequently associated with genetic syndromes, highlighting its role as a syndromic marker. In CHARGE syndrome—characterized by coloboma, heart defects, atresia of the choanae, retarded growth, genital anomalies, and ear abnormalities—over 50% of affected individuals exhibit choanal atresia, often bilateral and bony.[35] Similarly, it occurs in craniosynostosis disorders such as Apert syndrome, where premature fusion of cranial sutures accompanies choanal narrowing, contributing to facial dysmorphism and airway compromise.[15] Choanal stenosis, a less severe anomaly, involves partial narrowing of the choanae rather than complete blockage, leading to chronic nasal obstruction, snoring, and recurrent sinusitis that may persist into childhood.[32] Diagnosis typically relies on computed tomography (CT) imaging to assess the degree of narrowing and differentiate it from atresia, often revealing a conical or hourglass-shaped posterior nasal aperture.[8] Treatment for both choanal atresia and stenosis centers on surgical correction to restore patency, with approaches tailored to the anomaly type and patient age. For bilateral atresia, emergent management includes oral airway placement or intubation, followed by transnasal endoscopic repair in the first weeks of life to excise the obstructing tissue and widen the choana.[34] Unilateral cases and stenosis may undergo elective transnasal or transpalatal surgery, with stents sometimes used postoperatively to prevent restenosis; success rates exceed 90% with modern endoscopic techniques, though complications like scarring can necessitate revisions.[8]References
- https://en.wiktionary.org/wiki/choana