Multimodal Imaging and Analysis of the Neuroanatomical Organization of the Primary Olfactory Inputs in the Brownbanded Bamboo Shark, Chiloscyllium punctatum

The olfactory system of sharks and rays in numbers

Cartilaginous fishes have large and elaborate olfactory organs, but only a small repertoire of olfactory receptor genes. Here, we quantitatively analyze the olfactory system of 21 species of sharks and rays, assessing many features of the olfactory organ (OOR) (number of primary lamellae, branches of the secondary folds, sensory surface area, and density and number of sensory neurons) and the olfactory bulb (OB) (number of neurons and non-neuronal cells), and estimate the ratio between the number of neurons in the two structures. We show that the number of lamellae in the OOR does not correlate with the sensory surface area, while the complexity of the lamellar shape does. The total number of olfactory receptor neurons ranges from 30.5 million to 4.3 billion and the total number of OB neurons from 1.5 to 90 million. The number of neurons in the olfactory epithelium is 16 to 158 times higher (median ratio is 46) than the number of neurons in the OB. These ratios considerably exceed those reported in mammals. High convergence from receptor neurons to neurons processing olfactory information, together with the remarkably small olfactory receptor repertoire, strongly suggests that the olfactory system of sharks and rays is well adapted to detect a limited number of odorants with high sensitivity.

Ultramicroscopic organization of the exterior olfactory organ in Anguilla vulgaris in relation to its spawning migration

Background

Catadromous fishes have well-developed elongated olfactory organs with numerous lamellae and different types of receptor neurons related to their breeding migration.

Aim

The current study showed how the olfactory system adapted to the catadromous life. Our work declared the need of the migratory fishes for the sense of smell that is exhibited by a higher number of the olfactory lamellae and the receptor neuron verification in the olfactory epithelium.

Methods

Ten specimens of fully grown, but pre-matured, silver eels of Anguilla vulgaris were captured at the outlet of Edco Lake, overlooking the Mediterranean Sea, east of Alexandria. Olfactory rosettes were dissected and fixed for scanning electron microscope (SEM) and transmission electron microscope (TEM).

Results

Our study gave a morphological description of the olfactory system of A. vulgaris. At the ultrastructural level using SEM and TEM, one olfactory rosette was provided with 90-100 flat radial olfactory lamellae. The nasal configuration allowed water to enter and exit, transferring odorant molecules to olfactory receptor cells which comprise long cylindrical ciliated and microvillous receptors as well as rod-tipped cells. These cells are bipolar neurons with upward dendritic knobs. The olfactory epithelia also include crypt receptor cells. Interestingly, the olfactory neurons are delimited by nonsensory supporting cells, including long motile kinocilia and sustentacular supporting cells beside mucus secretory goblet cells and ionocytes or labyrinth cells that contribute to the olfaction process.

Conclusion

Olfaction is crucial in all vertebrates, including fishes as it involves reproduction, parental, feeding, defensive, schooling, and migration behaviors. Here, A. vulgaris is an excellent model for catadromous fishes. It has a well-developed olfactory organ to cope with the dramatic climate change, habitat loss, water pollution, and altered ocean currents effect during their catadromous life for reproduction.

Head anatomy of a lantern shark wet-collection specimen (Chondrichthyes: Etmopteridae)

In this study, we apply a two-step (untreated and soft tissue stained) diffusible iodine-based contrast-enhanced micro-computed tomography array to a wet-collection Lantern Shark specimen of Etmopterus lucifer. The focus of our scanning approach is the head anatomy. The unstained CT data allow the imaging of mineralized (skeletal) tissue, while results for soft tissue were achieved after staining for 120 h in a 1% ethanolic iodine solution. Three-dimensional visualization after the segmentation of hard as well as soft tissue reveals new details of tissue organization and allows us to draw conclusions on the significance of organs in their function. Outstanding are the ampullae of Lorenzini for electroreception, which appear as the dominant sense along with the olfactory system. Corresponding brain areas of these sensory organs are significantly enlarged as well and likely reflect adaptations to the lantern sharks' deep-sea habitat. While electroreception supports the capture of living prey, the enlarged olfactory system can guide the scavenging of these opportunistic feeders. Compared to other approaches based on the manual dissection of similar species, CT scanning is superior in some but not all aspects. For example, fenestrae of the cranial nerves within the chondrocranium cannot be identified reflecting the limitations of the method, however, CT scanning is less invasive, and the staining is mostly reversible and can be rinsed out.

Microstructure of the Bonnethead Shark (Sphyrna tiburo) Olfactory Rosette

Synopsis

The unusual shape of sphyrnid (hammerhead shark) heads has led to many functional hypotheses of potential sensory advantages and enhanced olfactory performance. Recent investigations into the flow of water within the sphyrnid olfactory chamber demonstrate that this complex structure exhibits a differential pressure system between the 2 nares that induces flow through the chamber. This leads to differential fluid velocities in different parts of the olfactory chamber. Particularly, lamellae at the medial end of the olfactory chamber experience a near-stagnant recirculation of water. The objectives of this study were to (1) describe the microstructure of the olfactory rosette of bonnethead sharks (Sphyrna tiburo) and (2) given the variability of water flow within the sphyrnid olfactory rosette, investigate differences of individual lamellae based on their positioning within the rosette. Specifically, we investigated degree of secondary folding, percent sensory area, and relative surface along the lateral-to-medial gradient. Both degree of secondary folding and percent sensory area may serve as proxies for olfactory sensitivity, providing connectivity between area devoted to sensitivity and water flow within the olfactory organ. We found that bonnethead sharks exhibited similar lamellar morphology to other shark species. We also described the projection of the olfactory nerve layer through an individual lamella. Additionally, we found that lamellae within the medial portion of the organ, which experience slower water velocities, had less secondary lamellar folds and less sensory area. These findings imply that these areas may be less sensitive. Future work should test for sensitivity differences within the rosette along the lateral-to-medial gradient.

Spanish

La forma inusual de las cabezas de los esfírnidos (tiburones martillo) ha llevado a muchas hipótesis funcionales de posibles ventajas sensoriales y unas mejores capacidades olfativas. Las investigaciones recientes sobre el flujo de agua dentro del órgano olfativo de los esfírnidos, demuestran que esta estructura compleja exhibe un sistema de presión diferente entre las dos fosas nasales que induce el flujo en el órgano. Esto conduce a velocidades de fluido diferentes en distintas partes del órgano olfativo. En particular, las láminas en el extremo medial del órgano olfativo experimentan una recirculación de agua casi estancada. Los objetivos de este estudio fueron 1) describir la microestructura de la roseta olfativa de los tiburones cabeza de pala (Sphyrna tiburo) y 2) considerando la variabilidad del flujo de agua dentro de la roseta olfativa de los esfírnidos, investigar las diferencias de las laminillas individuales, basadas en su posición dentro de la roseta. Específicamente, hemos investigado el grado de plegamiento secundario, el porcentaje del área sensorial y el área relativa de superficie a lo largo del gradiente de lateral a medial. El grado de plegamiento secundario y el porcentaje del área sensorial pueden servir como indicadores de la sensibilidad olfativa, proporcionando conectividad entre el área dedicada a la sensibilidad y el flujo de agua dentro del órgano olfativo. Descubrimos que los tiburones cabeza de pala exhibían una morfología laminar similar a la de otras especies de tiburones. También hemos descrito la proyección del estrato del nervio olfativo dentro de una lámina individual. Además, encontramos que las laminillas dentro de la porción medial del órgano que experimentan velocidades de agua más lentas, tenían menos pliegues laminares secundarios y una menor área sensorial. Estos hallazgos implican que estas áreas pueden ser menos sensitivas. El trabajo futuro debería evaluar las diferencias de sensibilidad dentro de la roseta a lo largo del gradiente de lateral a medial.

German

Die ungewöhnliche Kopfform der Sphyrniden (Hammerhaie) hat schon zu vielen funktionellen Hypothesen bezüglich möglicher sensorischer Vorteile und verbesserter olfaktorischer Leistung geführt. Kürzlich veröffentlichte Studien zur Wasserströmung innerhalb der olfaktorischen Kammern von Sphyrniden zeigen, dass diese komplexe Struktur unterschiedliche Drucksysteme zwischen den beiden Nasenlöchern erzeugt, welches eine Strömung durch die Nasenkammer erzeugt. Dies wiederum führt zu unterschiedlichen Flüssigkeitsströmungen in verschiedenen Abschnitten der olfaktorischen Kammer. Besonders bei den Lamellen am medialen Ende der olfaktorischen Kammer gibt es eine fast schon stillstehende Rezirkulation von Wasser. Die Ziele dieser Studie waren 1) das Beschreiben der Mikrostruktur der olfaktorischen Rosette des Schaufelnasen-Hammerhais (Sphyrna tiburo) und 2) wollten wir, aufgrund der Variabilität der Wasserströmung innerhalb der olfaktorischen Rosette der Sphyrniden, die Unterschiede von individuellen Lamellen basierend auf ihrer unterschiedlichen Position innerhalb der Rosette untersuchen. Wir untersuchten den Grad an sekundären Falten, den Prozentsatz an sensorischer Fläche und die relative Oberfläche entlang dem lateral-zu-medialem Gradienten. Sowohl der Grad an sekundären Falten wie auch der Prozentsatz an sensorischer Fläche mögen als Annäherung für die olfaktorische Sensibilität dienen, weil sie für eine Verbindung zwischen der Fläche, die dem Geruchssinn und der Strömung zwischen den olfaktorischen Organen sorgt. Wir fanden, dass die Schaufelnasen-Hammerhaie eine ähnliche lamellare Morphologie zeigen wie andere Hai-Arten. Wir beschreiben auch wie der Geruchsnerv durch eine individuelle Lamelle verläuft. Weiter fanden wir, dass die Lamellen innerhalb des mittleren Teils des Organs, welches geringe Strömungsgeschwindigkeiten erfährt, weniger sekundäre lamellare Falten enthält und weniger sensorische Fläche. Diese Entdeckungen implizieren, dass diese Bereiche weniger sensibel sind auf Gerüche. Zukünftige Arbeiten sollten die unterschiedlichen Sensibilitäten innerhalb der Rosette entlang des lateral-medialem Gradienten testen.

The Establishment of an Optimal Protocol for Contrast-Enhanced Micro-Computed Tomography in the Cloudy Catshark Scyliorhinus torazame

The purpose of this study was to determine the optimal imaging protocol for contrast-enhanced computed tomography (CECT) using micro-CT (μ-CT) for the posterior cardinal vein (PCV), dorsal aorta (DA), hepatic portal vein (HPV), kidney, liver, cephalic arteries (CAs), and gills of Cloudy Catsharks Scyliorhinus torazame. Additionally, we examined the availability of CECT screening for the coelomic organs. Different doses of iopamidol (100, 300, 500, and 700 mg iodine [mgI]/kg) were administered intravenously for 20 s in six sharks. The CT scans from the pectoral girdle to the pelvic girdle were performed at 0-600 s after administration. Contrast-enhanced CT imaging of the CAs, gills, and coelomic organs was examined. Assessment of the signal enhancement value revealed that the PCV was easily visualized with all contrast doses at 25 s. The CAs, gills, and DA were visible at a slightly higher dose (CAs and gills: 200 mgI/kg at 40 s; DA: 300 mgI/kg at 50 s). The HPV was obvious at a dose of at least 500 mgI/kg after a 150-s delay. The parenchyma of the kidney had a contrast effect at 300 mgI/kg, 150 s after the contrast effect of the renal portal system disappeared. The liver, which stores a lot of lipids, had poor overall contrast enhancement that was optimized at the highest dose of 700 mgI/kg. Contrast-enhanced CT screening at 700 mgI/kg and 150 s is likely to obtain the optimal imaging of the reproductive organs, such as the ovary, oviducal gland, uterus, and testis. The present findings can be applied not only to clinical practice but also to academic research and education on elasmobranchs in aquariums.