An additional bone in the sclera of the eyes of owls and the common potoo (Nictibius griseus) and its role in the contraction of the nictitating membrane

An Overview of the Penguin Visual System

Penguins require vision that is adequate for both subaerial and submarine environments under a wide range of illumination. Here we provide a structured overview of what is known about their visual system with an emphasis on how and how well they achieve these goals. Amphibious vision is facilitated by a relatively flat cornea, the power in air varying from 10.2 dioptres (D) to 41.3 D depending on the species, and there is good evidence for emmetropia both above and below water. All penguins are trichromats with loss of rhodopsin 2, a nocturnal feature, but only deeper diving penguins have been noted to have pale oil droplets and a preponderance of rods. Conversely, the diurnal, shallow-diving little penguin has a higher ganglion cell density (28,867 cells/mm²) and f-number (3.5) than those that operate in dimmer light. In most species studied, there is some binocular overlap, but this reduces upon submergence. However, gaps in our knowledge remain, particularly with regard to the mechanism of accommodation, spectral transmission, behavioural measurements of visual function in low light, and neural adaptations to low light. The rarer species also deserve more attention.

Anatomical study of the scleral ring and eyeball of the long-eared owl (Asio otus) with anatomical methods and diagnostic imaging techniques

BACKGROUND

The scleral ring in birds consists of ossicles that are fixed as small plates by cartilage joints and have no articulation to other parts of the skeleton.

OBJECTIVE

Due to inadequate examination of the scleral ring anatomy and its specific form in owls, this study aimed to investigate the exact structure of the scleral ring and some morphometric characteristics of the eyeball in a long-eared owl (Asio otus).

METHODS

The eyes of 20 alive and 10 dead male and female owls were examined. In addition to common anatomical methods, computed tomography scans and radiographic and ultrasonographic imaging techniques were used in this study.

RESULTS

The structure consisted of 15 ossicles. In the ventral part of the ring, these tubercles were observed in the scleral rings of all owls; in each ring, there were four bones with these tubercles. Additionally, there was no significant difference between the left and right eye parameters. Most ocular parameters in female owls were larger than those in males, but in the case of some parameters, such as optic nerve length and optic nerve sheath diameter, this difference was not observed.

CONCLUSIONS

According to this study, the scleral ring in the Asio otus has anterior and posterior parts, and the lens is in the immediate vicinity of the anterior part. The right and left scleral rings and eyeballs are bilaterally symmetrical in terms of the shape, size, and number of ossicles that form the ring.

Visual adaptation and retinal characterization of the Garganey (Anas querquedula): Histological and scanning electron microscope observations

The present study was designed to provide a complete morphological description of the eye of the migratory Garganey duck (Anas querquedula) and its visual adaptation with the different surrounding environmental conditions during its migration journeys using a stereomicroscope, scanning electron microscope (SEM), and light microscope. The current work depends on six adult Garganey ducks that were captured from the area near and on the shores of Edku city. The obtained results revealed that the eye of the migratory Garganey duck has the features of both diurnal and nocturnal birds. The histological examination reveals that the pigmented epithelium of the retina has long prolongations filled with melanin. The cone is the dominant photoreceptor, but simple rods are present. The inner nuclear and ganglion cell layers are thick. SEM examination shows that the arrangement of the collagen fibrils on the external surface was reticular in shape. The radial folds present as pledged structures on the pigmented epithelium covered with circular structures. The main lens body has flat with hexagonal outlines fibers. The edges and surfaces of these hexagonal fibers were studded with protrusions or elevations (balls) and depressions (sockets). The sockets and balls were either rounded or ellipsoid in shape. The balls were more on the edges and the sockets on the surface. In conclusion, our findings indicated a higher degree of functional adaptation between the morphological structure of the eye and the surrounding environmental conditions.

Skeletal elements of the penguin eye and their functional and phylogenetic implications (Aves: Sphenisciformes: Spheniscidae)

Scleral ossicles and other bony elements are present in the eyes of many vertebrates, including birds. In this study, the skeletal elements present in the penguin eye and orbit were imaged using macro photographs and micro-computed tomography (micro-CT), to help elucidate their function and significance. A total of 36 scleral rings and three whole skulls were imaged. King (Aptenodytes patagonicus), Fiordland crested (Eudyptes pachyrhynchus), Snares crested (Eudyptes robustus), royal (Eudyptes schlegeli) and yellow-eyed (Megadyptes antipodes) penguins had between 12 and 14 elements in their scleral ring while the gentoo (Pygoscelis papua) had 14 and 17; little penguins (Eudyptula sp.) consistently had between 10 and 12 elements. All had at least two elements that overlapped, usually totally, each neighbour, and two that were overlapped by each neighbour. The interior structure of all ossicles revealed a lattice-like arrangement of struts typical of cancellous bone, the whole being surrounded by thick cortical bone. The scleral ring of a 10 week gentoo chick was not completely ossified but rather had multiple small holes within it on micro-CT. A large os opticus was present in one king penguin but in another bird of the same age and gender there was no such bone. Much smaller accessory bones were found in the posterior pole of one Snares crested and one little penguin. We conclude that the penguin scleral ring not only maintains the shape of the eye but also provides protection and a site of insertion for rectus muscles. However, the extreme variability in the os opticus suggests that it is not essential to normal function.

The Ultrastructure of the Nictitating Membrane of the Little Penguin (Eudyptula minor, Aves)

The ultrastructure of the nictitating membrane in the little penguin Eudyptula minor was studied using both scanning and transmission electron microscopy to improve our understanding of the function of ocular adnexa in diving birds. Following euthanasia, eyes were enucleated and immersion fixed in Karnovsky's fixative. The nictitating membrane and conjunctiva were embedded in araldite and semi- or ultra-thin sections were stained and photographed using compound and transmission electron microscopes, respectively. Ultrastructural dimensions were measured directly from digital photographs. Surface ultrastructure was examined using scanning electron microscopy. The transparent nictitating membrane consists of a dense stroma surrounded by epithelia on both the external (conjunctival) and internal (bulbar) surfaces. The conjunctival surface of the membrane near the leading edge is covered by microvilli, which transition to microplicae and finally to microridges in the periphery. Beneath the epithelial cells, there is a well-developed basement membrane. Scattered throughout this epithelium are a few goblet cells. The surface of the bulbar epithelium is covered by microvilli near the leading edge, which become denser peripherally. The stroma consists of densely-packed collagen fibrils, which are randomly oriented in bundles near the leading edge but are aligned in the same direction parallel with the epithelial and corneal surfaces and with the leading edge, when the membrane is extended. The ultrastructure of the nictitating membrane in the little penguin differs from other birds and its function is predominantly protective, while preserving clear vision in both water and air.

Origin of the avian predentary and evidence of a unique form of cranial kinesis in Cretaceous ornithuromorphs

The avian predentary is a small skeletal structure located rostral to the paired dentaries found only in Mesozoic ornithuromorphs. The evolution and function of this enigmatic element is unknown. Skeletal tissues forming the predentary and the lower jaws in the basal ornithuromorph Yanornis martini are identified using computed-tomography, scanning electron microscopy, and histology. On the basis of these data, we propose hypotheses for the development, structure, and function of this element. The predentary is composed of trabecular bone. The convex caudal surface articulates with rostromedial concavities on the dentaries. These articular surfaces are covered by cartilage, which on the dentaries is divided into 3 discrete patches: 1 rostral articular cartilage and 2 symphyseal cartilages. The mechanobiology of avian cartilage suggests both compression and kinesis were present at the predentary-dentary joint, therefore suggesting a yet unknown form of avian cranial kinesis. Ontogenetic processes of skeletal formation occurring within extant taxa do not suggest the predentary originates within the dentaries, nor Meckel's cartilage. We hypothesize that the predentary is a biomechanically induced sesamoid that arose within the soft connective tissues located rostral to the dentaries. The mandibular canal hosting the alveolar nerve suggests that the dentary teeth and predentary of Yanornis were proprioceptive. This whole system may have increased foraging efficiency. The Mesozoic avian predentary apparently coevolved with an edentulous portion of the premaxilla, representing a unique kinetic morphotype that combined teeth with a small functional beak and persisted successfully for ∼60 million years.

O olho da coruja-orelhuda: observações morfológicas, biométricas e valores de referência para testes de diagnóstico oftálmico

Objetivou-se relatar características morfológicas do bulbo ocular e determinar valores de referência para testes oftálmicos selecionados em corujas-orelhudas (Asio clamator). Foram estudados 32 olhos de 16 corujas (Asio clamator), adultas e jovens, machos e fêmeas, de vida livre. Sendo compilados dados referentes a observações morfológicas do crânio, bulbo ocular e anexos, além de mensuração de testes oftálmicos, incluindo, Teste Lacrimal de Schirmer (TLS), cultura da microbiota normal da conjuntiva, estesiometria, pressão intraocular (PIO), espessura de córnea central (ECC), diâmetro horizontal da rima palpebral, diâmetro horizontal da córnea e oftalmoscopia indireta. Vinte e dois tipos de bactérias foram identificados em 12 corujas havendo predominância de microrganismos Gram-positivos. A média encontrada para o TLS foi de 5,03±3,28mm/min, para o diâmetro horizontal da rima palpebral em 16 corujas foi 21,24±1,17mm, e 15,7±2,74mm para o diâmetro horizontal da córnea. O valor médio para o teste de estesiometria foi de 0,80±0,59cm, a PIO média de 13,81±5,62mmHg e ECC média de 0,28±0,03cm. O estudo contribuiu para a caracterização da morfologia ocular e para o estabelecimento de valores de referências de testes diagnósticos oftálmicos em corujas-orelhudas, sendo necessário ainda o desenvolvimento de estudos complementares sobre histologia ocular desta espécie.

Osteologia craniana de Nyctibiidae (Aves, Caprimulgiformes)

A família Nyctibiidae (urutaus) apresenta sete espécies incluídas em um único gênero Nyctibius, distribuídas por toda a região neotropical desde o México até a Argentina, alcançando sua maior diversidade na região amazônica. São aves de hábito noturno caracterizadas por um distintivo mimetismo em troncos vegetais, onde permanecem imóveis durante o período diurno. Devido seus hábitos noturnos e comportamentos crípticos, o estudo de seus hábitos de vida é excessivamente difícil, o que faz desse grupo um dos menos conhecidos da região tropical. Logo, informações sobre comportamento e história natural da família são muito escassas, e raras são as contribuições a respeito de sua anatomia. O objetivo do presente trabalho foi descrever detalhadamente a osteologia craniana de seis das sete espécies de Nyctibiidae reconhecidas, incluindo Nyctibius griseus, N. grandis, N. aethereus, N. jamaicensis, N. leucopterus e N. bracteatus. Observa-se uma grande variação na osteologia craniana dentro da família, a qual apresenta um crânio bastante modificado e adaptado aos seus hábitos de vida, basicamente no que se refere a abrigar os olhos extremamente desenvolvidos e a proporcionar uma grande abertura bucal. Os ossos que formam o teto da cavidade bucal apresentam um achatamento dorso-ventral, particularmente nos ossos pterigóide e paraesfenóide, e o osso palatino é muito desenvolvido lateralmente. Na região de fusão da maxila com o arco jugal observa-se uma projeção, única entre as aves, a qual é vista até externamente com a ave em vida. O osso vômer tem grande variação dentro da família, apresentando um número variável de projeções rostrais entre as espécies. A região caudal do crânio é bastante larga, havendo grande distância entre os ossos quadrados, os quais são verticalmente posicionados e apresentam um reduzido processo orbital. A mandíbula, elástica e flexível, apresenta uma curta região sinfisiária e articulações sindesmóticas na região mediana dos ramos mandibulares. De forma geral, os Nyctibiidae apresentam um crânio bastante especializado, sendo possível muitas vezes estabelecer relação entre a forma das estruturas e os hábitos de vida das espécies. Mais ainda, a grande variação interespecífica no crânio observada ressalta a necessidade de uma revisão taxonômica das espécies, atualmente incluídas em um único gênero.

Skeletal elements in the vertebrate eye and adnexa: morphological and developmental perspectives

Although poorly appreciated, the vertebrate eye and adnexa are relatively common sites for skeletogenesis. In many taxa, the skeleton contributes to internal reinforcement in addition to the external housing of the eye (e.g., the circumorbital bones and eyelids). Eyeball elements such as scleral cartilage and scleral ossicles are present within a broad diversity of vertebrates, albeit not therian mammals, and have been used as important models for the study of condensations and epithelial-mesenchymal interactions. In contrast, other elements invested within the eye or its close surroundings remain largely unexplored. The onset and mode of development of these skeletal elements are often variable (early versus late; involving chondrogenesis, osteogenesis, or both), and most (if not all) of these elements appear to share a common neural crest origin. This review discusses the development and distribution of the skeletal elements within and associated with the developing eye and comments on homology of the elements where these are questionable.