Ultrastructural studies of epidermis keratinization in grass snake embryos Natrix natrix L. (Lepidosauria, Serpentes) during late embryogenesis

General aspects on skin development in vertebrates with emphasis on sauropsids epidermis

General cellular aspects of skin development in vertebrates are presented with emphasis on the epidermis of sauropsids. Anamniote skin develops into a multilayered mucogenic and soft keratinized epidermis made of Intermediate Filament Keratins (IFKs) that is reinforced in most fish and few anurans by dermal bony and fibrous scales. In amniotes, the developing epidermis in contact with the amniotic fluid initially transits through a mucogenic phase recalling that of their anamniotes progenitors. A new gene cluster termed EDC (Epidermal Differentiation Complex) evolved in amniotes contributing to the origin of the stratum corneum. The EDC contains numerous genes coding for over 100 types of corneous proteins (CPs). In sauropsids 2-8 layers of embryonic epidermis accumulate soft keratins (IFKs) but do not form a compact corneous layer. The embryonic epidermis of reptiles and birds produces small amount of other, poorly known proteins in addition to IFKs and mucins. In the following development, a resistant corneous layer is formed underneath the embryonic epidermis that is shed before hatching. The definitive corneous epidermis of sauropsids is mainly composed of CBPs (Corneous beta proteins, formerly indicated as beta-keratins) derived from the EDC. CBPs belong to a gene sub-family of CPs unique for sauropsids, contain an inner amino acid region formed by beta-sheets, are rich in cysteine and glycine, and make most of the protein composition of scales, claws, beaks and feathers. In mammalian epidermis CPs missing the beta-sheet region are instead produced, and include loricrin, involucrin, filaggrin and various cornulins. Small amount of CPs accumulate in the 2-3 layers of mammalian embryonic epidermis and their appendages, that is replaced with the definitive corneous layers before birth. Differently from sauropsids, mammals utilize KAPs (keratin associated proteins) rich in cysteine and glycine for making the hard corneous material of hairs, claws, hooves, horns, and occasionally also scales.

Cell adhesion and junctional proteins in the developing skin of snakes indicate they coordinate the differentiation of the epidermis

The development of scales and the sequence of epidermal layers during snake embryogenesis has been studied by immunofluorescence for the localization of cell adhesion, adherens, and communicating cell junctional proteins. At about 2nd/3rd of embryonic development in snakes the epidermis forms symmetric bumps at the beginning of scale formation, and they rapidly become asymmetric and elongate forming outer and inner surfaces of the very overlapped scales seen at hatching. The dermis separates a superficial loose from a deeper dense part; the latter is joined to segmental muscles and nerves, likely acting on scale orientation during snake movements. N-cam is present in the differentiating epidermis and mesenchyme of forming scales while L-cam is only/mainly detected in the periderm and epidermis. Mesenchymal N-cam is associated with the epidermis of the elongating dorsal scale surface and with the beta-differentiation that occurs in the overlapping outer surface of scales. Beta-catenin and Connexin-43 show a similar distribution, and they are mainly present in the periderm and differentiating suprabasal keratinocytes likely forming an intense connectivity during epidermal differentiation. Beta-catenin also shows nuclear localization in differentiating cells of the shedding and beta-layers at late stages of scale morphogenesis, before hatching. The study suggests that intensification of adhesion and gap junctions allows synchronization of the differentiation of suprabasal cells to produce the ordered sequence of epidermal layers of snake scales, starting from the shedding complex and the beta-layer.

Ultrastructural studies of developing egg tooth in grass snake Natrix natrix (Squamata, Serpentes) embryos, supported by X-ray microtomography analysis

The egg tooth development is similar to the development of all the other vertebrate teeth except earliest developmental stages because the egg tooth develops directly from the oral epithelium instead of the dental lamina similarly to null generation teeth. The developing egg tooth of Natrix natrix changes its curvature differently than the egg tooth of the other investigated unidentates due to the presence of the rostral groove. The developing grass snake egg tooth comprises dental pulp and the enamel organ. The fully differentiated enamel organ consists of outer enamel epithelium, stellate reticulum, and ameloblasts in its inner layer. The enamel organ directly in contact with the oral cavity is covered with periderm instead of outer enamel epithelium. Stellate reticulum cells in the grass snake egg tooth share intercellular spaces with the basal part of ameloblasts and are responsible for their nutrition. Ameloblasts during egg tooth differentiation pass through the following stages: presecretory, secretory, and mature. The ameloblasts from the grass snake egg tooth show the same cellular changes as reported during mammalian amelogenesis but are devoid of Tomes' processes. Odontoblasts of the developing grass snake egg tooth pass through the following classes: pre-odontoblasts, secretory odontoblasts, and ageing odontoblasts. They have highly differentiated secretory apparatus and in the course of their activity accumulate lipofuscin. Grass snake odontoblasts possess processes which are poor in organelles. In developing egg tooth cilia have been identified in odontoblasts, ameloblasts and cells of the stellate reticulum. Dental pulp cells remodel collagen matrix during growth of the grass snake egg tooth. They degenerate in a way previously not described in other teeth.

Structural and ultrastructural studies on the developing vomeronasal sensory epithelium in the grass snake Natrix natrix (Squamata: Colubroidea)

The sensory olfactory epithelium and the vomeronasal sensory epithelium (VSE) are characterized by continuous turnover of the receptor cells during postnatal life and are capable of regeneration after injury. The VSE, like the entire vomeronasal organ, is generally well developed in squamates and is crucial for detection of pheromones and prey odors. Despite the numerous studies on embryonic development of the VSE in squamates, especially in snakes, an ultrastructural analysis, as far as we know, has never been performed. Therefore, we investigated the embryology of the VSE of the grass snake (Natrix natrix) using electron microscopy (SEM and TEM) and light microscopy. As was shown for adult snakes, the hypertrophied ophidian VSE may provide great resolution of changes in neuron morphology located at various epithelial levels. The results of this study suggest that different populations of stem/progenitor cells occur at the base of the ophidian VSE during embryonic development. One of them may be radial glia-like cells, described previously in mouse. The various structure and ultrastructure of neurons located at different parts of the VSE provide evidence for neuronal maturation and aging. Based on these results, a few nonmutually exclusive hypotheses explaining the formation of the peculiar columnar organization of the VSE in snakes were proposed.

Species Identification of Fragmented or Faded Shed Snake Skins by Light Microscopy

Few convenient/expeditious methods for identifying the species of shed snake skins in specific areas have been developed. The scales on shed snake skins are permeable to light and can be examined by light microscopy (LM), which is of higher availability-especially for wild animal researchers and citizen scientists-than conventional approach which examines the scale microstructures by scanning electron microscopy. We collected and examined a total of 801 shed samples or scale specimens from 53 snake species in Taiwan and adjacent islands, and developed the first guide to identify the fragmented or faded shed skins of most snake species by LM. Morphological characters of scales can be examined by LM include the apical notch, apical pits, apical lobes, keels, scale symmetry, unpigmented spots (mechanoreceptor-like organs), interscale follicles, cross/longitudinal micro-ridge, oberhautchen cells, rows of spines, light/tiny dots, and other microstructures. The microstructures on the scale specimens prepared by the stripped method and the impression method were similar to those on shed skins when examined by LM. We investigated the variations of scale morphology associated with ontogeny, body region, and position on scales, discussed the character evolution of snake scale morphology, and certified that the interscale follicles and the unpigmented spots could also be useful characters for shed skin identification. The methods and results of this study could be applied to identify squamate skins/sloughs and even fecal remnants.

Ultrastructure of endocrine pancreatic granules during pancreatic differentiation in the grass snake, Natrix natrix L. (Lepidosauria, Serpentes)

We used transmission electron microscopy to study the pancreatic main endocrine cell types in the embryos of the grass snake Natrix natrix L. with focus on the morphology of their secretory granules. The embryonic endocrine part of the pancreas in the grass snake contains four main types of cells (A, B, D, and PP), which is similar to other vertebrates. The B granules contained a moderately electron-dense crystalline-like core that was polygonal in shape and an electron-dense outer zone. The A granules had a spherical electron-dense eccentrically located core and a moderately electron-dense outer zone. The D granules were filled with a moderately electron-dense non-homogeneous content. The PP granules had a spherical electron-dense core with an electron translucent outer zone. Within the main types of granules (A, B, D, PP), different morphological subtypes were recognized that indicated their maturity, which may be related to the different content of these granules during the process of maturation. The sequence of pancreatic endocrine cell differentiation in grass snake embryos differs from that in many vertebrates. In the grass snake embryos, the B and D cells differentiated earlier than A and PP cells. The different sequence of endocrine cell differentiation in snakes and other vertebrates has been related to phylogenetic position and nutrition during early developmental stages.

Nidovirus-Associated Proliferative Pneumonia in the Green Tree Python (Morelia viridis)

In 2014 we observed a noticeable increase in the number of sudden deaths among green tree pythons (Morelia viridis). Pathological examination revealed the accumulation of mucoid material within the airways and lungs in association with enlargement of the entire lung. We performed a full necropsy and histological examination on 12 affected green tree pythons from 7 different breeders to characterize the pathogenesis of this mucinous pneumonia. By histology we could show a marked hyperplasia of the airway epithelium and of faveolar type II pneumocytes. Since routine microbiological tests failed to identify a causative agent, we studied lung tissue samples from a few diseased snakes by next-generation sequencing (NGS). From the NGS data we could assemble a piece of RNA genome whose sequence was <85% identical to that of nidoviruses previously identified in ball pythons and Indian pythons. We then employed reverse transcription-PCR to demonstrate the presence of the novel nidovirus in all diseased snakes. To attempt virus isolation, we established primary cultures of Morelia viridis liver and brain cells, which we inoculated with homogenates of lung tissue from infected individuals. Ultrastructural examination of concentrated cell culture supernatants showed the presence of nidovirus particles, and subsequent NGS analysis yielded the full genome of the novel virus Morelia viridis nidovirus (MVNV). We then generated an antibody against MVNV nucleoprotein, which we used alongside RNA in situ hybridization to demonstrate viral antigen and RNA in the affected lungs. This suggests that in natural infection MVNV damages the respiratory tract epithelium, which then results in epithelial hyperplasia, most likely as an exaggerated regenerative attempt in association with increased epithelial turnover.IMPORTANCE Novel nidoviruses associated with severe respiratory disease were fairly recently identified in ball pythons and Indian pythons. Herein we report on the isolation and identification of a further nidovirus from green tree pythons (Morelia viridis) with fatal pneumonia. We thoroughly characterized the pathological changes in the infected individuals and show that nidovirus infection is associated with marked epithelial proliferation in the respiratory tract. We speculate that this and the associated excess mucus production can lead to the animals' death by inhibiting normal gas exchange in the lungs. The virus was predominantly detected in the respiratory tract, which renders transmission via the respiratory route likely. Nidoviruses cause sudden outbreaks with high rates of mortality in breeding collections, and most affected snakes die without prior clinical signs. These findings, together with those of other groups, indicate that nidoviruses are a likely cause of severe pneumonia in pythons.

Unique features of myogenesis in Egyptian cobra (Naja haje) (Squamata: Serpentes: Elapidae)

During early stages of myotomal myogenesis, the myotome of Egyptian cobra (Naja haje) is composed of homogenous populations of mononucleated primary myotubes. At later developmental phase, primary myotubes are accompanied by closely adhering mononucleated cells. Based on localization and morphology, we assume that mononucleated cells share features with satellite cells involved in muscle growth. An indirect morphological evidence of the fusion of mononucleated cells with myotubes is the presence of numerous vesicles in the subsarcolemmal region of myotubes adjacent to mononucleated cell. As differentiation proceeded, secondary muscle fibres appeared with considerably smaller diameter as compared to primary muscle fibre. Studies on N. haje myotomal myogenesis revealed some unique features of muscle differentiation. TEM analysis showed in the N. haje myotomes two classes of muscle fibres. The first class was characterized by typical for fast muscle fibres regular distribution of myofibrils which fill the whole volume of muscle fibre sarcoplasm. White muscle fibres in studied species were a prominent group of muscles in the myotome. The second class showed tightly paced myofibrils surrounding the centrally located nucleus accompanied by numerous vesicles of different diameter. The sarcoplasm of these cells was characterized by numerous lipid droplets. Based on morphological features, we believe that muscle capable of lipid storage belong to slow muscle fibres and the presence of lipid droplets in the sarcoplasm of these muscles during myogenesis might be a crucial adaptive mechanisms for subsequent hibernation in adults. This phenomenon was, for the first time, described in studies on N. haje myogenesis.

Ultrastructural studies of cilia formation during thyroid gland differentiation in grass snake embryos

The process of ciliogenesis that accompanies the differentiation of the thyroid gland in grass snake Natrix natrix L. embryos was studied ultrastructurally. Based on this study, it can be concluded that the ciliogenesis occurred in two waves and that new centrioles duplicated via centriolar pathways. The first wave of ciliogenesis started in the post-mitotic thyrocytes before their polarisation. It ended approximately halfway through the developmental period. The second wave of ciliogenesis took place after the polarization of thyrocytes and before the resting phase of the embryonic thyroid. This wave of ciliogenesis stopped shortly before hatching when fully differentiated thyrocytes restarted their activity. During the first half of thyroid differentiation, the cilia were formed "intracellularly" but during the second half, they differentiated "extracellularly" In the differentiating thyrocytes one cilium per cell was found; however, it could not be excluded that more than one cilium per cell may be formed. These cilia lacked central fibres and therefore they had a 9+0 formula that suggested that they were immotile.

Cross-immunoreactivity between the LH1 antibody and cytokeratin epitopes in the differentiating epidermis of embryos of the grass snake Natrix natrix L. during the end stages of embryogenesis

The monoclonal anti-cytokeratin 1/10 (LH1) antibody recognizing K1/K10 keratin epitopes that characterizes a keratinized epidermis of mammals cross-reacts with the beta and Oberhäutchen layers covering the scales and gastrosteges of grass snake embryos during the final period of epidermis differentiation. The immunolocalization of the anti-cytokeratin 1/10 (LH1) antibody appears in the beta layer of the epidermis, covering the outer surface of the gastrosteges at the beginning of developmental stage XI, and in the beta layer of the epidermis, covering the outer surface of the scales at the end of developmental stage XI. This antibody cross-reacts with the Oberhäutchen layers in the epidermis covering the outer surface of both scales and gastrosteges at developmental stages XI and XII just before its fusion with the beta layers. After fusion of the Oberhäutchen and beta layers, LH1 immunolabeling is weaker than before. This might suggest that alpha-keratins in these layers of the epidermis are masked by beta-keratins, modified, or degraded. The anti-cytokeratin 1/10 (LH1) antibody stains the Oberhäutchen layer in the epidermis covering the inner surface of the gastrosteges and the hinge regions between gastrosteges at the end of developmental stage XI. However, the Oberhäutchen of the epidermis covering the inner surfaces of the scales and the hinge regions between scales does not show cytokeratin 1/10 (LH1) immunolabeling until hatching. This cross-reactivity suggests that the beta and Oberhäutchen layers probably contain some alpha-keratins that react with the LH1 antibody. It is possible that these alpha-keratins create specific scaffolding for the latest beta-keratin deposition. It is also possible that the LH1 antibody cross-reacts with other epidermal proteins such as filament-associated proteins, i.e., filaggrin-like. The anti-cytokeratin 1/10 (LH1) antibody does not stain the alpha and mesos layers until hatching. We suppose that the differentiation of these layers will begin just after the first postnatal sloughing.