Light and electron microscope studies of the dermal capillaries in three species of hagfishes and three species of lampreys

Histology and morphometry of the skin of the trident goby Tridentiger brevispinis (Perciformes, Gobiidae)

The Korean trident goby, Tridentiger brevispinis, lives in adverse habitats that can easily become hypoxic due to low precipitation, regional dry periods, and high amounts of solar radiation. Histological and morphometric studies revealed the goby’s specialized skin (35.4–150.0 μm in thickness), consisting of an epidermis and dermis. The thicker epidermis comprises an outermost surface layer (having taste buds, stratified flattened cells, mucous cells, pigment cells, and stratified polygonal cells), middle layer (having stratified polygonal cells), and stratum germinativum (stratified columnar cells). In particular, the dermis has scales, well-developed vascularization, and a few blood capillaries just above the basement membrane, and a reduced diffusion distance was present in the lateral body. Consequently, adaptations such as thicker epidermis, well-developed vascularization, few blood capillaries, and a reduced diffusion distance may provide cutaneous respiration for survival in poorly oxygenated water during the periodic dry season.

Comparative biomechanics of hagfish skins: diversity in material, morphology, and movement

The baggy skins of hagfishes confer whole-body flexibility that enables these animals to tie themselves into knots without injury. The skin's looseness is produced by a subcutaneous blood sinus that decouples the skin and body core and permits the core to contort dramatically without loading the skin in tension or shear. Hagfish skin represents a biological composite material comparable in strength and stiffness to the conventionally taut skins of other fishes. However, our understanding of hagfish skin is restricted to only one of 78 species: The Pacific hagfish Eptatretus stoutii. To determine if other hagfish share similar characteristics with E. stoutii, we measured material properties and compared histological data sets from the skins of four hagfish species: E. springeri, E. stoutii, Myxine glutinosa, and M. hubbsi. We also compared these material properties data with skins from the American eel, Anguilla rostrata. We subjected skin samples from all species to uniaxial tensile tests in order to measure strength, stiffness, extensibility, and toughness of skins stretched along longitudinal and circumferential axes. We also used a series of equibiaxial tensile tests on skin samples from E. stoutii, M. glutinosa, and A. rostrata to measure stiffness of skins simultaneously strained along both axes. Significant results of uniaxial and biaxial tests show that the skins from Eptatretus are anisotropic, being stiffer in the longitudinal axis, and more extensible than the isotropic skins of Myxine. Skins of A. rostrata were stiffer in the circumferential axis and they were stronger, tougher, and stiffer than all hagfish skins examined. The skins of Eptatretus are histologically distinct from Myxine skins and possess arrays of fibers that stain like muscle. These interspecific differences across hagfish skins show a phylogenetic pattern with knotting kinematics and flexibility; both genera belong to distinct but major subfamilies within the Myxinidae, and Eptatretus is known for creating and manipulating a greater diversity of knotting styles than Myxine.

Flexible ammonia handling strategies using both cutaneous and branchial epithelia in the highly ammonia-tolerant Pacific hagfish

Hagfish consume carrion, potentially exposing them to hypoxia, hypercapnia, and high environmental ammonia (HEA). We investigated branchial and cutaneous ammonia handling strategies by which Pacific hagfish (Eptatretus stoutii) tolerate and recover from high ammonia loading. Hagfish were exposed to HEA (20 mmol/l) for 48 h to elevate plasma total ammonia (T_Amm) levels before placement into divided chambers for a 4-h recovery period in ammonia-free seawater where ammonia excretion (J_Amm) was measured independently in the anterior and posterior compartments. Localized HEA exposures were also conducted by subjecting hagfish to HEA in either the anterior or posterior compartments. During recovery, HEA-exposed animals increased J_Amm in both compartments, with the posterior compartment comprising ~20% of the total J_Amm compared with ~11% in non-HEA-exposed fish. Plasma T_Amm increased substantially when whole hagfish and the posterior regions were exposed to HEA. Alternatively, plasma T_Amm did not elevate after anterior localized HEA exposure. J_Amm was concentration dependent (0.05-5 mmol/l) across excised skin patches at up to eightfold greater rates than in skin sections that were excised from HEA-exposed hagfish. Skin excised from more posterior regions displayed greater J_Amm than those from more anterior regions. Immunohistochemistry with hagfish-specific anti-rhesus glycoprotein type c (α-hRhcg; ammonia transporter) antibody was characterized by staining on the basal aspect of hagfish epidermis while Western blotting demonstrated greater expression of Rhcg in more posterior skin sections. We conclude that cutaneous Rhcg proteins are involved in cutaneous ammonia excretion by Pacific hagfish and that this mechanism could be particularly important during feeding.

Rh glycoprotein immunoreactivity in the skin and its role in extrabranchial ammonia excretion by the sea lamprey (Petromyzon marinus) in fresh water

Aquatic organisms employ various strategies to excrete ammonia across the gills, skin, and (or) renal routes. During three different stages of their life cycle, we hypothesized that the basal vertebrate sea lamprey (Petromyzon marinus L., 1758) used the skin as a route for ammonia excretion. Measurements of ammonia excretion using divided flux chambers revealed that extrabranchial sites (skin plus renal) of ammonia excretion were quantitatively more important in larval sea lampreys, but following metamorphosis, the gills became the dominant route of excretion in juvenile sea lampreys. Despite the greater relative importance of the skin in the larval stage, Rh glycoprotein isoforms Rhbg, Rhcg1, and Rhcg2 were detected in the skin in all three sea lamprey life stages examined, but the patterns of expression were dependent on the life stage. We conclude that, during the relatively sedentary filter-feeding larval stage, extrabranchial routes play an equally important role as the gill in facilitating ammonia excretion. However, the gills by virtue of their extensive branchial vasculature become the dominant route of ammonia excretion following metamorphosis because of the need to offload greater amounts of ammonia arising from higher rates of basal ammonia production and the potential to excrete higher amounts of ammonia following ingestion of protein-rich blood in the parasitic stage.

It's all in the gills: Evaluation of O2 uptake in Pacific hagfish refutes a major respiratory role for the skin

Hagfish skin has been reported as an important site for ammonia excretion and as the major site of systemic oxygen acquisition. However, debate remains whether cutaneous O2 uptake is the dominant route of uptake; all evidence supporting this hypothesis has been derived using indirect measurements. Here we use separating chambers and direct measurements of oxygen consumption and ammonia excretion to quantify cutaneous and branchial exchanges in Pacific hagfish (Eptatretus stoutii) at rest and following exhaustive exercise. Hagfish primarily relied on the gills for both O2 uptake (81.0%) and ammonia excretion (70.7%). Following exercise, both O2 uptake and ammonia excretion increased, but only across the gill; cutaneous exchange was not increased. When branchial O2 availability was reduced by exposure to anteriorly-localized hypoxia (∼4.6 kPa O2), cutaneous O2 consumption was only slightly elevated on an absolute basis. These results refute a major role for cutaneous O2 acquisition in the Pacific hagfish.

Evolutionary origins of the blood vascular system and endothelium

Every biological trait requires both a proximate and evolutionary explanation. The field of vascular biology is focused primarily on proximate mechanisms in health and disease. Comparatively little attention has been given to the evolutionary basis of the cardiovascular system. Here, we employ a comparative approach to review the phylogenetic history of the blood vascular system and endothelium. In addition to drawing on the published literature, we provide primary ultrastructural data related to the lobster, earthworm, amphioxus, and hagfish. Existing evidence suggests that the blood vascular system first appeared in an ancestor of the triploblasts over 600 million years ago, as a means to overcome the time-distance constraints of diffusion. The endothelium evolved in an ancestral vertebrate some 540-510 million years ago to optimize flow dynamics and barrier function, and/or to localize immune and coagulation functions. Finally, we emphasize that endothelial heterogeneity evolved as a core feature of the endothelium from the outset, reflecting its role in meeting the diverse needs of body tissues.

Adaptations to in situ feeding: novel nutrient acquisition pathways in an ancient vertebrate

During feeding, hagfish may immerse themselves in the body cavities of decaying carcasses, encountering high levels of dissolved organic nutrients. We hypothesized that this feeding environment might promote nutrient acquisition by the branchial and epidermal epithelia. The potential for Pacific hagfish, Eptatretus stoutii, to absorb amino acids from the environment across the skin and gill was thus investigated. l-alanine and glycine were absorbed via specific transport pathways across both gill and skin surfaces, the first such documentation of direct organic nutrient acquisition in a vertebrate animal. Uptake occurred via distinct mechanisms with respect to concentration dependence, sodium dependence and effects of putative transport inhibitors across each epithelium. Significant differences in the absorbed amino acid distribution between the skin of juveniles and adults were noted. The ability to absorb dissolved organic matter across the skin and gill may be an adaptation to a scavenging lifestyle, allowing hagfish to maximize sporadic opportunities for organic nutrient acquisition. From an evolutionary perspective, hagfish represent a transitory state between the generalized nutrient absorption pathways of aquatic invertebrates and the more specialized digestive systems of aquatic vertebrates.

Phenotypic heterogeneity is an evolutionarily conserved feature of the endothelium

Mammalian endothelial cells (ECs) display marked phenotypic heterogeneity. Little is known about the evolutionary mechanisms underlying EC heterogeneity. The last common ancestor of hagfish and gnathostomes was also the last common ancestor of all extant vertebrates, which lived some time more than 500 million years ago. Features of ECs that are shared between hagfish and gnathostomes can be inferred to have already been present in this ancestral vertebrate. The goal of this study was to determine whether the hagfish endothelium displays phenotypic heterogeneity. Electron microscopy of the aorta, dermis, heart, and liver revealed ultrastructural heterogeneity of the endothelium. Immunofluorescent studies demonstrated marked differences in lectin binding between vascular beds. Intravital microscopy of the dermis revealed histamine-induced adhesion of leukocytes in capillaries and postcapillary venules, but no such adhesion in arterioles. Together, these data suggest that structural, molecular, and functional heterogeneity of the endothelium evolved as an early feature of this cell lineage.

Oxygen consumption by ammocoetes of the lamprey Geotria australis in air

When covered by moistened lint-free gauze, the larvae (ammocoetes) of the lamprey Geotria australis survived, without apparent discomfort, for 4 days in water-saturated air at 10, 15 and 20 degrees C. In air, the mean standard rates of O2 consumption of medium to large ammocoetes of G. australis (mean = 0.52 g) at 10, 15 and 20 degrees C were 14.5, 35.7 and 52.1 microliters.g-1.h-1, respectively. At 15 degrees C, the slope of the relationship between log O2 consumption (microliter O2.h-1) and log body weight for ammocoetes over a wide range in body weight was 0.987. The Q10s for rate of O2 consumption between 10 and 15 degrees C, 15 and 20 degrees C and 10 and 20 degrees C were 4.9, 2.9 and 3.6, respectively. Our results and observations of the ammocoetes suggest that, when out of water, larval G. australis derives most of its O2 requirements from cutaneous respiration, particularly at lower temperatures. This would be facilitated by the small size and elongate shape (and thus a relatively high surface-to-volume ratio), low metabolic rate, thin dermis, extensive subdermal capillary network and high haemoglobin concentration of larval G. australis.