Keratinization of fish skin with special reference to the catfish Bagarius bagarius

Summary Histochemical reactions indicating keratinization have previously been demonstrated in parts of the epidermis of Bagarius bagarius. Fluorescence histochemistry and electron microscopy have now confirmed these results. Elevated areas of the epidermis are capped by a layer of dead cells with altered contents. On the outer aspect of these cells a dense layer, 18 nm thick, beneath the plasma membrane corresponds to the resistant envelope found in keratinized cells in tetrapod vertebrates. In Bagarius this layer does not extend to all faces of the keratinized cells, but a similar envelope has been detected in two other sites of piscine keratinized epidermis investigated, namely in the breeding tubercles of Phoxinus phoxinus and in the teeth of Lampetra fluviatilis. In the elevated areas of Bagarius-epidermis, the epithelial cells undergo progressive changes in cytoplasmic organization as they become more superficial. The second tier from the surface is sealed by tight junctions and is separated from the overlying keratinized cells by a sub-corneal space resembling that found in keratinized amphibian epidermis. Histochemical evidence of a high lipid content in the outer layers of the epidermis correlates with the presence of lipid inclusions and lamellated membranous profiles in the material studied by electron microscopy. Histochemical results show that the fin skin of Blennius pholis is not keratinized, but secretes a cuticle, histochemically reactive for both proteins and glycoproteins.     

The skin surface of bony fishes

The cuticle described by light microscopists on the skin of various fishes, has been studied by electron microscopy in several species of teleost and in two other actinopterygian fish. Thelcuticle consists of an external coating layer, probably of mucopolysaccharide, which is secreted from the surface epidermal cells, not from the goblet mucous cells. The thickness of the cuticle is commonly of the order of 1 μm. It is particularly well developed in Trigla , where its thickness may vary from a fraction of a micron up to 50 μm, on different parts of the same individual. The cuticle has been detected in species from diverse orders, and is probably a normal constituent of the skin of all bony fishes. It is frequently lost during histological preparation. The external coat continues over the apertures of taste-buds, and may be continuous with the secretion at the mouths of chloride secreting cells. The secretion of the cuticle is partly from, or through, the outer membrane of the epidermal cells, but there is some evidence that cytoplasmic inclusions in the surface epidermal cells are also involved. There is striking variation in the appearance of these inclusions in electron micrographs of different species, and in some cases in different parts of the same fish, notably in Blennius. Certain of the inclusions are membrane-bounded vesicles whose contents are more electron-dense after staining with phosphotungstic acid than with lead citrate. In other cases, the inclusions are electron-transparent vacuoles. The cuticle is briefly compared with similar structures in other aquatic animals.     

Morphology and histology of the anterior dorsal fin of Gaidropsarus mediterraneus (Pisces Teleostei), a specialized sensory organ

Summary The morphology and fine structure of the vibratile anterior dorsal fin of the rockling Gaidropsarus mediterraneus are described. 60–80 fin rays project as a fringe from a reduced fin web; their lateral movement maintains the fin in almost constant rapid undulation, at a frequency of 3–4 beats per second. The fin can be laid back and with-drawn into a groove. Erector and depressor muscles, which are histologically distinct, move each ray. The fin support is modified, incorporating elastic cartilage, and enclosed in a capsule of collagenous connective tissue. The epidermis at the frontal and caudal aspect of each ray contains numerous receptor cells, over 100,000 per mm², which have an apical microvillus and synaptic connections with nerve fibres. The recurrent facial nerve sends a major branch to the dorsal fins, which is joined by dorsal ramuli of spinal nerves. It is calculated that there are three to six million receptor cells on the vibratile fin and in the epidermis of the dorsal groove, in individuals of average size. Taste buds do not occur in the skin of the groove, contrary to a previous report, nor on the vibratile fin rays, although they are present on the prominent most anterior fin ray and elsewhere on the fins and barbels. The undulatory motion of the fin draws sea water towards and through the vibratile rays and backwards as a perceptible current. The fin constitutes a specific sensory organ, a water sampler, peculiar to this rockling and related species.Abbrevations used in figures a aperture - am axial muscles - bl base of lepidotrichion - cc collagenous capsule - dlc dorsal longitudinal canal - dr distal radial - drs dorsal ramulus of a spinal nerve - e epidermal cell(s) - ec elastic cartilage - en extracapsular branch of the recurrent facial nerve - fm fin membrane - fr fin ray - frn fin ray nerve - in intracapsular branches of the recurrent facial nerve - l lepidotrichia - n nerve plexus - ns neural spine - pr proximal radial - rc receptor cell(s) - rdm radial depressor muscle - rem radial erector muscle - s scales - t tendons Dedicated to Professor Konrad Lorenz on the occasion of his 80th birthday     

Fine structure of the axillary gland in the brown bullhead (Ictalurus nebulosus)

The axillary glands of Ictalurus are lobulated invaginations of the epidermis, opening at a pore between the pectoral spine and the cleithrum. Holocrine cells lining a false lumen form a viscous secretion. The secretory cells originate in the tenuous basal layer of the gland wall. Secretion is initiated by the formation of compound vesicles in cells that become very large and have complex cytoplasm of a varied appearance. Golgi systems are well developed and the perinuclear cytoplasm may contain many mitochondria and sacs of ribosomal endoplasmic reticulum; some tracts of cytoplasm are vesicular and contain free ribosomes. Some cells contain numerous large lysosomes, and some have extensive contents of fibrillar masses imperfectly separated by membranes, that recall the appearance of the mucous secretion of goblet cells. The secretory cells break down, releasing the degenerating organelles, including the nuclei, into the false lumen. Some structures are still recognizable in the secretion even after it has been expelled, but the main part of the formed secretion consists of the mucus-like masses. Various leucocytes are found in the gland walls and embedded in the secretion. The fine structure differentiates the holocrine cells of the axillary gland from the club cells of the epidermis, and from the venom glands associated with the fin spines of catfishes. The function of the axillary gland secretion remains unknown.     

Endothelial layers in fish skin

Layers of cells limiting the deep face of the dermis and lining the scale pockets can be described as endothelial, using the term in the broad sense. A dermal endothelium has been found in lampreys and in teleosts of diverse form and habits; it consists of a single layer of modified fibrocytes joined by desmosomal and other junctions and having hemidesmosomes and numerous caveolae intracellulares . A fibrous zone interpreted as elastic tissue intervenes between the dermal endothelium and the collagen of the stratum compactum . The scale pocket lining consists of cells with caveolae, desmosomes, hemidesmosomes and usually with basement membrane. The lining may be one or two cells thick and may occur on both aspects of the scale pocket or only on the deeper side, depending on the species. The fine structure of these endothelial layers is compared with that of the vascular and lymphatic endothelia, the scale-forming cells, the perineurium and the peritoneal lining.     

Bar synapses in the end buds of lamprey skin

Summary The end buds of lamprey epidermis have been considered to be similar to taste buds, but the synapses on the receptor cells are of the type with a dense mass surrounded by lucent vesicles, which is not found in gustatory cells. It is suggested that the end buds may belong to the lateralis sensory system and be involved in the light sensitivity of lamprey skin.     

Oligovillous cells of the epidermis: sensory elements of lamprey skin

The epidermis cf Lampetra spp. contains several kinds of differentiated cell; one innervated variety is characterized by bearing a group of large apical microvilli which project from the surface of the skin. In Lampetra planeri such oligovillous cells are numerous under the oral hood of the ammocoete larva, on the papillae fringing the dorsal fin and bordering the gill vents of the adult, and at the tip of the male genital papilla. Elsewhere on the head, body and fins they are present but more scattered, which appears to be the condition also in adult anadromous Lampetra fluviatilis . There are differences in the number and dimensions of the microvilli found on oligovillous cells, but each is supported by a stout core of actin filaments extending a variable distance down into the cytoplasm. Under the apex of the cell there are microtubules and numerous vesicles which are thought to be concerned in the renewal of the membrane on the microvilli. Beside and proximal to the nucleus is a system of channels of rough endoplasmic reticulum, and a stack of membranous cisternae which appears to have been derived from the endoplasmic reticulum. A nerve fibre is associated with the base of the cell which is indented by a spur-like process from the neurite. Typical "synaptic vesicles" are not found in the cell but irregular vesicular or canalicular profiles are associated with the cell membrane adjoining the neurite spur. The space between the cell and neurite membranes contains extracellular material with a characteristic appearance of prickle-like densities on the cell side meeting densities on the neurite membrane. Variations in the cytology of oligovillous cells can be explained in terms of a cycle of development and de-differentiation. Certain cells with vesicles throughout the cytoplasm and with a narrow apex without microvilli are interpreted as degenerate examples. The oligovillous cells are thought to be chemosensory receptors.     

On the occurrence of merkel cells in the epidermis of teleost fishes

Summary The ultrastructure of a differentiated cell type in the epidermis of two species of teleost fish, Ictalurus melas and Phoxinus phoxinus, is described. This cell type has a synaptic association with nerve fibres, microvillus-like peripheral processes, and membrane-bounded inclusions, which together are the diagnostic features of the Merkel cells of tetrapod vertebrates. Other cytoplasmic features are shared with the epithelial cells. The appearance of the membrane-bounded granules depends on the fixative used; after fixation with glutaraldehyde the granules are of a size and electron-density comparable to that found in tetrapod Merkel cells, but after fixing in osmium tetroxide the granules are inconspicuous.Our thanks are due to Mr. A.C. Wheeler of the British Museum (Natural History) for help with the identification of the species of Ictalurus, and to Mr. E. Perry for technical assistance. One author (EBL) was supported by a SRC research studentship     

The location of silver in frog epidermis after treatment by ranvier's method,and possible implication of the flask cells in transport

Summary Frog skin, bladder wall, and sciatic nerve were treated by Ranvier's silver nitrate method and subsequently fixed and sectioned for electron microscopy. In the epidermis of the skin, more silver is found deposited in the flask cells than elsewhere, especially as a sub-apical plaque in the neck of the flask, which appears after the skin has been flooded with silver nitrate for 5 minutes. Mitochondria rich cells in the bladder also accumulate more silver than the surrounding epithelial cells, but do not show such a distinct sub-apical plaque. In myelinated nerve fibres treated similarly, silver accumulates in the paranodal regions of the axon, and outside the axon at the node. It is suggested that silver may accumulate near a site of ion transport, due to structural specialisations not visible by standard electron microscope techniques, and that the flask cells may therefore be implicated in transport in the frog skin.