Cellular responses in the skin of rainbow trout (Oncorhynchus mykiss) exposed to Rhine water

Trout, Oncorhynchus mykiss , were exposed to water from the Rhine for 24 days and their skin examined by light and electron microscopy. Relative to control fish mitotic figures were common and seen throughout the epidermis. Pavement cells in fish exposed to Rhine water contained significantly more secretory vesicles than control fish. Necrotic pavement cells were apparent after 24 h, and apoptotic cells from day 4 on. Mucous secretion was intense and the differentiation of mucous cells was stimulated. Some of these cells synthesized mucus of high electron density, probably of a serous composition. Leucocytes invaded the dermis and epidermis, and towards the end of the experiment many apoptotic and necrotic lymphocytes were found. In the dermis fibroblasts were abundant and actively producing collagen. Pigment containing cytoplasmic extensions of melanocytes penetrated into the epidermis. After 14 and 24 days of exposure many pigment cells, melanocytes, iridocytes and xanthocytes became apoptotic. Most of these changes are known from fish exposed to heavy metals, acid water or other stressful treatments, indicating that exposure to Rhine water is a stressful experience for trout.     

Characterization of primary culture of rainbow trout (Oncorhynchus mykiss) skin explants: Growth, cell composition, proliferation, and apoptosis

A trout (Oncorhynchus mykiss) epidermal skin primary explant system was evaluated over 8 d by light and electron microscopy. Three distinct regions of the explant outgrowth were identified on the basis of cell composition. The area immediately adjacent to the founder tissue contained mainly small migrating cells and mucous cells. Of the former. about 20% were mitotic and 6% apoptotic. The middle area was characterized by differentiated pavement cells and mucous cells, with fewer small migrating cells. Proliferation was approximately 30% and apoptosis 5%. Over time, total cell numbers halved as more pavement cells differentiated. The growing front contained many mucous and small migrating cells initially, with few pavement cells. About 50% of the cells were in the proliferative phase, and 5% were apoptotic. Later, there were fewer migrating and mucous cells, with a higher number of pavement cells. About 9% of the cells were apoptotic, and 70% of the cells were proliferating. As in vivo, pavement cells had apical microridges, although they were vacuolated and contained phagocytosed apoptotic bodies. The data and observations are based on the numbers of cell cultures prepared from separate trout giving the sample size n = 7. As this culture system is reproducible and closely approximates the epidermis of trout, it is a powerful tool to study the effects of pollutants, parasites, and endocrine factors on fish skin, eliminating whole-animal factors and reducing the number of experimental animals required.     

Cell renewal in the epidermis of the loach Misgurnus anguillicaudatus (Cypriniformes)

Cell kinetics of the epidermal cells of normal juvenile loach ( Misgurnus anguillicaudatus ) were studied with autoradiography. Fish were labelled with single tritiated thymidine injections and killed at regular time intervals. Three cell types are identified by light microscopy, namely the epithelial cells, the club cells and the mucous cells. Epithelial cells are the only cell type that is involved in cell proliferation and, like the epithelial cells in the epidermis of other teleosts, proliferation of these cells occurs at all epidermal layers. The club cells and the mucous cells seem to be differentiated from the epithelial cells. Based on the time-course study of the labelling index and the grain count halving method, the generation time of the epithelial cells is estimated to be 4 days. From the labelling index of double injections, the duration of the S phase is determined as 8.3 h. Significant cell loss from the outermost layer and cell translocation from the lower layer to the upper layer within 4 days are inferred from the fluctuations of the labelling index curve. The renewal of these cells in the tissue seems rapid in comparison to the epidermis of terrestrial vertebrates.     

The process of skin healing in experimentally wounded carp

The process of skin healing was studied in thin sections of adult mirror-carp, superficially wounded with a razor blade in a scaleless region. Shortly after wounding, epidermal cells from both sides of the wound moved towards the wound cavity. The cells moved as compact groups, without severing the normal intercellular desmosomes. The moving cells displayed phagocytotic activity of cellular debris during the migration. The phagosomes reacted with diaminobenzidine, revealing strong peroxidase content. The normally abundant pinocytotic vesicles from the basal layer of filament cells vanished during the first hour after wounding, and reappeared after 2 days; 24 h after wounding, desmosomes interconnected the filament cells from both sides of the wound. Due to profuse mucus secretion, the number of mucous cells from the epidermal stratum was markedly reduced. Rodlet cells appeared 1 h after wounding in the external region of the epidermis. There was pronounced increase in alkaline phosphatase content of the pavement cells 10 min after wounding; this enzyme appeared around the ridges of the pavement cells and inside the mucous cells 20 min later.
In the dermis, the region surrounding the wound was darkened, blood cells extravasated, and penetrated partially into the epidermis. After 2 days, dermal fibroblasts displayed intense phagocytosis; after 8 days they were particularly abundant in the region of regenerating tissue and were secreting large quantities of collagen. Marked changes in the relative frequency of the different types of leucocyte occurred during the post-wounding days.     

Changes in the mucous cells of the gills, buccal cavity and epidermis of the nine-spined stickleback, Pungitius pungitius L., induced by transferring the fish to sea water

A histochemical analysis of mucous cells in the gills, buccal cavity and epidermis of the nine-spined stickleback, Pungitius pungitius L., using alcian blue and PAS, showed that most cells contained more than one type of glycoprotein. In all regions, the cell content of sialylated glycoproteins increased, and of sulphated glycoproteins decreased during adaptation to sea water. The histochemical properties of the cuticle-secreting cells in the epidermis were less easy to study, but it seems that their content of neutral glycoproteins increased and their content of acid glycoproteins decreased, during adaptation to sea water. Mucous cells were generally less common in seawater-adapted animals, though in the epidermis of fish kept in sea water for 21 days, there were as many as in the freshwater controls. Sulphated glycoproteins may be more effective than others in freshwater osmoregulation for they are highly charged and could attract cations strongly. They may also help to combat bacterial and fungal infections, and perhaps these are less likely when laboratory fish are transferred to sea water.     

Temporal dynamics of epidermal responses of guppies Poecilia reticulata to a sublethal range of waterborne zinc concentrations

This study assessed the histological changes in the epidermis of guppies Poecilia reticulata induced by waterborne zinc (Zn). Laboratory‐reared P. reticulata fry were maintained individually in separate vessels containing artificial water (8 µg l^?1 Zn) to which 0, 15, 30, 60 or 120 µg l^?1 Zn was added. Their epidermal response to Zn was monitored regularly over 4 weeks. Compared with controls, mucus was rapidly released and mucous cell numbers decreased at all concentrations. Thereafter mucous release, epidermal thickness, numbers and size of mucous cells fluctuated at a rate that varied with Zn concentration, but fluctuations declined after day 18. Results clearly highlight the dynamic nature of the epidermal response to sublethal concentrations of waterborne Zn. In general, low concentrations of Zn induced a rapid response with reduced numbers and size of mucous cells and shift in mucin composition, and a subsequent thickening of the epidermis. Epidermal thickness and mucous cell area fluctuated over time but were normal after a month of exposure to low Zn concentrations. The number of mucous cells, however, remained low. Virtually all mucous cells from fish maintained in 15 and 60 µg l^?1 Zn contained acidic mucins throughout the month, whereas fish maintained at 30 µg l^?1 Zn responded by production of neutral mucins during the first 12 days followed by a mixture of neutral and acidic mucins. At 120 µg l^?1 Zn, the most dramatic effects were the gradual but sustained decrease in numbers and area of mucous cells, and the shift to acidic mucins in these cells. Thus, as concentration of Zn increased, the epidermal responses indicated a disturbed host response (dramatic decline in mucous cell numbers, with mixed composition of mucins), which may have been less effective in preventing Zn uptake across the epithelium.     

Changes in marine turbot (Scophthalmus maximus) epidermis and skin mucus composition during development from bilateral larvae to juvenile flat fish

Alike other flat fish, marine turbot has the particularity that changes from larvae with bilateral symmetry to adult with asymmetry, in terms of the position of the eyes. As expected, the skin configuration of this species is also affected by the development and transformation suffered by fish during metamorphosis. In this context, changes in the epidermis of marine turbot were studied using conventional staining and histochemical techniques using six lectins (UEA-I, PNA, RCA-I, WGA, Con A and SBA). During development from larvae to juvenile (3–300 days post-hatching), the epidermis increased in both thickness and the number of cell layers. In fact, the simple cuboidal epithelium observed in larvae at day 3 already became stratified at days 10–12, which sequentially increase in thickness with fish development. Turbot epidermis is composed basically of four cell types: epithelial and mucous or secretory cells that are present through the development, and pigmented cells and a type that the authors described as club-like cells that appear during and post-metamorphosis. The Alcian blue-periodic acid Schiff (AB-PAS) histochemical method revealed the presence of neutral glycoconjugates in mucous and club-like cells at post-metamorphic stages of fish. Accordingly, lectin analysis showed mucous cells containing glycoproteins rich in fucose (UEA-I labelling) and glycoconjugates rich in the sequence galactose-N-acetyl galactosamine (PNA and RCA-I labelling) when this cell type appears. Interestingly, melanophores were observed in the dorsal epidermis of post-metamorphic juveniles. This type of cell contains a black-to-brown pigment that provides the skin the typical colour of this fish species. Changes in mucous coat composition were observed during fish development, which was attributed to different roles of the glycoconjugates.     

Ultrastructural and histochemical study on gills and skin of the Senegal sole, Solea senegalensis

This study was undertaken to identify the normal ultrastructural features of gills and skin of the Senegal sole, Solea senegalensis, for a comparative measure to morphological alterations caused by environmental stressors such as reduced water quality and diseases. In the Senegal sole skin, four morphologically distinct layers were identified: cuticle, epidermis, dermis and hypodermis. The epidermis was composed of stratified epithelium containing three cellular layers: the outermost or mucosa layer, the middle or fusiform layer and the stratum germinativum or the basal layer. In the mucosa, two mucous cell types were differentiated: type A cells containing several round vesicles of different electron density and type B cells containing mucosomes of uniform electron density. Senegal sole have five pairs of gill arches, each containing two rows of well‐developed and compactly organized primary filaments and secondary lamellae. Fingerprint‐like microridges were observed on the surface of epithelial cells. The branchial lamellae epithelium consisted of different cell types: pavement, mucous and chloride. Between the chloride cells and the larger pavement cells, accessory cells were observed. Complexes of tight junctions and desmosomes were frequently observed between adjacent chloride and epithelial cells. Neutral mucosubstances and/or glycoconjugates were observed in the epidermis, dermis and hypodermis of S. senegalensis skin. Proteins rich in different amino acids, such as arginine and cysteine, reacted negatively or weakly positive in the epidermis, dermis and hypodermis. In gills, some mucous cells responded weakly positive to periodic acid‐Schiff (PAS) reaction but were strongly stained with Alcian Blue at pH 0.5, 1 and 2.5. When Alcian Blue pH 2.5–PAS reaction was performed, most mucous cells were stained blue (carboxylated mucins) and some mucocytes stained purple, indicating a combination of neutral and acid mucins. Proteins rich in cysteine‐bound sulphydryl (‐SH‐) and cystine disulphide (‐S‐S‐) groups were strongly detected in branchial and epidermal mucous cells, whereas lysine, tyrosine and arginine containing proteins showed very weak staining in both epidermal and branchial mucous cells. Protein reactions were strongly positive in the pillar cells, except for those rich in tryptophan, whereas the branchial cartilaginous tissue did not show an important reaction. The performed lipid reactions were negative in goblet and chloride cells. It is concluded from this study that ultrastructural and cytohistochemical features of the Senegal sole skin and gills may serve as control structures in both natural and aquaculture systems to monitor or detect environmental stress responses at the histological level.     

Rodlet cells in the epidermis of fish exposed to stressors

Rodlet cells (RC) were detected in the epidermis of carp (Cyprinus carpio) and trout (Oncorhynchus mykiss) exposed to stressors (e.g. acid water, heavy metals, thermal elevation, polluted water of the Rhine river, water loaded with organic manure, and distilled water), as well as after wounding. These cells were not found in the epidemis of fish kept under control conditions, thus suggesting that the appearance of RC in the epidermis is stressor-related. Immature RC, resembling mucous cells, were found in deeper epidermal zones while mature RC were found close to and at the skin surface. The latter contained several rodlets embraced within a capsule-like structure composed of filaments. At the skin surface, RC opened the filamentous capsule at the cell apex and a cytoplasmic tuft, containing the rodlets, protruded outwards into the surrounding water. Structures which appeared to be free rodlets were also observed in the water, near the ridges of the pavement cells. Cytochemical and immunocytochemical study revealed that the rodlets contain alkaline phosphatase at their periphery and peroxidase activity at their cores. These enzymes have been previously reported in mucous cells of stressed fish. All together, the location of RC, their structure, cytochemistry and direction of migration suggest that these cells form part of the non-specific defence mechanism of the skin, and possibly other epithelial tissues.     

Formation of desmosomes and other contact specializations in cultured skin of the frog (Rana esculenta)

Summary Small trypsinized explants from ventral skin of frogs (Rana esculenta) were maintained in culture for 4 days during which a newly formed epithelium differentiated along the cut edges of the dermis. During the first 6 h adjacent cells produced numerous interdigitating lamellipodia. After 2 days, epithelial polarity was restored by the formation of zonulae occludentes and the epithelial cells were joined by a few small newly formed desmosomes and by numerous interdigitations. Bipartite junctional complexes consisting of a zonula occludens, followed by a series of typical desmosomes, and characteristic of adult frog epidermis were formed only after 4 days. When cultured in the presence of an inhibitor of protein synthesis (cycloheximide) the trypsinized epidermis no longer formed desmosomes. Therefore pools of one or more crucial desmosomal proteins must be very low or non-existent. However, cycloheximide did not prevent the formation of cell contact specializations, consisting of a highly developed system of complex lamellar interdigitations, between adjacent cells.     

Ultrastructural differentiation of glandular stomach (proventriculus) in chick embryo

Cytochemical characterization of mucosubstances of chick glanular stomach (proventriculus) changes from 15 days of development to postnatal and adult stages was studied. To corroborate these data cytochemical, ultrastructural and ultracytochemical study of chick embryo proventriculus from 7 to 20 days of development was performed. At the 7th day several layers of undifferentiated cells formed an epithelium which covered the walls of the glandular stomach. Mocosubstances were not found. Between the 9th and 5th day a single layer of cylindrical cells was encountered forming invaginations which originated deep glands. Three types of cells were separated from the above mentioned layer, dark, clear and undifferentiated. The dark cells had organelles which are involved in protein synthesis and the clear ones were rich in mitochondria. Argentaffine cells appeared at 15th day instead mucosubstances formed a thin coat on the epithelium at 9th day which increased at the end of development in the apical cytoplasm and gland cells. These observations demonstrate that proventriculus of chick embryo has ultrastructurally differentiated cells involved with enzymatic and hydrochloric acid secretion after the 9th day. These progressive events are correlated with the digestion process of yolk during embryogenesis. At the end of development the proventriculus has completely organized the glandular layer.     

Degeneration and death,by apoptosis and necrosis,of the pavement and chloride cells in the gills of the teleost Oreochromis mossambicus

Summary Degeneration and death of branchial epithelial cells were studied in an African cichlid fish. In both freshwater and seawater fish the superficially located pavement cells are sloughed off at the end of their lifecycle. This process is preceded by degeneration via a process of cytoplasmic shrinkage and condensation related to apoptotic (physiologically controlled) cell death. The chloride cells are pleomorphic, i.e., accessory, mature, and degenerating cells. Degeneration of chloride cells mainly occurs by apoptosis. Degenerating cells show shrinkage and densification of cytoplasm and nuclei, and swelling of the tubular system; these cells are then separated from the ambient water by pavement cells. They are finally phagocytosed and digested by macrophages. Apoptosis of chloride cells, but not of pavement cells, is greatly stimulated when the fish are in seawater; this reflects an increase in cellular turnover of the chloride cells. Accidental cell death (necrosis) of pavement cells or chloride cells is rarely observed in fully adapted freshwater and seawater fish. Its incidence increases in the first few days following transfer of fish from fresh water to seawater.     

Proliferation of human embryonic and fetal epidermal cells in organ culture

The morphology of human embryonic and fetal skin growth in organ culture at the air-medium interface was examined, and the labeling indices of the epidermal cells in such cultures were determined. The two-layered epidermis of embryonic specimens increased to five or six cell layers after 21 days in culture, and the periderm in such cultures changed from a flat cell type to one with many blebs. The organelles in the epidermal cells remained unchanged. Fetal epidermis, however, differentiated when grown in this organ culture system from three layers (basal, intermediate, and periderm) to an adult-type epidermis with basal, spinous, granular, and cornified cell layers. Keratohyalin granules, lamellar granules, and bundles of keratin filaments, organelles associated with epidermal cell differentiation, were observed in the suprabasal cells of such cultures. The periderm in these fetal cultures formed blebs early but was sloughed with the stratum corneum in older cultures. The rate of differentiation of the fetal epidermis in organ culture was related to the initial age of the specimen cultured, with the older specimens differentiating at a faster rate than the younger specimens. Labeling indices (LIs) of embryonic and fetal epidermis and periderm were determined. The LI for embryonic basal cells was 8.5% and for periderm was 8%. The fetal LIs were 7% for basal cells, 1% for intermediate cells, and 3% for periderm. The ability to maintain viable pieces of skin in organ culture affords a model for studying normal and abnormal human epidermal differentiation from fetal biopsies and for investigating proliferative diseases.     

Cellular responses of the skin and changes in plasma cortisol levels of trout (Oncorhynchus mykiss) exposed to acidified water

The skin structure and the plasma cortisol levels of trout, Oncorhynchus mykiss, were examined during 7 days of exposure to water of pH 5. By day-4 and-7, the thickness of the epidermis was significantly (P<0.05) less in acid exposed fish than in controls, and degenerative cells were common in the upper epidermal layers. Many epidermal cells exhibited signs of necrosis, and by day-7 many apoptotic cells were also present. Secretory vesicles of high electron density were abundant in the filament cells of the 3–4 outermost layers of epidermis, and intercellular spaces had increased. Mitotic figures occureed throughout the epidermis, with the exception of the outermost cell layer. Mucous cells became elongated after day-1, and later, newly differentiating mucous cells could be seen close to the skin surface, and many mucocytes contained mucosomes of high electron density. Rodlet cells were occasionally seen. Chloride cells appeared similar to those of control fish. Many leucocytes, mainly macrophages and lymphocytes, had penetrated the epidermis via the highly undulating basal lamina, and at day-7, numerous apoptotic lymphocytes were found. In the dermis, melanosomes became dispersed in the cytoplasmic extensions of melanocytes which were present in the epidermis of all acid-exposed fish. Iridocytes were rate after day-4, while fibroblasts were abundant and secreted large amounts of collagen. After 1 day of exposure to acidified water, a significant (P<0.05) elevation of the plasma cortisol level had occurred, but this subsequently declined, and had returned to control values by day-7. The changes in skin structure, however, remained throughout the whole exposure period.     

九翅豆蔻种子的解剖学和组织化学研究

九翅豆蔻种子包括假种皮、种皮、外胚乳、内胚乳和胚．由外珠被发育而来的种皮可划分为外种皮、中种皮和内种皮．外种皮由一层表皮细胞构成,其壁增厚并略木质化．中种皮包括下皮层、油细胞层和含２—５层细胞的色素层;各为一层薄壁细胞的下皮层与油细胞层非常压扁．内种皮由一层石细胞构成,极厚,占种皮厚度的１／３—２／３,是种皮主要的机械层;内种皮整体外观呈波浪形,在珠孔端和合点端的内种皮除外．种子在珠孔端分化出珠孔领和孔盖,在合点端分化出下皮细胞垫、大型薄壁细胞区、维管束和合点端色素细胞区．外胚乳细胞内充满淀粉,内胚乳细胞含有大量蛋白质和多糖,胚细胞含有蛋白质、多糖和脂类物质．脂类物质不存在于油细胞中,而存在于胚细胞、部分假种皮细胞、外种皮细胞和内胚乳最外层细胞中．建议将油细胞（层）改称为半透明细胞（层）．     

The ultrastructure of the integument of the American eel,Anguilla rostrata

Summary The morphology and ultrastructure of the lateral body integument of the leptocephalus, glass eel, pigmented elver, and adult stages of the American eel, Anguilla rostrata, were examined with light and electron microscopy. The integument consists of an epidermis separated by a basal lamina from the underlying dermis. Three cell types are present in the epidermis in all stages. Filament-containing cells, which are the principal structural cell type, are increasingly numerous at each stage. Mucous cells, which secrete the mucous that compose the mucous surface coat, are also more numerous in each subsequent stage and are more numerous in the anterior lateral body epidermis than in the posterior lateral body epidermis of the adult. Club cells, whose function is unknown, are most numerous in the glass eel and pigmented elver. Chloride cells are common in the leptocephalus which is marine and infrequent in the glass eel. They are not present in the pigmented elver and adult which inhabit estuaries and fresh-water. Lymphocytes and melanocytes are also present in some stages. The dermis comprises two layers: a layer of collagenous lamellae, the stratum compactum, and an underlying layer of loose connective tissue, the stratum spongiosum.There is a progressive increase in epidermal thickness at each stage which is paralleled by an increase in the thickness of the stratum compactum. Rudimentary scales are present in the dermis of the adult. The increase in the number of epidermal filament-containing cells, epidermal thickness and stratum compactum thickness is correlated with an increased need for protection from abrasion and mechanical damage as the eel moves from a pelagic, oceanic habitat to a benthic, freshwater habitat. The increase in mucous cell numbers is likewise correlated with an increased need for the protective and anti-bacterial action of the mucous surface coat in the freshwater environment.This investigation was supported by NIH research grant NS-11276 from National Institute of Neurological Diseases and Stroke to Dr. J.D. McCleave and by N.S.F. Grant GD 38933 to the Bermuda Biological Station, St. Georges West, Bermuda. Bermuda Biological Station Contribution No. 668     

Seed coat development, anatomy and scanning electron microscopy of Harpagophytum procumbens (Devil's Claw), Pedaliaceae

Seed coat development of Harpagophytum procumbens (Devil's Claw) and the possible role of the mature seed coat in seed dormancy were studied by light microscopy (LM), transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM). Very young ovules of H. procumbens have a single thick integument consisting of densely packed thin-walled parenchyma cells that are uniform in shape and size. During later developmental stages the parenchyma cells differentiate into 4 different zones. Zone 1 is the multi-layered inner epidermis of the single integument that eventually develops into a tough impenetrable covering that tightly encloses the embryo. The inner epidermis is delineated on the inside by a few layers of collapsed remnant endosperm cell wall layers and on the outside by remnant cell wall layers of zone 2, also called the middle layer. Together with the inner epidermis these remnant cell wall layers from collapsed cells may contribute towards seed coat impermeability. Zone 2 underneath the inner epidermis consists of large thin-walled parenchyma cells. Zone 3 is the sub-epidermal layers underneath the outer epidermis referred to as a hypodermis and zone 4 is the single outer seed coat epidermal layer. Both zones 3 and 4 develop unusual secondary wall thickenings. The primary cell walls of the outer epidermis and hypodermis disintegrated during the final stages of seed maturation, leaving only a scaffold of these secondary cell wall thickenings. In the mature seed coat the outer fibrillar seed coat consists of the outer epidermis and hypodermis and separates easily to reveal the dense, smooth inner epidermis of the seed coat. Outer epidermal and hypodermal wall thickenings develop over primary pit fields and arise from the deposition of secondary cell wall material in the form of alternative electron dense and electron lucent layers. ESEM studies showed that the outer epidermal and hypodermal seed coat layers are exceptionally hygroscopic. At 100% relative humidity within the ESEM chamber, drops of water readily condense on the seed surface and react in various ways with the seed coat components, resulting in the swelling and expansion of the wall thickenings. The flexible fibrous outer seed coat epidermis and hypodermis may enhance soil seed contact and retention of water, while the inner seed coat epidermis maintains structural and perhaps chemical seed dormancy due to the possible presence of inhibitors.     

Seed structure and germination in Cuscuta pedicellata with some notes on C. campestris

Dry seeds of Cuscuta pedicellata have a deeply pitted surface due to invaginated epidermal cell walls. After water uptake these walls bulge outwards and the seed surface becomes papillose. The seed coat consists of an epidermis, two palissade cell layers, and a multiple layer of parenchyma cells. The epidermis contains starch and mucilage, the parenchyma cells are compressed but some contain starch. The endosperm consists of starch–filled cells, but has a peripheral aleuron layer. The endosperm cell walls are gelatinous. The variable structure of the seed coat epidermis is believed to function in wind dispersal and rapid water uptake. Seed dormancy is common in the genus, but does apparently not occur in C. pedicellata.     

A comparative study of the epidermis of the common carp and the three Indian major carp

A comparative study has been made of the mucogenic epidermis of the common carp, Cyprinus carpio var. communis, and the three Indian major carps, Catla catla, Labeo rohita and Cirrhina mrigala: on the basis of epidermis structural organization, these species are easily differentiated. The epithelial cells in the superficial layer, as in most fishes, show secretory activity, evidenced by positive histochemical reactions, which is high in C. carpio var. communis, moderate in C. catla and low in L. rohita and C. mrigala. The epithelial cells in the underlying two or three layers also give positive reactions, though their intensity is relatively weak. The mucous cells in C. carpio var. communis are distributed in large numbers arranged in several superimposed layers in the outer regions of the epidermis, whereas in C. catla they are fewer in number and are widely separated in the surface layers as well as in the deeper layers of the epidermis; in both species the mucous cells appear rounded, large, and open on the surface by wide pores. In contrast, in L. rohita and C. mrigala the mucous cells are smaller, restricted mainly to the superficial layer, close together in a single row, and open on the surface by narrow pores. The overall density of mucous cells in L. rohita and C. mrigala, as in C. catla, is much lower than in C. carpio var. communis. In the epidermis of C. carpio var. communis there are a large number of mucous cells, and the few club cells are restricted to the deeper layers. In contrast, in the epidermis of the three Indian major carp the overall density of the mucous cells is much lower and the club cells are very numerous. It is suggested that the high density of club cells compensates an overall low density of mucous cells as an adaptation for an effective defence mechanism. Increased mucus production in the epidermis of C. carpio var. communis, as evidenced by a large number of mucous cells in outer regions and high secretory activity of superficial layer epithelial cells, is associated with increased precipitation of mud held in suspension, needed as an adaptation to the species’peculiar bottom-scooping habits. The varied density of the taste buds in the epidermis of the four carp is associated with their feeding habits.     

Horny cell formation in the epidermis of Rana pipiens

Well preserved transitional cells were found between differentiated cells and horny cells of the frog epidermis, thus facilitating the study of the sequential events involved in horny cell formation. Autolysosomes appear to play an important role in the formation of horny cells. These structures preferentially digest those cytoplasmic components which are not necessary constituents of the terminal horny cell. The release of the contents of the small mucous granules into the intercellular spaces is one of the initial events in horny cell formation. Filaments and large mucous granules seem to be resistant to the lytic digestion and contribute to the bulk of the horny cell. Loss of fluids through the plasma membrane and consolidation of the remaining constituents, results in a flattened horny cell. The appearance of a thickened membrane around the horny cell signifies the completion of the transformation process.