Lips are specialized structures that cover the jawbones, and border the anterior orifice of alimentary canal, the mouth. In general, lips and structures associated with them in different fish species may be considered as mainly concerned with the selection, capture, deglutition and pre digestive preparation of food. The effectiveness of these structures is dependent on modifications in relation to food and feeding habits of the fishes and environmental niches inhabited by them.
Morphological data are also key to understanding fish nutrition in ecology and aquaculture, and during development as well as mechanisms for physiological adaptations to a changing environment. A number of the multifunctional roles of the fish lips and associated structures that are discussed incorporate distinctive morphological features that will be highlighted in this chapter. The lips and associated structures represent a significant vertebrate innovation and are highly diversified.
Therefore, present work was undertaken to investigate diversity of the epithelia of lips and associated structures in different fresh-water fish species with the aim to elucidate the surface architecture using Scanning Electron Microscope. The functional aspects of the lips and associated structures in family Gobiidae, Cobitidae, Belontiidae and few species of Cyprinidae show considerable variation and exhibit unique morphological modifications associated with their lips and other structures around the mouth regarding information on the level of surface architecture as seen under SEM in relation to various food and feeding habits and ecological niches.
When we started to survey, collect and organize the current knowledge on lips and associated structures for the invited chapter on SEM, we soon realized that such a study would lead to a greater understanding only if the lips were discussed as incorporate distinctive morphological features.
The successful maintenance of fish populations in challenging environments requires responsive adjustments in their behaviour, morphology and physiology and these have been reflected by modifications at the level of their organ systems, organs and tissues. The lips are no exception to this. The importance of food in daily life of a fish is obvious, and is reflected in the form of the mouth, lips, jaws and so on. These structures present more diverse modifications than any other organ of the body.
Lips and the structures intimately associated with them in different fish species are in direct contact with a complex ever-changing aquatic environment and ecological conditions in which fish inhabits. They are highly sensitive, serve a variety of functions and are characteristically modified in different groups of fishes. These modifications may be associated in some way either with the diet or the method of feeding.
In general, the upper and the lower jaws bear relatively simple and thin lips. These may be thick, fleshy and fimbriated or even unculiferous. In some cases one or both of the lips may regress or fail entirely to develop. The rostral cap is probably present in most fish species, although it may be so reduced as to be overlooked easily. In many forms it lies well above the upper lip and plays no direct role in feeding, while in others it is greatly enlarged, partially or completely overlies the upper lip and plays a major role in gathering food from the substrate. In the older literature the rostral cap and horny jaw sheaths frequently are confused with the lips.
The gross and fine structure of the lips, the rostral caps and the horny jaw sheaths is extremely varied. This involves, among other things, formation of unculiferous fimbriae, tubercles, unculi, papillae, or ridges and grooves of variable height and distribution on the lips and the rostral cap, and sharp cutting edge, cone shaped structure or unculi on the horny jaw sheaths.
Literatures pertaining to the morpho-anatomical structures of the lip in freshwater teleosts are fragmentary and many authors while studying the alimentary canal, briefly described the morphology and structural organisation of the lips of different fish species (Vanajakshi, 1938; Kapoor, 1958; Khanna, 1961, 1962; Pasha, 1964 a, b, c; Saxena & Bakhshi, 1964; Lal et al., 1964; Chitray, 1965; Sehgal, 1966; Moitra & Bhowmik, 1967; Lal, 1968; Sehgal & Salaria, 1970; Moitra & Sinha, 1971; Sinha, 1975; Sinha & Moitra, 1975, 1976, 1978; Kapoor et al.,1975). Suzuki (1956) described the histological organisation of lips of bottom feeding scythe fish
Agrawal & Mittal (1991, 1992 a, b, c) reviewed the literature and described the structural organisation of the epithelia of lips and associated structures of three Indian major carps – a surface plankton and detritus feeder,
Scanning Electron microscope (SEM) reveals the details of surface architecture of tissues to an extent not possible by other procedures. In spite of this fact, the review of literature reveals that not much attention has hitherto been paid to study the surface architecture of fish lips and structures associated with them using SEM. In view of this, Roberts (1982) who examined a variety of fish species using scanning electron microscope, reported that differences in morphology of the lips and associated structures include:
Degree of development and specialisation of the lips,
Degree of development and specialisation of the rostral cap,
Presence or absence of horny jaw sheaths on the jaws in addition to, or in place of the normal lips, and
Form and distribution of unculi on the rostral cap, lips and horny jaw sheaths.
Recently, Ojha & Singh (1992), using SEM described functional morphology of the anchorage system and food scrapers of
Our current knowledge on aspects of modifications in lips and associated structures in fish arises from too little data to arrive at general trends without running the risk of confusing variability, both real and experimental with adaptive phenomenon. What will be required to remedy this situation is a more extensive examination of a larger variety of species, particularly species adapted to different living conditions. This is an area of research for which the extreme biodiversity of fishes will be a powerful tool. Recently, the lips and associated structures of the Labeonini cyprinids have traditionally been identified as important characters in their classification (Yang & Mayden 2010).
The present work has, therefore, been undertaken with the aim to make a comparative study of the organisational pattern of lips and structures associated with them, at scanning electron microscopic levels, in certain fresh-water fish species having different feeding habits and inhabiting varied ecological niches, to address following specific questions:
Do the lips and the structures associated with them in fish species with different feeding habits and inhabiting varied ecological niches show modifications in their organisational pattern?
Does the surface architecture of epidermis of lips and associated structures of the fish species show adaptive modifications in relation to their ecophysiological status and varied feeding habits?
This chapter treats the morphology of lips and associated structures of the family Gobiidae, Cobitidae, Belontiidae and few species of Cyprinidae. The fresh-water fish species inhabiting different ecological niches and having different feeding habits selected for this study are
2. Materials and methods
Live specimens of
In all the fish species investigated the upper jaws, in general, show a variable degree of protrusion. The upper lip (UL), borne on upper jaw, is associated with the rostral cap (RC) (rostralkappe = Minzenmay, 1933), through a fold of skin (FSUR), that in turn continues with the dorsal head skin. The FSUR, in general, is thin and membranous, and shows remarkable capacity of extendibility. The FSUR, when mouth is closed, lies in a deep groove between the UL and the RC.
The lower jaws in the fish species, in contrast, are only slightly protrusive. The lower lip (LL) is borne on lower jaw. Generally, it continues with the ventral head skin directly at the narrow middle region and through a fold of skin (FSLS) at the lateral sides of the lower jaw. In
In the fish species investigated, in general, the epithelia of the UL, the RC, the FSUR, the LL, the FSLS, the AP, the FSLA and the FSAV are mucogenic. The epithelia of the HUJS and the HLJS, in contrast, are keratinized. In the epithelia of the RC and the AP in
In the present account unculi or keratinization are recorded in a total of 5 fishes, and SEM observations are recorded for all of the above mentioned 6 fishes. These observations may be resulted as follows:
The epithelia of the UL, the LL, the RC, the FSUR and the FSLS are mucogenic and are covered by a mosaic of irregularly polygonal epithelial cells of varied dimensions (Fig. 2 a, b, c). The surface architecture of the epithelial cells is characterised by the presence of a series of micro-ridges. The boundaries between adjacent epithelial cells are demarcated by smooth well-defined uninterrupted double row of closely approximated micro-ridges.
The oral cavity of
4.1. Epithelial cells
The surface of the epithelia of the UL and the LL characteristically appears like that of honeycomb. The epithelia appear folded and differentiated in to wide ridges separated by shallow and narrow furrows. The epithelial cells show a variable degree of invagination and thus their surfaces characteristically appear as concave depressions of varied depth (Fig. 2 a, b, c).
The micro-ridges on the surface of the epithelial cells in the epithelia of the UL and the LL appear smooth, extensive, uninterrupted, and are separated by wide furrows. In general, these appear systematically arranged parallel to each other often traversing towards the deeper regions of concave depressions in each cell (Fig. 2 b, c). The micro-bridges interconnecting the adjacent micro-ridges are prominent and are often located close to each other (Fig. 2 c).
The surfaces of the epithelial cells in the epithelium of the RC, in contrast, show only a slight concavity and appear as shallow depressions. The micro-ridges appear smooth, extensive, uninterrupted, at times branched and are separated by wide furrows. In general these are arranged systematically in a concentric manner, traversing almost parallel to the boundary of the cell forming intricate patterns. The micro-bridges interconnecting the adjacent micro-ridges are prominent similar to those in the epithelia of the UL and the LL (Fig. 3 a).
The epithelial cells in the epithelia of the FSUR and the FSLS, unlike those in the epithelia of the UL, the LL and the RC, appear flattened and do not show depressions at their surfaces. The micro-ridges on the surface of the epithelial cells in these regions though are extensive and often traverse parallel to each other are not interconnected by micro-bridges (Fig. 3 b).
4.2. Mucous cells
Interspersed between the epithelial cells in the epithelia of the FSUR and the FSLS mucous cell apertures of varied dimensions are observed similar to those in
4.3. Taste buds
There are two types of TBs in
On the other hand type II-big taste buds on the surface of the epithelium of the RC are observed. At intervals, rounded mounds of epithelial cells bearing this type of comparatively big taste buds (Fig. 4 a). In the central region, the microvilli are arranged in the form of characteristic rosettes (Fig. 4 a). The microvilli of each rosette probably represent sensory hairs originating from sensory cells of the taste buds (Fig. 4 a).
4.4. Superficial neuromasts
Superficial neuromasts sunk slightly in the epithelium are observed. The central region of each such superficial neuromast is characterised by the presence of a characteristic structure consisting of tall-elongated closely approximated projections. This could represent the cupula of the superficial neuromast. The epithelial cells surrounding the superficial neuromasts are concentrically arranged to form a characteristic ring like pattern. (Fig. 4 b).
5.1. Mucogenic epithelia
The epithelia of the UL, the LL, the RC, the FSUR and the FSLS are mucogenic. The epithelia of the UL and the LL are thrown in to distinctive protuberances of variable dimensions delineated by narrow furrows (Fig. 6 a, b). Further, the epithelia of the FSUR and the FSLS are characteristically pleated (Fig. 6 a, b, c).
5.1.1. Epithelial cells
The epithelia of the UL, the LL, the RC, the FSUR and the FSLS in
5.1.2. Mucous cells
Mucous cell openings, seen as wide, rounded apertures or crypts, often containing blobs of mucus, are interspersed between the epithelial cells. Generally, such apertures occur where the boundaries of 3 or more epithelial cells meet (Fig. 7 a, b).
5.1.3. Taste buds
The most conspicuous surface feature of the epithelia of the UL and the LL in
5.2. Keratinized epithelia
The epithelia of the HUJS and the HLJS are keratinized and resemble with each other in their surface architecture. The surface epithelial cells of the HUJS and the HLJS, in general, towards the proximal regions gradually get transformed in to truncated specialised structures - the unculi. Thus the surface architecture of the horny jaw sheaths at their distal regions i.e. the regions boarding the mouth is strikingly distinct from those at the proximal regions i.e. towards the buccal cavity (Fig. 9 a).
The surface of the epithelia at the distal regions of the HUJS and the HLJS are covered by a mosaic pavement of irregularly polygonal epithelial cells of varied dimensions (Fig. 9 b). The free surface of these cells is characterised by the presence of compactly arranged micro-ridges separated by narrow irregular spaces. The micro-ridges, in general, are short, sinuous, beaded, branched with abrupt ends and are irregularly interwoven to form maze like pattern (Fig. 9 c). The boundaries between adjacent epithelial cells are very prominent and appear slightly raised from the general surface of the epithelia (Fig. 9 c). These are demarcated by well-defined double rows of micro-ridges, which appear either lying very close to each other or fused. The central regions of these epithelial cells, in general, show rounded bulge at the surface. Each bulge is further demarcated by a narrow depression around them in the form of a ring (Fig. 9 b, c). These bulges could represent the nuclei of these epithelial cells, which appear greatly flattened in cross sections.
The surface of the epithelia at the proximal regions of the HUJS and the HLJS are studded with characteristic truncated, polygonal unculi (Fig. 9 d). The unculi, in general, appear uniform in dimensions and remain projected at the free surface. Each unculus represents modified surface relief of fine projections of a superficial layer epithelial cell
The epithelia of the UL, the LL, the RC, the FSUR and the FSLS are mucogenic. In contrast, the epithelia of the HUJS and HLJS are keratinized and the dead keratinized epithelial cells at the surface are commonly visualised to be lifted up from the underlying tissues. They are probably in the process of being exfoliated. Further, the jaw sheaths are characterised by the presence of papilliform teeth like structures, which protrude at intervals from their surfaces facing the mouth opening (Fig. 11 a, b, c). The surface of the UL epithelium shows slight infoldings, which are visible even in stretched conditions. The folds on the surface of the LL, in contrast, are more distinct and are distinguished in to prominent ridges separated by shallow gutter-like depressions. Generally, these ridges run parallel to each other along the surface bordering the mouth. (Fig. 11 a, c).
6.1. Mucogenic epithelia
6.1.1. Epithelial cells
The epithelia of the UL, the LL, the RC, the FSUR and the FSLS in
The micro-ridges on the surface of the epithelial cells in the epithelia of the UL and the LL are generally short, straight or sinuous and smooth often arranged in the form of small groups (Fig. 12 c; Fig. 13 b). Several such groups of micro-ridges may be observed on the surface of each cell. Adjacent groups of micro-ridges are delineated from each other by extensive micro-ridges, which are often branched and encircle each group. The micro-ridges within a group are generally arranged parallel to each other either linearly or concentrically. The adjacent micro-ridges are interconnected with each other by fine transverse connections, the micro-bridges (Fig. 12 b).
The micro-ridges on the surface of the epithelial cells in the epithelium of the RC, in contrast, appear smooth, extensive, uninterrupted, at times branched and are separated by wide furrows. In general these are arranged systematically in a concentric manner, traversing almost parallel to the boundary of the cell forming intricate patterns. In the narrow central region of these cells, the micro-ridges are often either indistinct or fragmented (Fig. 12 a, b). The micro-ridges on the surface of the epithelial cells in the epithelia of the FSUR and the FSLS are relatively few, extensive and are located parallel to each other at long intervals (Fig. 13 a). Further micro-bridges could not be located.
6.1.2. Mucous cells
Interspersed between the epithelial cells in the epithelia of the UL, the LL, the RC, the FSUR and the FSLS mucous cell apertures of varied dimensions are observed similar to those in
6.1.3. Taste buds
Taste buds are located on the ridges at the surface of the UL and the LL. Further, the epithelial surface in the regions where taste buds are located is thrown into papillae like projections protruding beyond the general surface of the epithelia. At the summit of each such papilla several microvilli representing the taste hairs of the taste buds are located. The arrangement of the epithelial cells at and around each papilla bearing a taste bud is concentric and the appearance of the taste buds at the summit of these papillae are similar to those in
6.2. Keratinized epithelia
The HUJS and HLJS are covered with a pavement of epithelia of close packed polygonal cells of irregular shape and size. The epithelial cells, however, show regional variations in their surface architecture.
At the proximal regions of the jaw sheaths, the surface of the epithelial cells appears scrawly. The micro-ridges are small, low, irregular and ill defined. Further, the pattern formed by these at the cell surfaces seems indistinctive. The boundary between the adjacent epithelial cells is delineated either by shallow separating clefts or by a double row of micro-ridges separated by distinct spaces (Fig. 14 a, b).
At the distal regions of the jaw sheaths the micro-ridges at the surface of the epithelial cells are frequently punctated (Fig. 14 c) and are separated by wide spaces. In addition short, sinuous, branched micro-ridges interwoven to form characteristic patterns are also observed (Fig. 15 a). The boundaries between adjacent epithelial cells are demarcated by double row of closely approximated micro-ridges.
At and near the apical margins of the horny jaw sheaths the epithelial cells generally, exhibited a surface relief of fine closely approximated micro-ridges such being often prominent in the central parts of the cells. The micro-ridges at the narrow peripheral portions of the cells were relatively short being more widely spaced and irregularly located. The boundaries of the epithelial cells were demarcated by prominent continuous marginal elevations of adjacent cells, sometimes with an inconspicuous gap between them.
The epithelial cells at the distal regions including the apical margins are frequently observed to be lifted up from the underlying tissues. They were probably in the process of being sloughed (Fig. 14 c; Fig. 15 a, b).
7.1. Mucogenic epithelia
The RC is very prominent and greatly enlarged. Its epithelium may be distinguished in to a keratinized belt towards the mouth opening and a major mucogenic region towards dorsal head skin. At the apical margins of the RC mucogenic islands are observed between the keratinized regions. These non-keratinized and keratinized regions show characteristic alternate arrangements.
The epithelium of the AP like that of the RC is distinguished in to mucogenic and keratinized regions. The epithelium of the major central region of the AP is mucogenic. The narrow peripheral regions of the AP are, however, keratinized.
7.1.1. Epithelial cells
The surface of the mucogenic epithelium of the RC is covered by a mosaic pavement of irregularly polygonal epithelial cells of varied dimensions. The free surface of each epithelial cell is characterised by the presence of a series of compactly arranged micro-ridges separated by narrow irregular spaces. The micro- ridges, in general, appear sinuous, having smoothed surface, short with abrupt ends and irregularly interwoven to form maze like patterns. The boundaries between adjacent epithelial cells are demarcated by well-defined double row of micro- ridges, which are often interwoven to give a braided appearance.
7.1.2. Mucous cells
Interspersed between the epithelial cells rounded or irregular shaped crypts could be observed. These crypts often contain blobs of mucus and represent mucous cell openings. Generally, these apertures are located at the points where the boundaries of three or more epithelial cells meet.
7.1.3. Taste buds
A large number of epithelial protrusion or elevations that extends beyond the epithelial surface are located at irregular intervals. Each epithelial elevation is characteristically associated with a taste bud. The epithelial cells covering the surface of the elevations and at their vicinity, in general, are arranged concentrically. At the apical surface of each epithelial elevation, closely packed microvilli are observed. These microvilli appear to represent the taste hairs originating from the sensory cells of the taste buds.
The mucogenic islands in between the keratinized regions at the apical margin of the RC are characterised by the presence of several stumpy epithelial protrusions lying close to each other. Each epithelial protrusion is associated with a taste bud.
The surface relief of the epithelial cells in the mucogenic region of the AP, in general, is similar to that of the epithelial cells in the mucogenic epithelium of the RC. Further, the mucogenic epithelium of the AP resembles with that of the RC in the distribution of a large number of taste buds and in the presence of mucous cell openings.
7.2. Keratinized epithelia
7.2.1. Rostral cap
The surface of the epithelium of the RC in the keratinized regions, in contrast, to that in mucogenic regions appears shaggy. In general, it is matted with rounded projections or excrescencies in an organised array that are separated by shallow grooves.
The surface of each excrescence is represented by a cluster of several (15-25 or even more) prominent somewhat curved spine like unculi each having a broad base and a narrow apical end. Each unculus is projected at the free surface and represents modified surface relief of fine projections of a superficial layer epithelial cell.
These projections show a gradual increase in their height from the peripheral margin to the centre of the cell and in general appear compactly arranged or fused. The apical end of an unculus is either blunt or conical and the surface is rough with vertically oriented micro-villous projections. Between the unculi, the boundaries of the adjacent epithelial cells demarcated by distinct rows of micro-ridges may be observed.
The epithelial cells in shallow grooves between the excrescencies also show modified surface relief of fine projections. These are, however, less prominent and are not differentiated in to unculi like structures.
7.2.2. Adhesive pad
The keratinized epithelium at the posterior and lateral margins of the AP is characterized by the presence of rounded projections or excrescencies similar to those in the keratinized epithelium of the RC. In the keratinized epithelium at the anterior margin of the AP, in contrast, these excrescencies appear relatively prominent and tall. In general, these appear inverted cone shaped or basket like, each with a narrow proximal base, which gradually becomes relatively wide at the distal region. Like in the keratinized epithelium of the RC, the distal surfaces of these projections are characterised with the presence of a cluster of unculi, which represent modified surface relief of fine projections of superficial layer epithelial cells. The boundaries of the adjacent epithelial cells are often clearly demarcated by well-defined uninterrupted rows of micro-ridges.
7.2.3. Horny upper jaw sheath & Horny lower jaw sheath
The surface sculpture of the epithelia of the HUJS and the HLJS is similar to each other and are characteristically studded with tall, truncated, polygonal unculi. These unculi, in general, appear uniform in dimensions and shape and remain projected at the free surface. The unculi are arranged diagonally in parallel rows in an organised manner to form a characteristic pattern on the surface of the horny jaw sheaths. Each unculus, like that of the RC and the AP, represents modified surface relief of fine projections of a superficial layer epithelial cell. In contrast, these projections appear more developed smooth and prominent at the margins of the cells and show a gradual decline in their height towards the central part of the cell. This results in the formation of a characteristic sharp edge at the margin and a deep depression at the central region of each unculus. Each unculus thus appear very much like a tooth.
The rudimentary UL and the LL, and the delicate FSUR, the FSLA, the FSACAP and the FSAV remain concealed and thus the surface architecture could not be visualized because these regions are deeper in position.
8.1. Mucogenic epithelia
8.1.1. Epithelial cells
Surface architecture of the mucogenic epithelia of the UL, the LL, the RC, the FSUR and the FSLS, in general, resembles with each other. The surface of the epithelia is covered by a mosaic pavement of irregularly polygonal epithelial cells of varied dimensions. The free surface of each epithelial cell is characteristically thrown in to a series of micro-ridges having smooth surface. The micro-ridges, in contrast to those of
8.1.2. Mucous cells
Crypts representing the mucous cell openings, often containing blobs of mucus, are frequently observed at the borders of 3 or 4 epithelial cells (Fig. 17 a). The crypts are relatively conspicuous, large, rounded and frequent in the epithelia of the UL, the LL, and the RC, than those of the FSUR and the FSLS.
8.1.3. Taste buds
In the epithelia of the UL and the LL a large number of taste buds are observed (Fig. 18 a, b). Each taste bud is situated on a small epithelial papilla projecting at the surface. The epithelial cells covering the surface of these papillae and at their vicinity, in general, are arranged concentrically (Fig. 17 c; Fig. 18 a, b). At the summit of each papilla closely packed microvilli are observed (Fig. 17 c). These microvilli appear to represent the taste hairs originating from the sensory cells of the taste buds.
8.2. Keratinized epithelia
The epithelial surface of the HUJS and the HLJS at their distal regions are studded with characteristic truncated, polygonal unculi (Fig. 18 b). The unculi, in general, appear uniform in dimensions and remain projected at the free surface (Fig. 18 c; Fig. 19 a). Each unculus represents modified surface relief of fine projections of a superficial layer epithelial cell and resemble in their shape and organisation to those in the epithelia at the proximal regions of the HUJS and the HLJS of
The unculi show a gradual decrease in their height towards the proximal regions of the HUJS and the HLJS. At these regions the surface relief of each unculus in contrast to those at the distal regions appears scraggy (Fig. 19 b, c). The micro-villous projections at the peripheral region on the surface of each cell appear fused to form the outer boundary of each unculus with scrawly surface (Fig. 19 c). The major central part of each unculus is occupied with micro-villous projections, which often appear fused awkwardly to give a scrambled or frothy appearance to the surface (Fig. 19 c).
Between the unculi both at the distal and proximal regions of the HUJS and the HLJS the boundaries of the adjacent epithelial cells demarcated by distinct rows of micro-ridges are observed (Fig. 19 a, c).
9.1. Mucogenic epithelia
9.1.1. Epithelial cells
The surface of the mucogenic epithelia of the UL, the LL, the RC, the FSUR and the FSLS, like those of
9.1.2. Mucous cells
Interspersed between the epithelial cells in the epithelia of the UL, the LL, the RC, the FSUR and the FSLS are observed wide, rounded crypts or pores of varied dimensions, like in
9.1.3. Taste buds
In the RC epithelium, small taste buds are located individually at long intervals (Fig. 22 a). In the epithelia of the UL and the LL, however, a large number of taste buds are observed, each located on a small epithelial papilla projecting at the surface (Fig. 22 a, b). The taste buds at the major distal potion of the lips are characteristically located in-groups arranged in parallel rows (Fig. 22 b, c). The arrangement of the epithelial cells at and around each papilla bearing a taste bud and the appearance of the taste buds at the summit of these papillae are similar to those in
9.2. Keratinized epithelia
The epithelial cells at the surface of the HUJS and the HLJS are studded with characteristic polygonal unculi (Fig. 23 a, b, c). The surface relief of each unculus appears truncated. Each unculus is raised significantly from the general surface as a narrow bend at the periphery and has a shallow depression at its major central region (Fig. 23 a, b, c). Further, the surface relief appears scraggy similar to those at the proximal regions of the HUJS and the HLJS of
Research on lips and associated structures began about 200 years ago, as described by Anson, 1929 in his manuscript “The comparative anatomy of the lips and labial villi of vertebrates”. He made an attempt to define lips and on Danforth’s interpretation of homology, homologous lips are found at certain stages of development in some representatives of all classes of vertebrates. The primary lips characteristic of selachians, after the maxillary and premaxillary bones have developed within the territory of the upper lip (toadfish, cod), may disappear (trout, Spelerpes), accompanied by a forward migration of the lower jaw. The secondary lips of higher forms are first indicated in certain teleosts and amphibians. Lips vary in structure to accord with their physiological functions, whether sensory, prehensile, or adhesive (Anson, 1929). By precise comparative morphology and gene expression analyses, a possibility was inferred that ammocoete lips may not be identical to gnathostome jaws (Kuratani, 2003).
The surface architecture of the superficial layer of epithelial cells in the lips and associated structures is characterised by specialised structures, the micro-ridges forming different patterns in different fish species investigated in this study. These structures in other fishes, have been described as cytoplasmic folds (Merrilees, 1974), microvilli (Harris & Hunt, 1975), microfolds (Hunter & Nayudu, 1978) or ridges (Iger et al., 1988). Insofar as these structures appeared as micro-ridges under SEM and microvilli under TEM. The term “microridges” is used in this study following Whitear & Mittal (1986), Whitear (1990), Suzuki (1992) and Whitear & Moate (1998) and seems appropriate.
The micro-ridges on the surfaces of the mucogenic epithelia form characteristic maze like patterns in different fish species. Fishelson, (1984) correlated the variations in micro-ridge pattern with the locomotory activity of the fish. He suggested that in faster swimming fishes, micro-ridges are most developed and serve to trap mucus on the epithelial surface. The present study, however, is not in support of this since micro-ridges are well developed and conspicuous on the free surface of the lips and associated structures in all the six fish species investigated showing significant difference in their habits and habitats.
The retention of secretion has been the most popular hypothesis of micro-ridge function (Hughes & Wright, 1970; Hughes, 1979; Tillman et al., 1977; Meyer-Rochow, 1981; Fishelson, 1984). Modifications in the pattern of micro-ridges can also be caused by various intrinsic, e.g. hormonal (Schwerdtfeger, 1979 a, b), or extrinsic factors e.g. temperature (Ferri, 1982), salinity (Ferri, 1983), mercury salts (Pereira, 1988), organic pollutants, (Iger et al., 1988), handling and ectoparasites (Whitear, 1990). Some speculations about the function of micro-ridges have centred on mechanical considerations (Lanzing & Higginbotham, 1974; Hawkes, 1974; Sibbling & Uribe, 1985). The provision of reserve apical membrane to allow for distortion was postulated by Zeiske et al., (1976) but Sperry & Wassersug (1976) found no change of pattern after stretching fish oesophageal epithelium and suggested that spread of mucus from goblet (mucous) cells might be guided by the direction of ridges.
Presence of conspicuous micro-ridges on the surfaces of the mucogenic epithelia in the fish species investigated could be considered to reflect high secretory activity of the surface epithelial cells in these regions. Secretion of glycoproteins (GPs), shown histochemically (Pinky et al., 2008, Tripathi & Mittal, 2010), in the surface epithelial cells in the mucogenic regions is in support of this. Further, Whitear (1990) proposed that the form of micro-ridges correlate with the type and rate of secretion at the cell apex. The development of micro-ridges would then be a consequence of arrival of new membranes as vesicles carrying the secretion fuse with the apical plasmalemma and high ridges would indicate a rapid sequence of arrival of secretory vesicles at the surface.
In the epithelia of lips and associated structures of all fish species studied
Secretions elaborated by the epithelial cells and the mucous cells in the mucogenic epithelia could be regarded as an adaptation to lubricate and protect the epithelia from abrasion (Pinky et al., 2002). The role of mucus was likewise postulated previously to inhibit the invasion and proliferation of pathogenic micro-organisms and to prevent their colonisation in fish epidermis (Nigrelli, 1937; Nigrelli et al., 1955; Hildemann, 1962; Liguori et al., 1963; Lewis, 1970).
In the oral cavity the lining mucous membrane becomes keratinized to varying degrees in different animals and also in different areas of the mouth (Adams, 1976). Some of the most dramatic advances made over the past 2-3 decades in epidermal research have come about through the utilization of newly developed biochemical investigative techniques, examples of which include the use of gene cloning to study the organization of the keratin gene family, and the use of immuno-fluorescence with monoclonal antibodies to discern when various keratin proteins appear during differentiation. In SEM studies, the Keratinized surfaces of the fishes studied are shaggy and are matted with an organised array of horny projections separated by shallow grooves. The boundaries of the adjacent epithelial cells (the surfaces of which are modified into unculi) are demarcated by well-defined and distinct rows of microridges.
Horny projections from single cells of lips and associated structures have been reported in a wide variety of fishes by various workers (Leydig, 1895; Rauther, 1911; 1928; Hora, 1922; Minzenmay, 1933; Saxena, 1959; Thys, 1961; Kaiser, 1962; Lal et al., 1966; Saxena & Chandy, 1966; Roberts, 1982). Girgis (1952), in a herbivorous bottom feeder
The present study shows that in
Keratinization occurs in the structures associated with lips of fish
In the epithelia of the RC and the AP in
The keratinized epithelia are mainly composed of the epithelial cells only. The mucous cells and the taste buds are not observed. The absence of the gland cells in the keratinized epithelia suggests an inverse relationship between the degree of keratinization and slime secretion.
In most vertebrates the sense of taste is used as a close range receptor for food item discrimination. Fish are unique among vertebrates in having taste buds widely distributed over various regions. The present study shows that
In the lips and associated structures of
Presence of large number of taste buds in
The taste buds in the lips and associated structures of all the six fish species investigated remain encircled by characteristic concentric whorls of epidermal cells. Harvey & Batty (1998), suggested that it was sometimes possible to locate and count taste buds by the presence of the characteristic ring of epidermal cells surrounding the sensory apex, even when the apex itself was damaged or missing.
Earlier SEM studies reported that fish taste buds fall into three categories based on their external surface morphology (Reutter et al., 1974; Ezeasor, 1982). In addition to the three types of taste buds previously described from various teleost fish, a fourth type comprising very small buds, was found in some cardinal fish (Fishelson, 2004)
The fishes studied are characterised by the peculiar trophic niche they occupy: many scrape epilithic or epiphytic algae and other food items from submerged substrates. This specialized feeding type is possible thanks to the remarkably formed, ventrally placed suckermouth of
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