MATERIALS AND METHODS

Living specimens of normal and healthy snow trout, Schizothorax curvifrons (25-30 cm in length, body weight 300-350 g) were collected from their natural habitat of the paradise vale. The fishes were killed on the spot with a blow to the head and dissected. Portions of the different parts of the intestinal tract (intestinal bulb, intestine proper and rectum) of 2 cm length were removed immediately after dissection. The different portions of the intestinal tract were incised to expose the mucosal surface, spread out and pinned with the mucosal surface uppermost on thin cork sheets and were rinsed in physiological saline to remove excess mucous and other adhered particles. After rinsing, the tissue samples were fixed in 2.5% glutaraldehyde in 0.1 M sodiumcacodylate buffer (pH 7.3) for 24 h at 0-4°C (Primary fixation). Following primary fixation the samples were transferred to 0.1 M sodiumcacodylate buffer for 24 h and post fixed in 1-4% osmiumtetraoxide in 0.1 M sodiumcacodylate buffer (pH 7.3) for 1-2 h at 0-4°C (Secondary fixation). The post fixed samples were then dehydrated in increasing concentrations of ethanol (30-100%) and critical point dried with liquid carbon dioxide. The serosal surface of each tissue sample was then mounted on the metal stubs with a paste of colloidal sliver paste and coated with a thin conductive film of gold in a sputtering coater and examined under LEO 435 VP scanning electron microscope at All India Institute of Medical Sciences (AIIMS), New Delhi, India.

RESULTS

The mucosa of the intestinal bulb and intestine is thrown into numerous longitudinal major or primary mucosal folds showing zigzag pattern of arrangement. The major mucosal folds are associated with many minor or secondary mucosal folds. The major mucosal folds of the intestine are observed to be smaller in size than the intestinal bulb. A quite distinct concavity is noted to be present between a primary and secondary mucosal folds of the intestinal bulb and the same type of concavity is present between two major mucosal folds of the intestine (Fig. 1, 2). The mucous secreting goblet cells and the columnar epithelial cells bearing microridges are also prominently observed on the mucosal folds of the intestinal bulb and the intestine (Fig. 3-5). On the other hand, the rectal mucosal surface exhibits highly irregular pattern of major mucosal folds, the secondary mucosal folds and the concavity are not observed during the present study (Fig. 6). However, a thin film of mucous is found spread over the major mucosal folds and well defined beautiful minute pores through which the mucous cells discharge their contents are also found in the rectal mucosa.

Fig. 1:	Scanning electron micrograph of intestinal bulb showing primary mucosal folds (MF1) associated with secondary mucosal folds (MF2) and a distinct concavity (¯) between primary and secondary folds X400

Fig. 2:	SEM of intestinal mucosal surface showing primary mucosal folds (MF1), secondary mucosal folds (MF2) and a distinct concavity (¯) between major mucosal folds X400

Fig. 3:	SEM of intestinal bulb showing columnar epithelial cells (CEPC) and mucous cells (MC) on the primary mucosal folds X800

Fig. 4:	Scanning electron micrograph of intestinal bulb showing distinct microridges (®) X3000

Fig. 5:	Scanning electron micrograph of intestinal mucosa showing prominent microridges (®) and mucous cells (MC) X3000

Fig. 6:	Scanning electron micrograph of rectal mucosa showing irregular arrangement of major mucosal folds (MF1) covered with thin film of mucin (®) and pores (<) through which mucous cells discharge their contents. X450

DISCUSSION

Scanning electron microscopic studies of the intestinal bulb and intestine revealed that the mucosa is thrown in to zigzag pattern of mucosal folds. However, the mucosal folds of the intestinal bulb are observed to be much larger and more complex than that of the intestine. These observations are in good agreement with the findings made earlier in other teleosts (Vickers, 1962; Sinha, 1983; Caceci, 1984). Al-Husaaini (1949) in Cyprinus carpio reported that the mucosal folds of the intestinal bulb are thrown in various directions, branch and reunite and the direction of the caudal intestine is predominantly transverse but some zigzagging is present. However, no such branching and reunion has been observed in the intestinal bulb of the fish studied, this may be probably due to the fact that the intestinal tract of Schizothorax curvifrons is approximately 2-3 times longer than the body length. The complex folding of the mucosa of the intestinal bulb with the resultant increase in the surface area might aid in the mixing of food with the hepatopancreatic digestive juice and neutral and acid mucin secreted by the mucous cells, so essential for maintaining the optimum pH for the proper functioning of various enzymes. The arrangement of the mucosal folds of the intestine enable the fish to increase the total mucosal surface area 3-5 times as compared to the ordinary epithelial cells, this increase in total surface area of the mucosa is of vital utility for the effective functioning towards the herbivorous diet of Schizothorax curvifrons. The present observations further revealed that a distinct concavity is noticeable in between the mucosal folds of the intestinal bulb and intestine. The presence of major mucosal folds along with the minor ones aid in increasing the mucosal surface of the intestinal bulb and intestine, whereas, the distinct cavities in between the mucosal folds serve in the retention of ingested food for longer periods, so essential for the effective functioning of the intestine. The presence of mucous cells is a common feature in the intestinal tract of teleosts. The mucous secreted by the mucous cells differ both from species to species and along the intestinal tract. The presence of mucous cells in the intestinal bulb and intestine of Schizothorax curvifrons is suggestive of the fact that the mucin secreted by the mucous cells keeps them moist and also provide lubrication to the food thus enabling easy transport of the ingested material apart from protecting the epithelial cells from mechanical injury.

The rectal mucosa of the Schizothorax curvifrons is thrown into irregularly arranged mucosal folds, minor mucosal folds and the concavity being totally absent. The mucosal folds are covered with a distinct film of mucin and the pores through which the mucous cells expel their contents are also quite distinct. The secretion of mucin to the exterior through pores has been reported by various workers in different teleosts (Kapoor, 1958; Khanna, 1964, 1968; Moitra and Sinha, 1971, 1972; Sinha and Moitra, 1975, 1976; Moitra and Ray, 1977, 1979). The occurrence of major mucosal folds and the absence of minor mucosal folds as well as well defined cavities in the rectal mucosa of Schizothorax curvifrons are suggestive of its insignificant role in food storage, digestion and absorption. However, a distinct film of mucin observed in the rectal mucosa indicates its role as a lubricant helping thereby in easy defecation of the undigested or underdigested food.

Sinha (1983) in adult major carp, Labeo rohita and Sinha and Chakrabarti (1985) in Catla catla have reported the presence of microridges on the luminal plasma membrane of the columnar epithelial cells of the intestinal tract which is also evident from the present study. Microridges are not only restricted to the intestinal segment but, have also been observed in the fish skin (Hawkes, 1974; Lanzing and Hugginbotham, 1974; Harris and Hunt, 1975; Hunter and Nayudu, 1978; Bereiter-Hahn et al., 1979), in the epithelial cells of gills (Rajbanshi, 1977; Olson and Fromm, 1973; Mattey et al., 1979; Mir and Channa, 2009) and in the stratified epithelial cells of buccopharynx and oesophagus (Mallatt, 1979; Sis et al., 1979; Ezeasor and Stokoe, 1980). Therefore, the occurrence of microridges on a variety of teleost epithelial cells probably indicates some common functions. Sinha (1983) reported that microridges represent a mechanical adaptation which in the anterior intestine of adult Labeo rohita would withstand the trauma resulting from ingested materials. Sperry and Wassersug (1976) also reported similar functions of microridges in the esophageal region of trout. In addition of the aforesaid functions, these microridges also help in holding and spreading a thin film of mucin secreted by the adjacent mucous cells of the Schizothorax curvifrons.

CONCLUSION

It may be concluded from the present investigation that the surface architecture of the gut mucosa as revealed by Scanning Electron Microscopy (SEM) under different magnifications throws light on the modifications of mucosal epithelium and morphological characteristics of the various cells lining the gut. These observations are useful in correlating morphological features of the region concerned with functions, like the presence of high primary folds along with numerous secondary folds is an adaptation towards herbivores diet of the adult Schizothorax curvifrons. On the other hand, deep concavities formed by the anastomosis of the mucosal folds serve in the retention of ingested food for longer periods. Similarly, variations in the pattern of microridges in columnar epithelial cells in the different regions of the intestinal tract can be related to their functional significance.

ACKNOWLEDGMENTS

The authors are indebted to whole of the technical staff of the electron microscopy center of All India Institute of Medical Sciences (AIIMS) New Delhi, India, for their excellent technical assistance and Miss Sumaira Nabi, research scholar P.G. Department of Zoology, University of Kashmir, Srinagar, India, for her kind help in carrying out the tissue fixation. They also thank Dr. T.C. Nag (Associate Prof. Deptt. of Anatomy AIIMS) not only for his helpful suggestions but also his sharing of knowledge on the intestinal tract throughout this study.