Summary. The activity of alkaline phosphatase in the various tissue components of the regenerating skin of a cat-fish has been studied. A marked increase in alkaline phosphatase activity in the cells of migrating epithelium has been correlated with their highly active state. High alkaline phosphatase in the basal cells after 2 days has been found to have played an important role in cell multiplication and differentiation. The functional significance of this enzyme is discussed in relation to the granulation tissue formation.Considerable attention has been paid in recent years to investigating changes in alkaline phosphatase activity histochemically in fish skin 1-5. In spite of the fact that alkaline phosphatase plays a significant role in various processes concerned with the regeneration and repair of different tissues 64, such studies in fish skin still remain almost unattempted. The present investigation is to elucidate the changes in alkaline phosphatase activity in the various cellular components of the regenerating skin of a fresh water cat-fish, Heteropneustes fossilis (Bloch) (Heteropneustidae, Pisces). (approximately 16-18 cm in length) were collected from a local pond at Dibrugarh, Assam and were acclimatized to laboratory conditions before the experiment began. Materials and methods. Live specimens of Heteropneustes fossilisIncised wounds approximately 5 mm long and 2-3 mm deep, parallel to the longitudinal axis of the body, were made with a sharp scalpel blade between dorsal fin and the lateral line canal. Skin fragments each containing a wound were removed at various intervals and fixed in 10% neutral formalin at 4~ for 18 h and frozen sections were cut at 15 gm with a freezing microtome. Alkaline phosphatase activity was visualized using the calcium-cobalt method 9 l0 and the coupling azo-dye method . Control sections were prepared by incubating the sections in the absence of the substrates or by incubating the sections after inactivation of the enzyme by 10-min treatment with boiling waterlL

Localization and characterization of different lipids in the cellular constituents of the skin of Heteropneustes fossilis has been made using several histochemical techniques. High contents of cholesterol, its esters and phospholipids have been correlated with the metabolically active state of the basal cells undergoing cell proliferation and differentiation. The polygonal cells in the outermost layer of the epidermis, though rich in phospholipid contain small amounts of cholesterol and its esters. This has been correlated with the metabolically less active state of theses cells. Neutral lipids and phospholipids in the polygonal cells of the outermost layer may contribute to the contents of surface slime and act as an efficient barrier for the penetration of water through the skin. The deposits of neutral lipids in the subcutis may provide energy during the period of fasting, act as a barrier for water diffusion through the skin and serve as shock absorbing pads protecting the fish from mechanical injury.

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Incisional wounds of the same length and depth were made in the skin of catfish Heteropneustes fossilis, and the response in terms of levels of glycogen in cells during healing was studied histochemically. After injury, glycogen disappeared completely from the cells in the migrating epidermis. This disappearance is correlated with the migration of the epidermal cells toward the wound gap for epithelialization and the increased production of mucous cells at the initial stages of healing. The gradual accumulation of glycogen in the epithelial cells and the basal cells after epithelialization of the wound during the early stages of healing suggests that in these cells gluconeogenesis predominates. The gradual disappearance of glycogen from the epithelial cells 72 h after injury and from the basal cells 36 h after injury indicates the gradual resumption of their metabolic activities. The increased gradients of glycogen in club cells that are undergoing vacuolization and disintegration are discussed. No significant levels of glycogen were observed during the formation of granulation tissue and biosynthesis of the sub‐cutis. The depletion of glycogen in the degenerating muscle bundles is correlated with the synthesis of lactic acid which contributes to local acidity and favours the autolysis of muscles.