الفهرس 
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Abstract
The skin is the outermost covering of the body, so it is in direct contact with environment. for this reason, it constitutes the primary primary physical barrier between fish and its external environment. It is responsible for maintaining the normal physiological function, its useful to reflect the health condition of the fish. This study aimed to characterize the skin structure in fresh and brackish water fish and make comparison between them. The current study was performed on thirty specimens fresh and brackish water fish. Three types of fresh water fish (common carp, catfish and Nile tilapia) and three types of brackish ware fish (grey mullet, sea bass and red porgy) were used. the study based on the micro-anatomical investigation of the skin fragments from the back of the fish. The results of the current study revealed that the skin the investigated fish consisted on three main layers, represented by the epidermis, the dermis and the hypodermis. The epidermis was formed of basal layer, middle layer and outermost layer. The mean thickness of the epidermis was in the investigated fish varied significantly. The epidermal thickness was higher in fresh water fish compared to brackish water fish. There are two types of unicellular glands in the epidermis, such as alarm cells (club cells) in the epidermis of common carp and catfish and mucous cells in the epidermis of all studied fish. The mean count of mucous cells was significantly varying between the studied fish. The skin of brackish water fish had higher density of mucous cells than that of fresh water fish. The dermis was consisted of the outer spongy layer and the inner compact layer. The mean thickness of the dermis in the present study differed significantly between the investigated fish. The dermis of fresh water fish was thicker than that of brackish water fish. The melanocytes were more numerous in fresh water fish than in brackish water fish. This is an adaptation to turbidity of fresh water. The hypodermis consisted of a layer of connective tissue containing blood vessels and adipocytes. The current study concluded that fish farms should raise brackish water fish because of the low incidence of diseases in them because they contain many mucous cells that protect them from pathogens. Also, the lack of pigment cells in it make it attractive to customers.