الفهرس 
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Abstract
The present work aimed at throwing some light on histological, scanning electron microscopy (SEM) and immunohistochemical structure of the skin of catfish. To achieve such purpose, specimens from head (dorsal and ventral aspect), trunk (dorsal and ventral aspect) and tail of 90 random mature healthy catfish were collected during different seasons which were subjected to different investigations. For light microscopy, specimens were fixed in 10% buffered neutral formalin and processed until paraffin sections were prepared. In addition, specimens were fixed in 2.5 % glutaraldehyde and processed for scanning electron microscope examination. Imunohistochemical study was carried out using BAX.
The skin consisted of three layers, epidermis, dermis and hypodermis. The epidermis consisted of stratified squamous epithelium containing Malpighian cells, mucous cells and club cells (alarm cells). The Malpighian cell was the main component of the epidermis of fish skin. In the basal layer of epidermis, they appeared as columnar cells, while in the superficial layer the cells became flat. The mucous cells were varied from round to columnar or goblet shape with basal basophilic nuclei and acidophilic cytoplasm. They located at middle and superficial layer of the epidermis. Some of mucous cells opened to the outer surface of the skin and release mucous secretion which form mucus layer. The club cells had different shapes; round, oval and elongated with spherical central basophilic nucleus. They located at the middle layer of epidermis.
The dermis contained stratum spongiosum or adiposum depending upon body region and seasonal variation beside stratum compactum. Stratum compactum composed of bundles of collagen fibers. Stratum spongiosum comprised of loose connective tissue contain collagen, reticular fibers, blood vessels and melanocytes. Stratum adiposum composed of upper layer of connective tissue contain melanocytes and lower layer of clusters of adipose cells which were surrounded by capsule of connective tissue. Hypodermis comprised of loose connective tissue containing adipose cells.
SEM investigation showed elevations of epidermal cells which contained openings of mucous cells. Mucous cells showed lobulated elevations, while club cells showed large elevations. The dermis contained bundles of collagen fibers.
The immunohistochemical reactions of BAX in the catfish cells were varied. It was positive in the middle epidermal cells, faint positive in the club cells, while it was negative in the mucous cells.
During different seasons, it was found that the skin of head during summer was highly folded than that of the winter, spring and autumn. The head epidermis composed of stratified squamous epithelium containing mucous cells more than that of the club cells during summer, while club cells more than the mucous cells during autumn and winter. The dermis of head consisted of stratum adiposum and stratum compactum. The epidermis during spring all over the fish body contained numerous club cells and mucous cells were few.
The epidermis of dorsal and ventral trunk during summer, ventral trunk during autumn and winter contained mucous cells more than the club cells, while the epidermis of the dorsal trunk during autumn contained club cells more than that of the mucous cells. The dermis of dorsal trunk during summer and winter formed from stratum spongiosum and stratum compactum. On the other hand, stratum adiposum and stratum compactum are the principle layers of the dermis of trunk during autumn and spring and ventral trunk during summer and winter. The dermis of dorsal trunk contains numerous melanocytes more than that of ventral trunk.
Generally, the skin of tail was thinner than that of head. The skin of tail during autumn is thinner than the skin of head in this season and thinner than the tail skin of the other seasons. The tail epidermis characterized by more numerous mucous cells in summer than club cells, while the club cells were more abundant than mucous cells in autumn and winter. The dermis consisted of stratum spongiosum and stratum compactum.