Histology of gill , gonad , kidney, liver and digestive system of Persian sturgeon (Acipenser persicus)

Sturgeons are valuable species of the Caspian Sea and the Persian sturgeon Acipenser persicus is an endemic species of the south Caspian Sea. Possessing the highest breeding percentage in sturgeon hatcheries and catch numbers of this species is of particular importance in Iran regarding rehabilitation of stocks and fingerling release. The main aim of this study was to understand the histological structure of this valuable species and to make use of this information in other basic and applied research. The present study was conducted from 1998 through 2001 on a total of 34 A. persicus specimens (18 male and 16 female) with an average weight of 13.75 kg and an average length of 146.3 cm. Fishes under study were caught using gillnets and beach seine in the fishery zones 1 and 2. Biometric measurements of fish specimens were carried out on catch and primary gonadal staging was assessed by means of macroscopic examination. Tissue samples including gill (primary and secondary ray), male and female gonads (anterior, central and posterior), digestive tract (oesophagus, stomach and intestine), liver (different parts of the large and small lobules) and kidney (anterior, central and posterior) measuring a few millimeters in thickness and a few millimeters in width were prepared and fixed individually in buoin solution. Tissue samples were dehydrated in different dilutions of alcohol and chloroform solution until they were clear. They were then embedded in paraffin and sectioned at 5-7 µm using a Leitz microtome and stained following the hematoxylin and eosin (H & E) method. Distinct samples were photographed using a light microcopy in 10 microscopic fields. The microscopic examination of gills revealed the presence of chloride cells, secondary cartilage, erythrocytes, gill lamella, epitheleum, polar cells, respiratory cells and respiratory spines. Ovary in A. persicus was similar in structure with that of other sturgeons and a major part of it was comprised of stroma tissues. Follicles were the main structural and functional indices in ovary. The main distinguishing feature of the ovary in the early stage of growth is the presence of ovarian grooves in gonads. Grooves were observed in stage I of sexual maturity and oogonia were seen in the primary growth of protoplasmic ovary. They are attached densely to the dorsal part of the abdominal cavity and their development is slow. Oocyte cells are observed assembled in ovary sacs. In the II stage of maturity gonads are yellow to pink in color with distinct lateral folds. Fat tissue is the main energy source for oocytes. Duration of this stage is very long. Primary oocytes are single layered, follicle shaped and could be observed with naked eyes as white spots in an ovary section. The III stage of maturity is characterized with trophoplasmic growth of oocytes, reduction in fat reserves, increase in oocyte volume, appearance of pigments under cell membranes and growth of arc cilia on gelatinous part of follicles. Nucleus is located away from the center and surrounded by several nucleoli. The granulosa layer occurs at this stage. In the IV stage of maturity oocytes growth is increased and nucleus is seen migrating toward the animal pole. Tiny yolk granules are observed in the animal pole and larger yolk granules and fat droplets in the vegetative pole. Growth of external layers of oocyte continues while protoplasmic growth is completed at this stage. At this stage germinal vesicle reaches its final growth and oocyte is covered with follicle, jelly coat, zona radiata external, zona radiata internal, fatty layer and pigments. The stage V of maturity is characterized with intensive bleeding in ovaries, residues of non-ovulated oocytes, appearance of stage II oocytes and decomposition of residues of non-ovulated oocytes. In the stage VI of maturity oocytes resembling stage II of maturity are clearly observed. Ruptured follicles, pigment reserves and oocytes representing various maturity stages are also present. Testis in Persian sturgeon in the first stage of maturity includes spermatogony cells in its germinal parts. Male gonads are situated along notochord and attached to the dorsal diaphragm or primary area of kidney formation through short mesenchymes. In stage II of male gonad maturity, spermatogonia cells are situated in gonad channel walls in a single layer without wrinkles. Primary and secondary spermatocytes and sertoli cells are present. In stage III of male gonad maturity, spermatogony cells intensively divide and reproduce. Blood vessels are observed in the form of condensed blood cells. Development and formation of primary and secondary spermatocytes, meiotic divisions and presence of spermatids are characteristics of this stage. Stage IV is distinguished by an active cycle of spermatogenesis, further meiotic divisions and entrance of sperms into the sperm channels. In stage V intensive bleeding occurs in testis. Spermatozoids, sperm channels and numerous blood vessels are also observed in this stage. In stage IV of male gonad maturity large epithelial cells of follicle and emptied sperm channels are observed. Kidney in Persian sturgeon is comprised of three sections; anterior (pronepherose including malpighian bodies and nephritic segments) middle and posterior parts. Nephrons include malpighian bodies, glomerula, Buman capsule, renal tubes or urinary tubes, neck part, spiral section, middle part, distant spiral section and shows various patterns in microscopic histological studies. Liver tissue is composed of hepatocytes, biliary tubes, lymphatic vessels, liver capillaries or sinusoid, kupfer cells and liver cells. Liver cells are roughly circular and polygonal in structure with a single and distinct nucleus and single nucleolus. Endoplasmic reticulum and other essential cell organs are present in its cytoplasm and considerable fat and glycogen reserves are observed in cell cytoplasm. Therefore large numbers of theses vacuoles are observed in a stained liver cells. Pancreas in Persian sturgeon is comprised endocrine and exocrine parts. The exocrine part releases pancreatic excretions and the endocrine section (Langerhans islands) releases hormones. The digestive system is comprised of alimentary canal that includes the mouth, pharynx, esophagus and intestine with epithelial and muscular layers. The epithelial layer sustains epithelial coat, parenchyma, condensed layer, muscular epithelium, sub- epithelial layer, muscular layer including ring and longitudinal layer and serum membrane. The pharynx epithelium is stratified in anterior part and simple in posterior part. Epithelial cells of pharynx include mucus like matrix in cytoplasm. These cells lack muscular epithelium and a complex and condensed connecting tissue is present beneath the epithelium. Stomach has two independent sections of glandular structure for collecting food and commencement of chemical digestion and non-glandular structure with strong muscular layer specified to create stomach movement and mechanical digestion. Epithelial parts are simple or wrinkled in structure. Intestine stretches from the pyloric section of stomach to the anal part and includes duodenum, anterior and posterior intestine and rectum. Duodenum is a part of intestine where biliary canals, pancreas and pyloric ceaca enter. Intestinal epithelium is comprised of a single layer of stretched columnar cells that encompasses epithelial cells. Rectum is the last section of intestine and a sphincter separates it from posterior intestine. Anal is the terminal opening of the alimentary canal. Goblet shaped epithelium cells are observed in epithelial structure of rectum while simple tubular glands without large intestinal branched glands are present in Parin. Results obtained in this study along with characterizing microscopic structure of studied organs in Persian sturgeon facilitated implementation of other related specific researches.

Iranian Fisheries Science Research Institute

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