Seasonal changes of pronephros lymphoid tissue in grass carp (Ctenopharingodon idella): a histometrical and histological study

The major lymphoid tissues in teleost fish are the kidneys, thymus, spleen and mucosa-associated lymphoid tissue including the skin, gills and intestine. The head of kidney (pronephros) is an important haematopoietic organ and has morphological similarities with the bone marrow in higher vertebrates. In this study, during 12 months from August 2002 to July 2003, 120 grass carp (10 fish/month) were harvested from 3 fish culture ponds in Ahvaz. The water, weather temperatures and light duration of days (photoperiod) were recorded. After biometrical examination of age, weight and the length of the fishes, the whole kidneys were removed from abdominal cavity and samples were fixed in bouin’s solution for light microscopy and in glutaraldehyde for electron microscopy. The results showed that the lymphoid tissue distribution has significant changes during different seasons so that, the mean ± SEM of lymphoid tissue distribution in hot seasons (33.53 ± 0.59 mm2/cm2) was more than cold seasons (19.20 ± 0.65 mm2/cm2). Also the statistical results showed that water temperature has more significant (P<0.05) effect on lymphoid tissue. Degenerated lymphoid cells with pale and vacuolated cytoplasm were observed. A reversed correlation was found between the number of normal and degenerated lymphocytes. Electron microscopy revealed that degenerated lymphocytes were devoid of membranous organelles, their cytoplasms were vacuolated and the nuclear envelope had some alterations.