Zebrafish kidney marrow and effects of exposure to low temperature. (a) Gross appearance of normal and cold zebrafish kept in 26.5°C or 17°C water, respectively, for 1 week. Dotted green line indicates location of anterior and posterior kidneys (arrows). (b) Histology of the kidney marrow of cold zebrafish stained with toluidine blue O. In 26.5°C water, hematopoietic cells (HCs) were observed between renal tubules (RTs) (left). Similarly aged zebrafish kept in 17°C water for 7 months exhibit abnormal RTs (right). Erythrocyte clusters (arrow) were observed only in the kidney marrow of cold zebrafish. (c) Expression of erythropoiesis-related genes in the kidney marrow of normal (26.5°C, white bars) and cold-exposed (17°C, gray bars) zebrafish. Gene expression was quantified by real-time polymerase chain reaction during the second week of cold exposure. Expression levels of genes encoding transcription factors important for erythropoiesis (gata1a and klfd), the adult hemoglobin gene α-globin (hbaa1), and β-globin (ba1) were downregulated, whereas the epo and epor expression levels were slightly decreased. Low expression of hbaa1, ba1, klfd, and gata1a indicates anemic status of cold zebrafish. These frames are modified from our previous report . ba1, globin ba1; epo, erythropoietin; epor, erythropoietin receptor; gata1a, GATA-binding protein 1a; hbaa1, hemoglobin alpha adult-1; klfd, Kruppel-like factor d. These figures are modified from .