In higher vertebrates (reptiles, birds, and mammals) development of the kidney proceeds in three stages. The first kidney to form in the embryo is the pronephros. It consists of paired glomeruli and two separate pronephric ducts that drain waste products into the cloaca. The cloaca in teleosts is a small embryologic structure, which is lost in the adult. Later on in embryogenesis the pronephros is replaced by the mesonephros, which in turn is replaced by the metenephros or the adult kidney. In teleosts, kidney development ends with the mesonephros, no metenephric kidney forms.

In zebrafish, the pronephros consists of two nephrons emanating from a centrally fused paired glomerulus. In further development, the nephrons of the pronephros degenerate but the structure remains as the head kidney and becomes the site for the immune system and for the steroidogenic and chromaffin cells, which are homologous to the adrenal gland of mammals. The mesonephros becomes the adult kidney in the zebrafish and contains the nephrons responsible for filtering blood wastes and for salt and water uptake.

The kidney extends the length of the body cavity (Figures 2.3, 2.4, and 10.1). The anterior or head kidney contains lymphoid, hematopoietic, steroidogenic, and endocrine cells. The head (cranial) kidney is the dominant site of hematopoiesis and is composed predominately of hematopoietic elements. The immature hematopoietic cells lie between the nephron tubules. Distinct areas of lymphopoiesis and red blood cell formation are seen. The posterior or tail kidney contains nephrons with surrounding lymphoid tissue (Figure 10.2). The kidney can be seen occupying space between the vertebral column and the gas bladder. The head kidney contains both a left and right side but they become fused in the tail kidney.