The zebrafish pancreas is similar to the pancreas in higher vertebrates. It serves both as an exocrine and endocrine organ. The endocrine function resides in cells composing the islets of Langerhans. Pancreatic neuroendocrine cells in the islets include the alpha, beta, and delta cells, which produce the hormones glucagon, insulin, and somatostatin, respectively. The exocrine function of the pancreas is a responsibility of the acinar cells. These cells synthesize digestive enzymes that collect in the centrally located acinar ducts, then flow into the pancreatic duct and finally into the intestine. Interestingly, recent data suggest that the zebrafish pancreas may also be the site for the developing B cells of the immune system.
In routine histologic sections, unlike in higher vertebrates, the zebrafish pancreas is seen as a diffuse collection of acini and islets scattered in fatty tissue and located around the liver and intestine (Figure 8.1). Sometimes pancreatic tissue appears embedded in the liver, which has given rise to the term hepatopancreas. The pancreas develops from dorsal and ventral buds. The dorsal bud appears to be strictly endocrine while the ventral is predominately exocrine.
At least two types of islets, Brockmann body/principal islet (Figure 8.2) and the diffuse islets (Figure 8.3), can be recognized histologically. The Brockman or principal islet is a large collection of pancreatic neuroendocrine cells surrounded by a small amount of acinar tissue. The neuroendocrine cells are arranged in ribbons and the islet is highly vascularized (Figure 8.4). The smaller, diffuse islets are scattered throughout pancreatic acinar tissue. A third type, the single beta cell, can be detected only with immunofluorescent techniques.