Liver

The pancreas has a thin capsule of connective tissue, from which septa extend to cover the larger vessels and ducts and to separate the parenchyma into lobules.

This gland produces digestive enzymes in its exocrine portion and a number of hormones in its endocrine portion (islets of Langerhans).

Blood flow into the pancreas is arranged in such a fashion that the acini receive arterial blood from vessels dedicated to these structures and the islets of Langerhans also receive blood dedicated to the islets.

Additionally, acini also receive venous blood drained from the islets of Langerhans, so that hormones, such as somatostatin, released by those islet cells can reach the acinar cells immediately after release.

The exocrine pancreas is a serous compound tubuloacinar gland.
1. Pancreatic acinar cells
  1. Pancreatic acinar cells are pyramidal serous cells arranged around a central lumen.
  2. They possess a round basal nucleus, abundant rough endoplasmic reticulum (RER), an extensive Golgi complex, numerous mitochondria, and many free ribosomes.
  3. Zymogen (secretory) granules are membrane bound and densely packed in the apical region of pancreatic acinar cells. They contain enzymes and proenzymes packaged in the Golgi complex.
  4. Their basal plasmalemma has receptors for cholecystokinin and acetylcholine.
  5. The secretory acini are surrounded by a basal lamina that is supported only by a delicate sheath of reticular fibers with a rich capillary network.
7. Pancreatic ducts
  1. The initial intra-acinar portion of the intercalated ducts is formed by centroacinar cells, which are low cuboidal with a pale cytoplasm.
  2. From the initial portion, the intercalated ducts converge into a small number of intralobular ducts, which in turn empty into large interlobular ducts that empty into the main (or accessory) pancreatic duct.
  3. The main pancreatic duct fuses with the common bile duct, forming the ampulla of Vater, which delivers secretions of the exocrine pancreas and the contents of the gallbladder into the duodenum at the major duodenal papilla.
8. Exocrine pancreatic secretions
  1. Enzyme-poor alkaline fluid
    1. Enzyme-poor alkaline fluid is released in large quantities by intercalated duct cells stimulated by secretin in conjunction with acetylcholine.
    2. Function. It probably neutralizes the acidic chyme as it enters the duodenum.
  2. Digestive enzymes
    1. Digestive enzymes are synthesized and stored in the pancreatic acinar cells. Their release is stimulated by cholecystokinin (previously known as pancreozymin) and costimulated by acetylcholine released by postganglionic parasympathetic fibers.
    2. Digestive enzymes are secreted as enzymes or proenzymes that must be activated in the intestinal lumen.
    3. Enzymes include pancreatic amylase, pancreatic lipases, ribonuclease, and deoxyribonuclease; proenzymes include trypsinogen, chymotrypsinogen, procarboxypeptidase, and proelastase.
    4. In order to protect themselves from the digestive enzyme trypsin, these cells manufacture trypsin inhibitor so that trypsinogen cannot be converted to trypsin within the cytosol.

Exocrine secretions from acini flow successively through intercalated ducts, intralobular ducts, interlobular ducts and finally into the duodenum through the main pancreatic duct.

Embedded within the pancreatic exocrine tissue are Islets of Langerhans, the endocrine component of the pancreas. Islets contain several cell types and are richly vascularized.

Structure and Function