exocrine and endocrine
The pancreas is covered in a tissue capsule that partitions
the gland into lobules. The bulk of the pancreas is composed of pancreatic exocrine cells, whose ducts are arranged in clusters
called acini (singular acinus). The cells are filled with secretory granules containing the pre-cursor digestive
enzymes (mainly trypsinogen, chymotrypsinogen, pancreaiic lipase and amylase) that are secreted into the lumen of
the acinus. These granules are termed zymogen granules (zymogen referring to the inactive precursor enzymes).
Zymogen granules are localized to the subapical area
of pancreatic acinar cells. After fusion with the apical membrane, they are flushed into the duodenum, where enterokinases
(bound to enterocytes but facing the lumen of the duodenum) catalyze the activation of trypsinogen into trypsin. Trypsin,
an endopeptidase, cleaves amino acids from chymotrypsinogen to produce an active endopeptidase, chymotrypsin. These in turn
can 'chop up' polypeptides, released from stomach, into absorbable units. They also activate the other enzymes released. It
is important to synthesize inactive enzymes in the pancreas to avoid autodegradation, which can lead to pancreatitis.
The pancreas is the main source of enzymes for digesting
fats (lipids) and proteins - the intestinal walls have enzymes that will digest polysaccharides. Pancreatic secretions from
ductal cells contain bicarbonate ions and are alkaline in order to neutralize the acidic chyme that the stomach churns
out. Control of the exocrine function of the pancreas are via the hormones gastrin, cholecystokinin andsecretin, which are
hormones secreted by cells in the stomach and duodenum, in response to distension and/or food and which cause secretion of
The two major proteases the pancreas secretes are trypsinogen
and chymotrypsinogen. These zymogens are inactivated forms of trypsin and chymotrypsin. Once released in the intestine,
the enzyme enterokinase present in the intestinal mucosa activates trypsinogen by cleaving it to form trypsin. The free trypsin
then cleaves the rest of the trypsinogen and chymotrypsinogen to their active forms.
Pancreatic secretions accumulate in intralobular ducts
that drain to the main pancreatic duct, which drains directly into the duodenum.
Due to the potency of its enzyme contents, it is a very
dangerous organ to injure and a puncture of the pancreas tends to require careful medical intervention.
Scattered amongst the acini are the endocrine cells of
the pancreas, in groups called the islets of Langerhans. They are the B (glucagon producing) cells, the a
(insulin producing) cells, the delta (somatostatin producing) cells and a small number of F cells secreting Pancreatic polypeptide.
The hormones produced by these cells have a critical role in regulating blood glucose levels in the fed and fasted states.