Anatomy, Physiology, and Embryology of the Pancreas

Basic Anatomy

The human pancreas is a long, tapered, glandular organ, divided into four anatomic domains, the head, neck, body, and tail, along with one accessory lobe or uncinate process, with the head nestled within the curve of the second part of the duodenum. It is situated in a retroperitoneal position, lying obliquely across and behind the stomach with a firm and lobulated smooth surface, extending transversely toward the hilum of the spleen, measuring between 12 and 15 cm long in adults. The digestive enzymes and bicarbonates are secreted by the exocrine-acinar tissue, which then drain into tiny pancreatic tubular ductal networks, eventually joining to form the main pancreatic ducts (ducts of Wirsung and Santorini) to drain into the duodenum via the major duodenal papilla (ampulla of Vater). The main pancreatic duct (derived from the ventral anlage to become the duct of Wirsung and the distal duct of Santorini) may have a separate accessory pancreatic duct (derived from the proximal duct of Santorini), which drains the uncinate process and lower part of the head of the pancreas into the duodenum via the minor duodenal papilla. Incomplete fusion of the dorsal and ventral pancreatic ducts results in pancreas divisum, but numerous anatomic variations of the pancreatic ductal drainage system exist ( Fig. 90.2 ). The endocrine hormones are produced in the islets of Langerhans (which are scattered throughout the pancreas) and drained by a network of capillaries that invade the islet, and are thus delivered into the bloodstream. The distribution of endocrine cells within the islet is species dependent. In rodents, the core of the islet is occupied by the β cells surrounded in the periphery by a ring of α cells, whereas in humans and monkeys, all the endocrine cell types are intermingled. Nonendocrine cells also exist in the islet, including endothelial cells, neurons, dendritic cells, macrophages, and fibroblasts. Pancreatic surgery requires a good understanding of the anatomic relationships of the pancreas with other structures. The pancreatic head lies in front of the inferior vena cava, right renal artery, both renal veins, and the superior mesenteric vessels, whereas the uncinate process lies posterior to the superior mesenteric vessels. The neck of the pancreas lies directly over the portal vein and vertebral bodies L1 and L2. Because the neck lies over those vertebrae, anterior-posterior blunt trauma can lead to pancreatic trauma with possible ductal injury from a direct blow to the epigastrium. The common bile duct passes in a deep groove on the posterior aspect of the pancreatic head until it joins the main pancreatic duct at the ampulla of Vater in the pancreatic parenchyma. The stomach lies anterior to the body and tail of the pancreas, whereas the aorta, left adrenal gland, and left kidney lie posterior to the body of the pancreas. The tail lies in the hilum of the spleen with the splenic artery, which is often tortuous, running along the superior border of the pancreas. The major blood supply for the pancreas arises from multiple branches of the celiac trunk and superior mesenteric arteries forming arterial arcades within the body and tail of the pancreas, with the inferior and superior pancreaticoduodenal artery running along the head of the pancreas. Approximately one in every five patients has major variations in the arterial anatomy, such as having the right hepatic artery, which usually originates from the celiac trunk, arising from the superior mesenteric artery (also known as a replaced right hepatic artery) traveling posterior to the pancreatic head toward the liver. Discerning this arterial anomaly is important in preoperative computed tomography (CT) scans to avoid injury ( Fig. 90.3 ).

Physiology

Despite the disparate functions of the endocrine and exocrine parts of the pancreas, the two different components coordinate to regulate and respond to food digestion by secreting different hormones and digestive enzymes, with a regulatory feedback system in place. The pancreas regulates the body's energy metabolism through the endocrine islet cells of Langerhans, which constitute nearly 2% of the total pancreas. This regulation is delicately balanced through the actions of the hormones insulin and glucagon. Insulin is the hormone of energy storage; it induces an increase in amino acid uptake and facilitates glucose uptake into cells, which increases protein synthesis and decreases lipolysis and glycogenolysis, especially after a meal or in a hyperglycemic state. Glucagon, on the other hand, is viewed as the hormone of energy release; it stimulates higher blood glucose levels by stimulating hepatic gluconeogenesis, glycogenolysis, and lipolysis in the setting of hypoglycemia, and thus counteracts the effects of insulin.

β cells secrete insulin based on blood glucose levels as well as neural and humoral stimuli. The stimulus for insulin release into the bloodstream is far greater when glucose is ingested enterally compared to the parenteral route, indicating that a feed-forward mechanism in the digestive tract is activated, anticipating the rise in blood glucose. This anticipation is mediated by incretins . There are two main incretin hormones, glucose-dependent insulinotropic peptide, also known as gastric inhibitory peptide (GIP), and glucagon-like peptide-1 (GLP-1). Both are secreted by endocrine cells located in the small intestinal epithelium when the luminal concentration of glucose increases in the digestive tract, and subsequently they stimulate the β cells to secrete more insulin. Hence, the great interest in the pharmaceutical industry to develop incretin-based therapies to treat diabetes, particularly type 2 diabetes, because of its potent secretagogue effect on β cells. Unlike traditional medications that stimulate β cells to secrete insulin regardless of blood glucose level, incretins augment the β-cell response to blood glucose levels in a glucose-dependent manner, in addition to GLP-1's inhibitory effect on glucagon secretion and the ability to increase food transit time in the stomach. Humoral inhibitors for insulin release include somatostatin, amylin, leptin, and pancreastatin. The vagus nerve generally stimulates insulin release, whereas the sympathetic nervous system inhibits it, mediated by various peptidergic molecules secreted from nerve fibers such as substance P, vasoactive intestinal peptide (VIP), and neurotensin.

Exocrine secretion is stimulated by the hormones secretin and cholecystokinin (CCK), and by parasympathetic vagal discharge. The exocrine function is traditionally divided into three phases: (1) the cephalic phase, which is triggered by the sight and smell of food, comprises 10% to 20% of pancreatic excretion; (2) the gastric phase, which is triggered by food entering the stomach and gastric distention, comprises 15% to 20% of enzyme excretion; and (3) the intestinal phase, which is triggered by acidification of the duodenum and proximal jejunum, comprises 60% to 70% of meal-stimulated pancreatic excretion. The exocrine portion of the pancreas is comprised of a ductal tree along with a mass of acinar cells. Acidification and entry of fatty acids along with bile salts in the duodenum stimulate secretin and VIP, in turn leading to the release of a bicarbonate-rich fluid from ductal cells. Vagal stimulation and the entry of either peptides or fatty acids into the duodenum cause release of CCK and acetylcholine, producing the secretion of a digestive enzyme–rich fluid from the acinar cells.

Currently, the most widely accepted model of bicarbonate secretion from the ductal cells involves the diffusion of carbon dioxide into the cell from the circulation, where it is hydrated by carbonic anhydrase to form H ₂ CO ₃ . H ₂ CO ₃ dissociates into H ⁺ and . The bicarbonate is transported into the ductal space by a chloride/bicarbonate exchanger. Secretin binds to receptors on the basolateral membrane, activating adenylate cyclase to produce cyclic adenosine monophosphate (cAMP). cAMP in turn activates the cystic fibrosis transmembrane regulator (CFTR) on the luminal cell surface, allowing for the passage of chloride into the ductal space. The passage of bicarbonate and chloride across the ductal cell membrane generates an ionic and osmotic gradient causing sodium and water to follow. Defects in CFTR lead to both acute and chronic pancreatitis through ductal and glandular obstruction secondary to the inability to hydrate the ductal molecules in the lumen. The lack of chloride ions flowing into the lumen prevents the formation of an ionic and osmotic gradient. Therefore, sodium and water do not cross into the lumen, producing a low volume, thickened secretion and subsequent blockage. Pancreatitis is rarely a complication in individuals with mutations of both CFTR alleles because this results in rapid destruction of the pancreas beginning in utero. Patients experience the loss of acinar cells, which are a necessary nidus for pancreatitis, leading to pancreatic insufficiency.

Along with bicarbonate secretion, the second arm of pancreatic exocrine function involves the release of digestive enzymes from the acinar cells. Digestive enzymes are synthesized in their inactive form within acinar cells and are packaged into zymogen granules. The granules migrate to the cell surface and fuse to the cell membrane releasing their contents in response to vagal stimulation, peptides, and fatty acids. Some enzymes, including amylase, lipase, RNase, and DNase are synthesized in their active forms, but most (trypsinogen, chymotrypsinongen, procarboxypeptidase, and proelastase) are inactive upon release. The intestinal brush border enzyme, enteropeptidase, cleaves trypsinogen to its active form, trypsin. Trypsin cleaves and activates the remaining digestive enzymes. More than 40 mutations in cationic trypsinogen (PRSS1) , the gene that encodes trypsin, have been uncovered. The mutations often cause the premature activation of trypsinogen to trypsin, producing a condition characterized by recurrent episodes of pancreatitis ultimately leading to pancreatic insufficiency.

Serum amylase is usually measured to diagnose pancreatitis, which is usually 2.5 times normal within 6 hours after the onset of an acute episode, and then returns to normal within 3 to 7 days. However, the major limitation of serum amylase measurement to diagnose pancreatitis is the lack of specificity because several clinical conditions can result in elevated amylase. In addition, a normal serum amylase certainly does not exclude pancreatitis. The amylase-to-creatinine ratio (ACR) may help in differentiating acute pancreatitis from other conditions using the following equation:

An ACR greater than 5% suggests acute pancreatitis, and ratios less than 1% suggest macroamylasemia. Serum lipase levels, on the other hand, are believed to be more specific in diagnosing pancreatic tissue damage because lipase is only produced in the pancreas. Lipase tends to be higher in alcoholic pancreatitis and the amylase level higher in gallstone pancreatitis, hence the lipase-to-amylase ratio has been suggested as means to distinguish between the two.

Basic Pancreatic Embryology

The embryonic pancreas is known to pass through three stages of development. The first is the undifferentiated stage where the endoderm evaginates to initiate pancreatic morphogenesis, with only insulin and glucagon genes being expressed at this stage. The second phase involves epithelial branching morphogenesis with simultaneous formation of primitive ducts. This stage involves the separation of islet progenitors beginning to differentiate and losing their attachments to the basement membrane. The third and final stage begins with the formation of acinar cells at the apices of the ductal structures, with the development of zymogen granules containing enzymes. Acinar cells usually commence enzyme secretion shortly after birth.

During early development, the pancreas initiates by the regional specification of the undifferentiated primitive endodermal foregut tube by these transcription factors: pancreatic and duodenal homeobox 1 (Pdx1), which marks the prepancreatic endoderm, and pancreas-specific transcription factor 1a (PTF1a), where both are expressed in multipotent pancreatic progenitor cells. The pancreas first appears morphologically as a mesenchymal condensation at the level of the duodenal anlagen, distal to the stomach on the dorsal aspect of the foregut tube at embryonic day (E) 9.0 in mice. All cells expressing Pdx1 and PTF1a in the endoderm will eventually give rise to the epithelial cells in the adult pancreas, which includes endocrine, acinar, and duct cells. At around E9.5 gestation in mice and the 26th day of gestation in humans, the dorsal bud begins to evaginate into the overlying mesenchyme while retaining luminal continuity with the gut tube. Approximately 12 hours later in mice, and 6 days after dorsal bud evagination in humans, the ventral bud begins to arise. Gut rotation will bring the ventral lobe dorsally, ultimately fusing with the dorsal pancreatic bud (this event corresponds to around the sixth to seventh week of gestation in humans or E12 to E13 in mice) contributing to the formation of the uncinate process and inferior part of the head of the pancreas, while the rest of the pancreas arises from the dorsal pancreatic bud. The entire ventral pancreatic duct and the distal part of the dorsal pancreatic duct fuse together to form the main pancreatic duct of Wirsung. The remaining proximal part of the dorsal pancreatic duct is either obliterated or persists as a small accessory pancreatic duct of Santorini. This fusion and evagination of the two buds is followed by elongation of the pancreatic bud stalk region (precursor to the main pancreatic duct) and branching morphogenesis of the apical region of the bud. Unlike the usual branching morphogenesis growth patterns seen in the developing kidney, lung, and salivary gland, in which the branching morphogenesis occurs at 90-degree angles, the pancreas grows in an acute-angled branching pattern, which leads to the exclusion or “squeezing out” of mesenchyme from between the closely apposed branches of epithelium ( Fig. 90.4 ). This exclusion of mesenchyme may influence epithelial-mesenchymal interactions and lineage selection. The pancreas then undergoes major amplification of the endocrine cell population through two distinct waves of differentiation within the pancreatic epithelium during embryogenesis, an early primary wave (pre E13.5 in mice), followed by the secondary wave of differentiation (E13.5 to E16.5 in mice). Over a similar gestational window, the exocrine pancreatic precursors undergo an exponential increase in branching morphogenesis and acinar cell differentiation.

Dorsal and Ventral Pancreatic Bud Development

It is important to note that although the morphologic development of the ventral and dorsal pancreatic buds may be similar, they differ markedly at the molecular level, with various lines of evidence suggesting that there are differences in the specification between the pancreatic rudiments, with the notochord playing a key role. Sonic hedgehog (Shh), which is a potent intercellular patterning molecule, is expressed along the entire foregut, but is noticeably suppressed in the prospective pancreatic endoderm. This suppression of Shh appears to be necessary for dorsal pancreatic development, permitting the expression of pancreas-specific genes including Pdx1 and insulin. Deletion of the notochord in chick embryo cultures leads to ectopic Shh being seen in the pancreatic region of the foregut endoderm, with subsequent failure of the pancreas to develop. Activin-βB (a member of the transforming growth factor-β family) and fibroblast growth factor (FGF) 2 both mimic notochord activity in inducing pancreatic genes. In stark contrast to the dorsal bud, developmental gene expression in the ventral pancreatic anlage is not affected when the notochord is removed. The ventral pancreas, on the other hand, develops under the control of signals from the overlying cardiogenic mesenchyme, which also produces prohepatic signals (FGFs) to induce liver formation. Lack of prohepatic FGF signaling in regions of the cardiogenic mesenchyme will lead to the endoderm differentiating into ventral pancreas by default. When ventral foregut endoderm is cultured in the absence of cardiac mesoderm or FGF, it fails to activate liver-specific genes, with Pdx1 being expressed instead. Cardiac mesoderm, through FGF, induces liver formation from the ventral endoderm, and simultaneously inhibits pancreatic development. Further differences between ventral and dorsal pancreas are demonstrated in Hlxb9 mutant mice. The homeobox gene Hlxb9, which is transiently expressed in the endoderm in the region of the dorsal and ventral pancreatic anlage, when inactivated in mice, only dorsal pancreatic development is blocked. Hematopoietically expressed homeobox 1 (Hex1) is an early marker of the anterior endoderm and is expressed at E7.0 in the cells that will subsequently give rise to the ventral pancreas and liver. Hex-null mutant embryos have specific failure of ventral pancreatic bud development, with the dorsal bud developing normally. These examples underscore the significantly different molecular controls governing dorsal and ventral pancreatic bud development.

Initiation of the Pancreas With Endodermal Patterning

The signaling molecules that govern the specification of the primitive gut tube into different specialized domains remain yet to be fully elucidated. The pancreas and other endoderm-derived organs develop through a series of reciprocal interactions between the endoderm and the surrounding mesenchyme, which is a critical step in initiating organ specification or endodermal patterning along the anterior-posterior axis of the foregut endoderm. Endodermal patterning is manifested by the regional expression of transcription factors in the primitive gut tube; for example, Hex1 and Nkx2.1 (also known as thyroid transcription factor 1) are expressed at E8.5 in defined foregut domains along the anterior-posterior axis of the primitive gut tube, giving rise to liver and lung/thyroid, respectively. Pdx1 and PTF1a are coexpressed in the foregut-midgut endoderm boundary, defining the pancreas and duodenum, whereas Cdx1, and Cdx4 (early markers of posterior endoderm) are expressed in the posterior midgut and hindgut domains that will give rise to the small and large intestines. Thus, various domains of the primitive gut tube are specified ( Fig. 90.5 ).

Mesenchyme-induced endodermal patterning is necessary before the initiation of organogenesis. When the pancreatic mesenchyme is removed from the pancreatic epithelium in explant cultures, it results in disrupted pancreatic cell differentiation, with the endocrine lineage being favored over exocrine.

The primitive gut tube is divided into three domains, foregut, midgut, and hindgut regions, each of which will give rise to specialized structures. This subdivision into presumptive gut tube domains is governed by different molecular markers in the gastrula stage endoderm (E7.5). The endoderm toward the anterior side of the embryo generates the ventral foregut, which will later give rise to the liver, lung, thyroid, and the ventral pancreas. The dorsal region of the definitive endoderm, on the other hand, contributes to the formation of the esophagus, stomach, dorsal pancreas, duodenum, and intestines. The pancreas has been found to form as a result of the actions of some key specific transcription factors and signaling pathways. For example, FGF4 and Wnt signaling from the posterior mesoderm are specifically inhibited in the anterior endoderm to allow foregut development. FGF signaling is required to initially determine, and then to maintain gut tube domains, as demonstrated with cultured mouse endoderm and by in vivo studies in chick embryos. FGF4 is normally expressed in the mesoderm and ectoderm adjacent to the developing midgut-hindgut endoderm, and when isolated mouse endoderm was cultured in the presence of high concentrations of FGF4, a posterior (intestinal) endoderm was induced. On the other hand, lower concentrations of FGF4 induced a more anterior (pancreas-duodenal) cell fate. Similarly, when chick embryos are treated in vivo with FGF4, the Hex1 (anterior endodermal marker) expression domain was reduced, whereas CdxB (posterior endodermal marker) expression expanded anteriorly, inhibiting the development of the foregut. Therefore, FGF4 plays a critical role in endodermal patterning by repressing anterior (foregut) fate and promoting posterior (intestinal) endoderm fate. Another molecular pathway that has linked endodermal patterning to the initiation of pancreatic development is Wnt/β-catenin signaling, as demonstrated in frog ( Xenopus ) studies. β-catenin repression in the anterior endoderm is specifically necessary to initiate liver and pancreas development, and to maintain foregut identity. Conversely, forcing high β-catenin activity in the posterior endoderm promotes intestinal development and inhibits foregut development into liver and pancreas. McLin et al. demonstrated that forced β-catenin expression in the anterior endoderm (where β-catenin is usually repressed) led to downregulation of Hhex, as well as other foregut markers for liver (for1) , pancreas (pdx1) , lung/thyroid (nkx2.1) , and intestine (endocut) , resulting in inhibition of foregut fate, namely liver and pancreas formation. Repressing β-catenin in the posterior endoderm (future hindgut that normally expresses β-catenin) induced ectopic liver and pancreas markers (hhex, Pdx1, elastase, and amylase) with subsequent ectopic liver bud initiation and pancreas development. The homeobox-containing gene Hhex, is a direct target of β-catenin, is one of the earliest foregut markers, and is essential for normal liver and ventral pancreas development in mice. Hex expression was noted to have an important role in the specification and differentiation of the ventral pancreas, where Hex ^−/− null mutant mouse embryos lacked a ventral pancreas, and lacked liver, thyroid, and parts of the forebrain.

Retinoic acid (RA) signaling has been implicated as an important molecule for endodermal patterning in zebrafish. RA signaling is necessary for specification and differentiation of both liver and pancreas. As with RA, bone morphogenetic protein (BMP) signaling has also been shown to have a role in endodermal patterning and in the normal development of the pancreas in zebrafish, but neither RA nor BMP affect the induction of endodermal precursors. Targeted disruption of the Pdx1 gene in mice also prevented pancreatic development. A critical role for Pdx1 in pancreatic initiation and patterning of foregut endoderm in mice was further demonstrated by humans with Pdx1 mutations being apancreatic. Despite our growing knowledge of many molecular signals mediating cross-talk between the pancreatic mesenchyme and the epithelium, most pathways remain poorly understood.

Pancreatic Mesenchyme

The pancreatic mesenchyme, which envelops the pancreatic epithelium after regional specification, contains important factors that are pivotal for pancreatic morphogenesis. These factors promote growth and differentiation of the developing pancreas, specifically inducing growth of the endocrine cell population and rapid branching morphogenesis. The pancreatic mesenchyme helps regulate lineage selection by the pancreatic epithelium between the endocrine and exocrine lineages during early stages of pancreatic development. This interaction between pancreatic mesenchyme and epithelium is a vital process for pancreatic development. Pure pancreatic epithelium (E11) cultured without its mesenchyme failed to develop at all; however, the epithelium grew into a fully differentiated pancreas (acinar, ductal, and endocrine structures), when cultured with its mesenchyme. The pancreatic mesenchyme has a pro-exocrine effect on the epithelium through cell-cell contact, and also a pro-endocrine effect, mediated by diffusible factors secreted from the mesenchyme. Mesenchymal contact with the epithelium both enhances notch signaling (Hes1), which favors the acinar lineage, and also inhibits neurogenin 3 (Ngn3) expression leading to the suppression of endocrine differentiation. The default differentiation of the pancreatic epithelium in the absence of mesenchyme is endocrine. Interestingly, culturing pure pancreatic epithelium in a basement membrane–rich gel, without its mesenchyme, led to the predominant formation of ductal structures. These results suggest that the basement membrane has factors or components that are conducive to ductal development. To further illustrate the importance of the mesenchyme and mesenchymal signaling in embryonic and organ development, when the normal embryonic separation that occurs between the spleen and the pancreas-associated mesenchyme does not occur in Bapx1-null mutant embryos, the dorsal pancreatic bud becomes intestinalized. Activin A, which is expressed in the splenic mesenchyme, is a possible mediator for this transdifferentiation because exposing pancreatic buds to activin A in an in vitro culture system also leads to intestinalization.

Some signaling pathways have been implicated in mediating this epithelial-mesenchymal interaction, such as the FGFs. Specifically, FGFs 1, 7, and 10, which are expressed in the pancreatic mesenchyme, mediate their effects through FGF receptor 2B (FGFR2B), which is expressed in the pancreatic epithelium. Mesenchymal FGF signaling has been shown to induce epithelial proliferation, favoring exocrine differentation. Similarly, null mutations for the receptor FGFR2B or the ligand FGF10 lead to blunting of early branching pancreatic morphogenesis, with inhibition of proliferation of endocrine progenitor cells and premature endocrine differentiation, indicating that FGF10 normally induces proliferation of epithelial cells and prevents endocrine differentiation. Despite the positive role that FGF plays in dorsal pancreatic development, it seems to play a different role in ventral pancreatic bud development. FGFs that are secreted from the cardiogenic mesenchyme inhibit ventral pancreatic bud formation and favor liver development. BMP ligands in pancreatic mesenchyme induce epithelial branching and inhibit endocrine differentiation.

Key Signaling Molecules