The pancreas is a composite gland having exocrine acini which discharge their secretions into the duodenum to assist in digestion, and groups of endocrine cells, the islets of Langerhans, whose role is in carbohydrate metabolism. In shape the gland resembles the upper end of a thick walking-stick or hook, lying sideways with the handle or hook on the right and turned downwards (Fig. 5.26). Its length is about 15cm.
The gland is of firm consistency, and its surface is finely lobulated. Its big head on the right is connected by a short neck to the body, which crosses the midline and tapers to a narrow tail on the left. The head and tail incline towards the paravertebral gutters, while the neck and body are curved boldly forward over the inferior vena cava and aorta in front of the first lumbar vertebra. The gland lies somewhat obliquely, sloping from the head upwards towards the tail behind the peritoneum of the posterior abdominal wall. The transpyloric plane (L1) is the guide to the surface marking; the neck lies on the plane, which passes across the head and body, and below the tail.
The head, the broadest part of the pancreas, is moulded to the C-shaped concavity of the duodenum, which it completely fills. It lies over the inferior vena cava and the right and left renal veins, mainly at the level of L2 vertebra. Its posterior surface is deeply indented, and sometimes tunnelled, by the terminal part of the bile duct. The lower part of the posterior surface has a hook-shaped extension upwards and to the left, behind the superior mesenteric vein and artery, in front of the aorta; this is the uncinate process of the head (Fig. 5.26). The transverse mesocolon is attached across the anterior surface of the head, which lies in both supracolic and infracolic compartments (Fig. 5.14).
The neck is best defined as the narrow band of pancreatic tissue that lies in front of the commencement of the portal vein, continuous to the right with the head and to the left with the body. At the lower margin of the neck the superior mesenteric vein is embraced between the neck and the uncinate process of the head, and behind the neck the splenic vein runs into its left side to form the portal vein. The transverse mesocolon is attached towards the lower border of the neck.
The body of the pancreas passes from the neck to the left, sloping upwards across the left renal vein and aorta, left crus of the diaphragm, left psoas muscle and lower part of left suprarenal gland, to the hilum of the left kidney. The body is triangular in cross-section with posterior, anterosuperior and anteroinferior surfaces separated by superior, anterior and inferior borders. The superior border crosses the aorta at the origin of the coeliac trunk (Fig. 5.39); the splenic artery passes to the left along the upper border of the body and tail, the crests of the waves showing above the pancreas, the troughs out of sight behind it (Fig. 5.26). The inferior border, alongside the neck, crosses the origin of the superior mesenteric artery. A slight convexity, the omental tuberosity, projects upwards from the right end of the superior border; above the lesser curvature of the stomach, this touches the omental tuberosity of the left lobe of the liver, with the lesser omentum intervening. The splenic vein lies closely applied to the posterior surface and the inferior mesenteric vein joins the splenic vein behind the body of the pancreas in front of the left renal vein, where it lies over the left psoas muscle. The transverse mesocolon is attached along the anterior border; the body lies, therefore, behind the lesser sac, where it forms part of the stomach bed.
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Figure 5.39 CT scan of the upper abdomen, viewed from below. 1, gallbladder; 2, liver; 3, portal vein; 4, inferior vena cava; 5, pancreas; 6, spleen; 7, left kidney; 8, T12 vertebra; 9, abdominal aorta; 10, right crus of diaphragm; 11, coeliac trunk. |
The tail of the pancreas passes forwards and to the left from the anterior surface of the left kidney. Accompanied by the splenic artery, vein and lymphatics it lies within the two layers of the splenorenal ligament and thus reaches the hilum of the spleen.
The pancreatic duct (of Wirsung) is a continuous tube running from the tail to the head, gradually increasing in diameter as it receives tributaries. At the hepatopancreatic ampulla (Fig. 5.40D) it is joined at an angle of about 60° by the bile duct and the manner of their joint opening into the duodenum is described on page 266. In intubation of the ampulla for endoscopic retrograde cholangiopancreatography (ERCP), the catheter preferentially enters the pancreatic duct. It drains most of the pancreas except for the uncinate process and lower part of the head, which drains by the accessory pancreatic duct (of Santorini). This opens into the duodenum at the minor duodenal papilla situated about 2cm proximal to the major papilla. The two ducts communicate with one another.
Blood supply
The main vessel is the splenic artery, which supplies the neck, body and tail. One large branch is named the arteria pancreatica magna. The head is supplied by the superior and the inferior pancreaticoduodenal arteries. Venous return is by numerous small veins into the splenic vein and, in the case of the head, by the superior pancreaticoduodenal vein into the portal vein and by the inferior pancreaticoduodenal vein into the superior mesenteric vein.
Lymph drainage
Lymphatics from the pancreas follow the course of the arteries. To the left of the neck the pancreas drains into the pancreaticosplenic nodes which accompany the splenic artery. The head drains from its upper part into the coeliac group and from its lower part and uncinate process into the superior mesenteric group of preaortic lymph nodes.
Nerve supply
Parasympathetic vagal fibres, which are capable of stimulating exocrine secretion, reach the gland mainly from the posterior vagal trunk and coeliac plexus, but, as with the gallbladder, hormonal control is more important than the neural. Sympathetic vasoconstrictor impulses are derived from spinal cord segments T6–10 via splanchnic nerves and the coeliac plexus, the postganglionic fibres running to the gland with its blood vessels. As with other viscera, pain fibres accompany the sympathetic supply, so that pancreatic pain may radiate in the distribution of thoracic dermatomes 6–10.
Structure
The pancreas is a lobulated gland composed of serous acini that produce the exocrine secretion, and the endocrine islets of Langerhans. The cells of the serous acini show the cytoplasmic basophilia typical of protein-secreting cells. Under the influence of secretin and CCK produced by neuroendocrine cells of the small intestine, the pancreatic acinar cells secrete various digestive enzymes, in particular trypsin and lipase, and some (the centroacinar cells) produce bicarbonate. The pale-staining islets are rounded groups of cells scattered among the acini. Special staining or electron microscopy is necessary to distinguish between the α-islet cells which secrete glucagon and the β-cells secreting insulin. There are also δ-cells that produce somatostatin.
Development
The pancreas develops as two separate buds, each an outgrowth of the endoderm at the junction of foregut and midgut (Fig. 5.40A). A ventral bud grows into the ventral mesogastrium in common with the outgrowth of the bile duct and a dorsal bud grows independently from a separate duct into the dorsal mesogastrium. The duodenal portion of the gut subsequently rotates and becomes adherent to the posterior abdominal wall, lying with the pancreatic outgrowths, behind the peritoneum. The duodenal wall grows asymmetrically; the openings of the two ducts, originally diametrically opposite, are thus carried around into line with each other (Fig. 5.40B, C), and the two parts of the gland fuse into the single adult pancreas. The duct systems of the two buds anastomose and there is eventually some interchange of drainage areas. The end result is that the duodenal end of the dorsal duct becomes the accessory pancreatic duct, and the duodenal end of the duct of the ventral bud joins with the remainder of the dorsal duct to form the main pancreatic duct (Fig. 5.40D). The tail, body, neck and part of the head of the pancreas develop from the dorsal bud; the rest of the head and the uncinate process develop from the ventral bud. Rarely the ventral bud may fail to rotate normally around the duodenum, resulting in a ring of pancreatic tissue encircling the second part of the duodenum.
The pancreatic acini develop by growth of cells from the terminal parts of the branching ducts. The islet cells appear to have an identical origin, but become separated from their parent ducts and undergo a complete change of secretory function.
Surgical approach
The head of the pancreas and the adjacent duodenum can be mobilized by incising the peritoneum along the right edge of the second part of the duodenum and turning the duodenum medially (Kocher's manoeuvre). This procedure gives access to the posterior surface of the duodenum and head of pancreas, and to the lower part of the bile duct. The inferior vena cava, ureter and gonadal vessels must not be damaged when peeling the duodenum and pancreatic head forwards. Resections of the head of the pancreas with the C-shaped duodenal loop involve restoring continuity by joining the bile duct to the end of the jejunum and the stomach and remaining pancreas to the side of the jejunum. The portal vein, which must be free of pathological involvement, has to be safeguarded during this resection. Pseudocysts of the pancreas (fluid accumulations following pancreatitis) bulge into the lesser sac, usually behind the stomach, and can be drained intra-gastrically by incising the anterior wall of the stomach and then entering the cyst by incising the posterior gastric wall.
