Anatomy and physiology

Surgical anatomy

The name ‘pancreas’ is derived from the Greek ‘pan’ (all) and ‘kreas’ (flesh). It was originally thought to act as a cushion for the stomach. The gland weighs approximately 80 g and is situated retroperitoneally. It is divided into a head, which occupies 30 per cent of the gland by mass, and a body and tail which comprises 70 per cent of the whole organ. The head lies within the curve of the duodenum overlying the body of the second lumbar vertebra and the vena cava, with more medially the aorta beneath the neck of the gland. The neck of the pancreas is that part which has the superior mesenteric vessels as a posterior relation. Coming off the side of the pancreatic head and passing to the left and behind the superior mesenteric vein is the uncinate process of the pan­creas. Behind the neck of the pancreas, near its upper border, the superior mesenteric vein joins the splenic vein to form the portal vein (Figs 55.1 and 55.2).

There are nine key processes that occur during pancreatic embryogenesis (Table 55.1).

 Malrotation of the ventral bud in the fifth week results in an annular pancreas, while the mode of ductular fusion in the seventh week produces the various possible ductular patterns. Between the 12th and 40th weeks of foetal life, the pancreas differentiates into exocrine and endocrine elements. Of the pancreatic mass, 8 0—90 per cent is composed of exocrine tissue. The primitive ducts and their ductules are responsible for the lobular arrange­ment of the pancreas. The main pancreatic duct is lined by columnar epithelium which becomes cuboidal in the duc­tules. Congenital anomalies of the pancreas are varied and arise during the early phase of development (Table 55.2).

Pancreatic acinar tissue is organised into lobules. The main duct ramifies into intralobular and interlobular ducts, ductules and, finally, acini. Acinar cells are clumped around a central lumen to form an acinus which communicates with the duct system. The pancreas thus consists of a network of fine ducts lined by secretory cells. The islets of Langerhans are distributed throughout the pancreas. Islet cells consist of differing cell types: 75 per cent are B cells (producing insulin), 20 per cent A cells (producing glucagon), and the remainder are D cells (producing somatostatin) and a small number of pancreatic polypeptide cells. Within an islet, the B cells form an inner core surrounded by the other cells. Capillaries draining the islet cells drain into the portal vein forming a pancreatic portal system .The pancreatic duct anatomy is variable as a result of the primordial bud development. The dorsal duct is expressed in a variable manner in the adult, as outlined in Fig. 55.3. Approximately 10 per cent of patients will have a significant flow from the main duct through the accessory papilla. The anatomy of the main papilla is also variable (Fig. 55.4) with the outlet of each duct protected by a complex sphincter mechanism (Fig. 55.5). Pancreatic anatomy has an implica­tion on disease, and thus a grasp of the development and variations is important in the assessment of the patient.

Surgical physiology

In response to a meal, the pancreas secretes digestive enzymes in an alkaline (pH 8.4) bicarbonate-rich fluid. Spontaneous secretion is minimal, the hormone secretin which is released from the duodenal mucosa evokes a bicarbonate-rich fluid. Cholecystokinin-pancreozymin (CCK) is released from the duodenal mucosa in response to food: CCK produces no increase in the volume of secretion, but is responsible for enzyme secretion. Vagal stimulation increases volume. Protein is synthesised at a greater rate (per gram of tissue) in the pancreas than in any other tissue, with the possible exception of the lactating mammary gland. About 90 per cent of this protein is exported from the acinar cells as a variety of digestive enzymes. Approximately 6—20 g of diges­tive enzymes enter the duodenum each day. Nascent proteins are synthesised as pre proforms and during their transit through the rough endoplasmic reticulum and Golgi cisterni, the newly synthesised proteins undergo modification in a sequence of steps. The proteins move from the rough endothelial endoplasmic reticulum to the Golgi complex, where lysosomes and mature zymogen storage granules containing proteases are stored, and move to the ductal surface of the cell from which they are extruded by exostosis. During this phase the proteolytic enzymes are in an inactive form, the maintenance of which is important in preventing pancreatitis.