BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. COORDINATION AND REGULATION IN ANIMALS

17.6. The Endocrine System

17.6.6. The Pancreas

Structure OF THE Pancreas

The structure of the pancreas under low and high magnifications of a Light Microscope is shown in Fig. 17.56, A and B, while a diagram of its Cellular Organization is presented in Fig. 17.56, C. This organ performs both endocrine and exocrine Functions. The bulk of the pancreas consists of tissue formed by acinar Cells; these cells make up rounded structures called acini (Fig. 17.56, C), from which numerous ducts branch off, eventually merging into a single main pancreatic duct, giving the overall structure a resemblance to a bunch of grapes (from Latin acinus — berry or grape cluster). Acinar cells are exocrine. They secrete pancreatic juice Enzymes, which are transported via the duct into the duodenum. The endocrine cells will be discussed below.

Class="center">

Fig. 17.56. Structure of the pancreas. A. Light micrograph at low magnification; the islets of Langerhans are visible among the mass of acinar cells. B. The same at higher magnification, showing part of a single islet of Langerhans. C. Diagram of an islet of Langerhans surrounded by acinar cells. D. Light micrograph of an islet of Langerhans. Note that the Insulin-secreting beta cells (light) occupy a central position, whereas the Glucagon-producing alpha cells (dark) have a peripheral Location.

Discovery of Insulin

In 1868, the German scientist Paul Langerhans noticed numerous small patches up to 0.5 mm in diameter within the pancreatic tissue. Microscopic examination of these patches revealed that they consisted of specialized cells and were richly supplied with Blood capillaries. They were later named the islets of Langerhans (Fig. 17.56, C). Each islet is formed by a small number of so-called α-cells, more numerous β-cells, and capillaries.

The function of the islets of Langerhans remained unknown until the last century. Eventually, it was discovered that surgical removal of The Pancreas in an experimental animal (such as a dog) leads to a condition very similar to human Diabetes Mellitus; this led to the hypothesis that the pancreas produces a hormone regulating blood sugar levels. This hormone was named insulin (from Latin insula — island), although its existence still had to be proven. This was achieved in 1921 by the Canadian researchers Banting, Best, and Macleod, who isolated the substance. Insulin injections cured diabetes in pancreatectomized dogs. These same scientists demonstrated that the hormone extracted from the pancreases of cattle and pigs (readily available as slaughterhouse waste) also helps human diabetes patients. The discovery by the Canadian scientists saved millions of lives worldwide. Nowadays, human insulin is produced using transgenic Bacteria (Section 25.2.1).

Two Hormones

It is now known that two main hormones are synthesized in the islets of Langerhans: insulin (by β-cells) and glucagon (by α-cells). These two hormones exert opposing effects on blood glucose levels.

Insulin

Insulin is a small polypeptide consisting of 51 amino acid residues. The Introduction/19.html">Primary Structure of insulin (Fig. 3.28) was determined in 1950 by Fred Sanger in Cambridge. This hormone is secreted in response to a rise in blood glucose concentration above 90 mg% (i.e., 90 mg/100 mL). Insulin is transported in the Blood Plasma bound to β-globulin and acts on all Organs, although its strongest effects are exerted on The Liver and Muscles. The binding of insulin to receptors on the outer Cell membrane alters its permeability and activates several enzyme systems. This triggers the following intracellular effects:

1) acceleration of glucose polymerization into Glycogen (Glycogenesis), occurring primarily in the liver and muscles (granules of this polysaccharide are clearly visible under an Electron microscope; see Fig. 5.12);

2) acceleration of glucose uptake by cells, especially Skeletal Muscle cells;

3) promotion of glucose utilization (rather than other high-calorie substances such as fat) as an energy source for cellular Respiration;

4) acceleration of The conversion of glucose into Fatty acids and fat for storage;

5) acceleration of cellular amino acid uptake and stimulation of Protein Synthesis;

6) inhibition of Gluconeogenesis (The formation of glucose from non-carbohydrate precursors).

Regulation of Insulin Production

Insulin production is regulated by a negative feedback mechanism. An increase in blood sugar concentration is detected by the β-Cells of the pancreas, which respond by increasing the Synthesis and Secretion of insulin. Elevated insulin levels trigger the rapid removal of glucose from the blood via the pathways described above. A drop in glucose concentration suppresses β-cell activity, leading to reduced hormone production.

Insulin secretion is vital for survival, as it is the sole factor responsible for lowering blood glucose levels. A deficiency of this hormone leads to a metabolic disorder known as diabetes mellitus. In diabetic patients, blood glucose levels are so high that it cannot be fully reabsorbed (retained) by the Kidneys and is consequently excreted in the urine.

The effects of insulin deficiency and excess are summarized in Table 17.14.

Table 17.14. Some symptoms of insulin deficiency and excess

Glucagon

Glucagon is a polypeptide consisting of 29 amino acid residues and secreted, like several Other Hormones, in response to a drop in blood glucose levels below normal. This typically occurs during increased Energy Expenditure, such as physical exertion. Glucagon raises blood sugar concentration, thereby increasing its availability to Tissues. The primary target organ for glucagon is the liver, where it stimulates The breakdown of glycogen into glucose (Glycogenolysis). In addition, it stimulates the conversion of Proteins, fats, and lactic acid into glucose. This process is called gluconeogenesis (from the Greek glykýs — sweet, néos — new, génesis — origin) (Section 19.6.2).

The binding of glucagon to receptors on the membrane of liver cells activates adenylate cyclase and leads to the formation of cAMP. The action of glucagon is similar to that of adrenaline: in both cases, cAMP activates phosphorylase enzymes that catalyze the breakdown of glycogen into glucose (see Fig. 17.48). Glucagon does not trigger this process in muscles. The regulation of its secretion, much like that of insulin, is based on a negative feedback mechanism, except that the response is mediated not by β-cells, but by α-cells, reacting not to an increase in glucose levels, but to a decrease.

Generalized diagrams illustrating The Role of these hormones in Carbohydrate METABOLISM are shown in Fig. 19.4 and 19.22.