In this article we will discuss about the meaning and development of pancreas in human body with the help of suitable diagram.
Meaning of Pancreas:
The pancreas is both an endocrine as well as exocrine gland. It is devoid of distinct connective tissue capsule and is covered by a fine layer of the loose tissue which passes into the gland as septa and subdivides the gland into many lobules. In lower animals (cat) the lobules are completely separated from one another by the connective tissue.
The exocrine component portion of the pancreas is a compound tubular gland. The terminal secretory portions of this gland are known as acini or alveoli which are tubular and somewhat convoluted and secrete pancreatic juice. These acini resemble those of serous (serozymogenic) alveoli of salivary glands and do not contain myoepithelial cells.
The main excretory duct of the gland is the duct of Wirsung, which extends the entire length of the gland, giving out several intralobular ducts or intercalated ducts. The duct of Wirsung opens in company with the common bile duct into the ampulla of Vater, which opens into the second part of the duodenum.
An accessory pancreatic duct, duct of Santorini, is often present. Within the lumina of many acini, one or more cubical cells are lying in contact with the apices of the secreting cells. These cells are known as centro-acinous (-acinar) cells. The cytoplasm of this cell does not possess any secretory granules. In each secretory cell, there are two well-marked zones—an inner (apical) zone towards the lumen and the outer (basal) zone towards the basement membrane.
In the inner zone there are numerous course zymogen granules and their number are varying with the functional activity of the cell. Their number are diminished during digestion but increased during rest. The basal zone contains the nucleus as well as basophilic or chromophilic substance. Electron microscopic structure shows highly developed rough-walled endoplasmic reticulum and a supranuclear Golgi apparatus.
Surface membranes are studded with ribosome granules which give basophilic staining. In resting stages the granular zone gradually increases up to three-fourth of the cell, but during activity the granular zone gradually diminishes in size. The pancreatic excretory duct is also similar to that of the salivary gland. Larger ducts contain elastic fibres and plain muscles which, when contract, may prevent the flow of juice into the duodenum. Near the duodenum small mucous glands are seen in the lamina propria.
Zymogen granules contain pro-enzyme. The pro-enzymes which became activated on secretion to form enzymes. These enzymes are responsible for latter stages of digestion of proteins, carbohydrates and fats.
Between acini or alveoli there are found groups of solid cells, termed as interacinous (interalveolar) cell islets (or islets of Langerhans) which are the endocrine component portion of the pancreas. The islets (islands) vary in size and are shown by drawing from the exocrine tissue by a thin basement membrane.
The endocrine pancreatic tissue cells are formed with much less cytoplasm than those of the exocrine pancreatic tissue cells. These islets are scattered throughout the pancreas but more numerous in the tail than elsewhere. There are two main types of cells in the islets – α-and β-cells (Fig. 9.38). The β-cells contain numerous insulin granules and secrete insulin.
Recent studies by Steiner and his colleague Oyer showed the existence of a biosynthetic precursor of insulin termed pro-insulin.
The concept of pro-insulin is supported by the following evidences:
i. Oxidative regeneration of reduced pro-insulin is much more efficient than that of reduced insulin.
ii. Pancreas slices incorporate labelled amino acids into pro-insulin fraction first.
iii. Low concentration of trypsin readily cleaves sensitive bonds in pro-insulin giving a insulin-like product.
The α-cells contain glucagon granules and secrete glucagon. A few ϒ- and δ-cells are scattered in the islets.
According to Hellerstrom et al it reveals that α-cells can be classified as α1 which is argyrophilic and α2, non-argyrophilic but rich in protein-bound tryptophan.
Development of Pancreas:
Pancreas develops as an outgrowth of the endoderm of the small intestine and ultimately becomes hollowed out to form the alveoli or acini. The islets of Langerhans develop as buds from the ducts and remain as solid clumps of cells. During development in most cases, the connections with the duct vanish and the islets become completely isolated. But in a few cases the connections persist as solid chords.