Brief account of digestive glands in vertebrate
Answers
Explanation:
Study Unit on DIGESTIVE GLANDS
INTRODUCTION
Most of the glands associated with the digestive tract are embryonic outgrowths of endoderm although in the stomodeal and proctodeal regions they are derived from ectoderm. This laboratory is concerned only with the gall bladder and the large, extramural glands such as liver, pancreas and the major salivary glands. The intramural glands are considered together with the digestive tract proper.
The liver is the largest internal organ in the body and the most diverse in function. It serves as a major storage organ (e.g. glycogen) and has numerous metabolic functions. Furthermore, it is both an endocrine and an exocrine gland. As an endocrine gland the liver releases lipids, glucose, proteins, glycoproteins and glycolipids into the bloodstream. Major blood proteins such as albumin, fibrinogen, complement, transferrin, and the apoproteins of the lipoproteins, are synthesized and secreted by this organ. As an exocrine gland the liver secretes bile into a system of canaliculi and ducts convey their content to the gall bladder, where it is stored and concentrated, before release into the digestive tract.
The pancreas is a large gland that is connected to the duodenum via an excretory duct. The organ contains several distinct cell types: some exocrine, some endocrine. The main function of pancreatic exocrine cells is secretion of various hydrolytic enzymes needed for digestion in the intestine (see accompanying Table, DG-7). The pancreatic ducts secrete bicarbonate ions and water to neutralize the pH of the chyme as it enters the duodenum. The endocrine portion of the pancreas is organized in well-deliniated cell aggregates (islets of Langerhans) that are interspersed among the acini formed by the exocrine cells. The hormones secreted by the endocrine pancreas regulate metabolic functions throughout the body, in the digestive tract, or in the islets themselves. The islets of Langerhans contain three major cell types, alpha, beta and delta cells, that secrete glucagon, insulin and somatostatin, respectively. The minor cell types secrete hormones having effects on the digestive tract and glands, including the pancreas itself. Subpopulations of delta cells (D1) secrete vasoactive intestinal peptide; F cells (PP cells) secrete pancreatic polypeptide; EC cells produce secretin, motilin and substance P. Gastrin is secreted by G cells in the pancreas of some animals, but has not been localized in the human pancreas.
The major salivary glands, the parotid, submandibular and sublingual, are found as paired sets. Their ducts empty into the oral cavity. Their main function is to secrete saliva, which contains mucin, water and ions, as well as a few digestive enzymes, such as amylase and RNAse (see accompanying Table, DG-7). Salivary glands are distinguished by the morphology of their secretory elements and duct structure, as well as by the predominant type of product they synthesize. They are called serous, protein secreting, or mucous, glycoprotein secreting (mucin secreting). Although serous cells do secrete glycosylated proteins and, therefore, can be considered to be seromucous cells, this morphological terminology is still used because of the much greater amount of carbohydrate in the mucous secretions synthesized by the mucous cells