19-03-2011, 10:45 AM
Presented By
R.Ramya
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ISLETS OF LANGERHANS,PINEAL GLAND,THYMUS GLAND
ENDOCRINE PANCREAS
FUNCTIONAL ANATOMY:
The endocrine part of the pancreas comprises numerous rounded collection of cells known as pancreatic islets or the islets of langherhans. These are embedded within the exocrine part.
These are approximately 1 million islets in the pancreas, most numerous in the tail, and they constitute 1 to 5% of the human pancreatic mass.
ISLETS OF LANGERHANS :
The islets are very richly supplied with the blood through dense capillary plexus (as in all ductless glands) and inervated by vagal and sympathetic fibres.
HUMAN PANCREAS
CELLULAR STRUCTURE:
Each islets contains, an average 2500 cells which are of four types and arranged as:
Beta(β) cells, make up to 60 to 70% of the total cells, and the constitute the central core of the islet. These cells secrete insulin.
Alpha(α) cells, form about 20% of the total cells and constitute the outer rim of the islet. These cells secrete glucagon.
Delta(δ) cells, form about 10% of total cells and are intermixed. These are the source of somatostatin.
• PP cells(F cells) : These are also pheripherlly placed scattered amongst the α cells. These are source of pancreatic polypeptide.
Gap junctions page link β cells to each other , α cells to each other, and β cells to α cells for rapid communication
FUNCTIONS OF INSULIN:
• Insulin secreted by the β cells is a polypeptide containing 51 amino acids with molecular weight of about 5807.
• Main functions is to lower the raised blood glucose levels.
• Proinsulin, is transported from the endoplasmic reticulum to the Golgi complex where it is packaged into granules and cleaved into insulin and residual connecting peptide, or c peptide.
Glucose is in excess of immediate needs insulin promotes their storage by:
• Acting on cell membrane and stimulating uptake and use of glucose by muscle and connective tissue cells.
• Increases the conversion of glucose to glycogen(glycogenesis) especially in liver and skeletal muscles.
• Promoting synthesis of fatty acid and storage of fat in adipose tissue (lipogenesis).
• Decreases glycogenolysis and prevents breakdown of protein and fat.
Major Stimulants:
• Increase blood glucose levels
• Parasympathetic stimulation
• Raised blood amino acid and fatty acid
• Gastrointestinal harmones (eg. Gastrin, secretin and cholecystokinnin-pancreozymin cck-pz )
• Sulfonylurea compounds and β-sympathetic fibres
Major inhibitors:
• Sympathetic stimulation
• Glucogon, adrenaline, cortisol and somatostatin(secreted by the pancreatic δ cells)
• Amylin,pancreastatin and α-sympathetic fibres
Insulin deficiency - diabetes mellitus
Diabetes mellitus is a disorder in which the following features are present : polyuria, polydipsia, polyphagia but weight loss, hyperglycemia and glycosuria.
Insulin excess – hypoglycemia
affects cortical function, discharge of sympathetic nervous system.
causes increase in the secretion of adrenaline, glucagon,glucocorticoids and growth harmone, all of which increase the blood sugar.
GLUCAGON:
Glucagon is secreted by α cells of islets of langherans is a single chain polypeptide, composed of 29 amino acids of molecular weight 3500.
Main function is to increase the blood glucose level.
ACTION OF GLUCAGON:
Increases blood sugar levels by glycogenolysis.
Promotes lipolysis.
Stimulates the secretion of insulin, growth harmone, pancreatic somatostatin and hepatic bile secretion.
Increases myocardial contractility.
Major stimulants:
low blood glucose levels
Amino acids,cholinergic fibres and β-sympathetic fibres
Exercise
Major inhibitors:
Somatostatin and insulin
Glucose
α- sympathetic fibres
SOMATOSTATIN
Pancreatic somatostatin is a neuropeptide containing 14 amino acid, synthesized by δ cells and also by intestinal cells.
ACTIONS:
Inhibits the secretion of glucagon and insulin.
Increases the motility of stomach, deodenum and gall bladder.
Decreases secretion of Hcl, pepsin, gastrin, secretin, intestinal juices and pancreatic juices.
Inhibits absorption of glucose, xylose and triglycerides across the mucosal membrane.
Inhibits the release of growth harmones and almost all peptides.
Inhibits gastric,pancreatic and biliary secretions.
Used to treat both endocrine and exocrine functions.
PANCREATIC POLYPEPTIDE:
Pancreatic polypeptide has 36 amino acids synthesized by PP cells of islets.
ACTIONS:
Inhibit exocrine pancreatic secretion.
Increases the plasma levels of PP serve as markers for the presence of islet cell tumuors and response to treatment.
A failure of plasma pancreatic polypeptide to increase when plasma glucose is sharply reduced suggest the loss of cholinergic pancreatic islets innervations.
PINEAL GLAND
Pineal gland is also known as epiphysis.
It is a small structure (5mm*7mm)
It shaped like a pine cone.
It is situated in the groove between the superior colliculi in diencephalic area of the brain above the hypothalamus
It forms the posterior boundary of third ventricle and lies the posterior end of corpus callosum.
It has a stalk which divides anteriorly into two laminae.
The superior lamina contains the habenular commissure while the inferior lamina has the posterior commisure.
The pineal stroma has two types of cells:
Neuroglial
Parenchymal
• Parenchymal cells of the pineal synthesize and secrete an indole called melatonin from serotonin, which is derived from the essential amino acid tryptophan.
MELATONIN RECEPTORS:
It has two types of receptors:
ML-1 : high affinity melatonin binding site.
ML-2 : low affinity melatonin binding site.
FUNCTIONS OF MELATONIN:
It has a important role in circardian rhythm of the body.
Both inhibitory and facilitatory effects of melatonin on the gonads are described depending upon the species.
An inhibitory effect of melatonin, on MSH(melanocyte stimulating harmone) and ACTH( cortico trophic harmone) secretion has reported.
Other actions of melatonin include the induction of sleep, and inhibition of puberty.
ABNORMALITIES:
Pineal gland tumours (pinealomas) are rare.
Increase melatonin secretion causing delay in puberty.
Decrease melatonin secretion causing precocious puberty.
Because of the location, large pineal tumours cause accumulation of cerebrospinofluid and increase in intracranial pressure, and neurological symptoms.
THYMUS GLAND
FUNCTIONAL ANATOMY:
Thymus is a small lymphoid structure located in the lower part of the neck in front of the trachea, below the thyroid gland.
At birth, it is small (weighing 10-12gm), gradually increases till the puberty when it weight 20-30gm, and then it starts decreasing in size and in the old age weighs about 3-6 gm.
The sex glands exerts a depressed effect on the thymus, therefore, castration(removal of gonads) prolongs the period of persistance of the thymus.
Thymus consists of inner medulla and outer cortex.
(i)Medulla : It comprises reticular epithelial cells, a few lymphocytes and
concentric corpuscles of Hassall.
(ii)Cortex : it includes actively multiplying, closely packed lymphocytes and contains no Hassall’s corpuscles
Thymus has two functions:
Immunological functions
Endocrinal functions
IMMUNOLOGICAL FUNCTIONS:
Formation of blood cells.
Development of immunologically competant T-lymphocytes is the essential function of thymus.
ENDOCRINAL FUNCTIONS:
Thymus tissue secretes two harmones, thymosin and thymin.
THYMOSIN : It is a peptide, promotes proliferation of T-lymphocytes in the thymus and pheripheral lymphoid tissue.
THYMIN : It is also called thymopoietin, inhibits acetylcholine release at motor nerve ending and thus suppresses neuromuscular activity.
ABNORMALITIES:
Hyperactivity of Thymus occurs myasthenia gravis.
Effect of removal of Thymus:
Thymectomy in new born mice causes:
Reduction in number of lymphocytes in blood and lymphoid tissues.
loss of cell-mediated immune response (due to deficiency of T cells).
Decrease ability to produce antibodies. The animals succumb to infection.
Failure to grow at normal rates, and development of a wasting diseases.