Alpha cell
Encyclopedia
Alpha cells are endocrine
cell
s in the islets of Langerhans
of the pancreas
. They make up 33-46% of the human islet cells and are responsible for synthesizing and secreting the peptide
hormone
glucagon
, which elevates the glucose
levels in the blood
.
To elevate glucose levels, glucagon binds to receptors on hepatocyte
s (liver cells) and some other cells (e.g. kidney cells). This activates an enzyme, glycogen phosphorylase
, inside the hepatocyte to hydrolyse glycogen
to glucose. This process is called glycogenolysis
.
In rodents, alpha cells are located in the periphery of the islets, however in humans the islet architecture is generally less organized and alpha cells are frequently observed inside the islets as well. When being viewed by an electron microscope
, alpha cells can be identified by their characteristic granules
with a large dense core and a small white halo.
Alternative and more common spellings: alpha-cell or α-cell.
Endocrine system
In physiology, the endocrine system is a system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. It derives from the Greek words "endo"...
cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s in the islets of Langerhans
Islets of Langerhans
The islets of Langerhans are the regions of the pancreas that contain its endocrine cells. Discovered in 1869 by German pathological anatomist Paul Langerhans at the age of 22, the islets of Langerhans constitute approximately 1 to 2% of the mass of the pancreas...
of the pancreas
Pancreas
The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as a digestive organ, secreting pancreatic juice containing digestive enzymes that assist...
. They make up 33-46% of the human islet cells and are responsible for synthesizing and secreting the peptide
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
glucagon
Glucagon
Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...
, which elevates the glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
levels in the blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
.
To elevate glucose levels, glucagon binds to receptors on hepatocyte
Hepatocyte
A hepatocyte is a cell of the main tissue of the liver. Hepatocytes make up 70-80% of the liver's cytoplasmic mass.These cells are involved in:* Protein synthesis* Protein storage* Transformation of carbohydrates...
s (liver cells) and some other cells (e.g. kidney cells). This activates an enzyme, glycogen phosphorylase
Glycogen phosphorylase
Glycogen phosphorylase is one of the phosphorylase enzymes . Glycogen phosphorylase catalyzes the rate-limiting step in the degradation of glycogen in animals by releasing glucose-1-phosphate from the terminal alpha-1,4-glycosidic bond...
, inside the hepatocyte to hydrolyse glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
to glucose. This process is called glycogenolysis
Glycogenolysis
Glycogenolysis is the conversion of glycogen polymers to glucose monomers. Glycogen is catabolized by removal of a glucose monomer through cleavage with inorganic phosphate to produce glucose-1-phosphate...
.
In rodents, alpha cells are located in the periphery of the islets, however in humans the islet architecture is generally less organized and alpha cells are frequently observed inside the islets as well. When being viewed by an electron microscope
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...
, alpha cells can be identified by their characteristic granules
Granule (cell biology)
In cell biology, a granule is a small particle. It can be any structure barely visible by light microscopy. The term is most often used to describe a secretory vesicle.-Leukocytes:...
with a large dense core and a small white halo.
Alternative and more common spellings: alpha-cell or α-cell.