Granule (cell biology)

In cell biology, a granule is a small particle.[1] It can be any structure barely visible by light microscopy. The term is most often used to describe a secretory vesicle.

Leukocytes

A group of leukocytes called granulocytes contain granules and play an important role in the immune system. The granules of certain cells, such as natural killer cells, contain components which can lead to the lysis of neighboring cells. The granules of leukocytes are classified as azurophilic granules or specific granules. Leukocyte granules are released in response to immunological stimuli during a process known as degranulation.

Platelets

The granules of platelets are classified as dense granules and alpha granules.

Germline granules

In 1957, André and Rouiller first coined the term "nuage".[2] (French for "cloud"). Its amorphous and fibrous structure occurred in drawings as early as in 1933 (Risley). Today, the nuage is accepted to represent a characteristic, electrondense germ plasm organelle encapsulating the cytoplasmic face of the nuclear envelope of the cells destined to the germline fate. The same granular material is also known under various synonyms: dense bodies, mitochondrial clouds, yolk nuclei, Balbiani bodies, perinuclear P granules in Caenorhabditis elegans, germinal granules in Xenopus laevis, chromatoid bodies in mouse, and polar granules in Drosophila. Molecularly, the nuage is a tightly interwoven network of differentially localized RNA-binding proteins, which in turn localize specific mRNA species for differential storage, asymmetric segregation (as needed for asymmetric cell division), differential splicing and/or translational control. The germline granules appear to be ancestral and universally conserved in the germlines of all metazoan phyla.

Many germline granule components are part of the piRNA pathway and function to repress transposable elements.

Insulin granules
Beta cell with insulin granules, which are the dark black spots surrounded by a white area called a halo.

A specific type of granule found in the pancreas is an insulin granule. Insulin is a hormone that helps to regulate the amount of glucose in the blood from getting too high, hyperglycemia, or too low, hypoglycemia.

Insulin granules are secretory granules, which can release their contents from the cell into the bloodstream. The beta cells in the pancreas are responsible for the storage of insulin and release of it at appropriate times. The beta cells closely control the release, and use unusual mechanisms to do so.[3]

Insulin granule maturation process

Immature insulin granules function as a sorting chamber during the maturation process listed below. Insulin and other insoluble granule components are kept within the granules. Other soluble proteins and granule parts then bud off from the immature granule in a clathrin-coated transport vesicle.[4] The process of proteolysis, removes the unwanted parts from the secretory granule resulting in mature granules.

Insulin granules mature in three steps: (1) the lumen of the granule undergoes acidification, due to the acidic properties of a secretory granule; (2) proinsulin becomes insulin through the process of proteolysis. The endoproteases PC1/3 and PC2 aid in this transformation from proinsulin to insulin; and (3) the clathrin protein coat is removed.[5]

Plant cells

Granules are one of the non-living cell organelle of plant cell (the others-vacuole and nucleoplasm). It serves as small container of starch in plant cell.

Starch

In photosynthesis, plants use light energy to produce glucose from carbon dioxide. The glucose is stored mainly in the form of starch granules, in plastids such as chloroplasts and especially amyloplasts. Toward the end of the growing season, starch accumulates in twigs of trees near the buds. Fruit, seeds, rhizomes, and tubers store starch to prepare for the next growing season.

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See also

References
  1. "granule" at Dorland's Medical Dictionary
  2. André J, Rouiller CH (1957) L'ultrastructure de la membrane nucléaire des ovocytes del l'araignée (Tegenaria domestica Clark). Proc European Conf Electron Microscopy, Stockholm 1956. Academic Press, New York, pp 162 164
  3. Goginashvili, A.; Zhang, Z.; Erbs, E.; Spiegelhalter, C.; Kessler, P.; Mihlan, M.; Pasquier, A.; Krupina, K.; Schieber, N.; Cinque, L.; Morvan, J.; Sumara, I.; Schwab, Y.; Settembre, C.; Ricci, R. (19 February 2015). "Insulin secretory granules control autophagy in pancreatic cells". Science. 347 (6224): 878–882. doi:10.1126/science.aaa2628. PMID 25700520.
  4. (Hou et al., 2009)
  5. (Hou et al., 2009)


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