Granule (cell Biology)
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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 Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form nat ...
.


In leukocytes

A group of
leukocytes White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from mult ...
, called granulocytes, contain granules and play an important role in the
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splint ...
. 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 granule An azurophilic granule is a cellular object readily stainable with a Romanowsky stain. In white blood cells and hyperchromatin, staining imparts a burgundy or merlot coloration. Neutrophils in particular are known for containing azurophils load ...
s or
specific granule Specific granules are secretory vesicles found exclusively in cells of the immune system called granulocytes. It is sometimes described as applying specifically to neutrophils, and sometimes the term is applied to other types of cells. These gr ...
s. Leukocyte granules are released in response to immunological stimuli during a process known as degranulation.


In platelets

The granules of platelets are classified as
dense granule Dense granules (also known as dense bodies or delta granules) are specialized secretory organelles. Dense granules are found only in platelets and are smaller than alpha granules.Michelson, A. D. (2013). ''Platelets'' (Vol. 3rd ed). Amsterdam: Aca ...
s and alpha granules. α-Granules are unique to platelets and are the most abundant of the platelet granules, numbering 50–80 per platelet 2. These granules measure 200–500 nm in diameter and account for about 10% of platelet volume. They contain mainly proteins, both membrane-associated receptors (for example, αIIbβ3 and P-selectin) and soluble cargo (for example, platelet factor 4 F4and fibrinogen). Proteomic studies have identified more than 300 soluble proteins that are involved in a wide variety of functions, including hemostasis (for example, von Willebrand factor WFand factor V), inflammation (for example, chemokines such as CXCL1 and interleukin-8), and wound healing (for example, vascular endothelial growth factor EGFand fibroblast growth factor GF 3. The classic representation of α-granules as spherical organelles with a peripheral limiting membrane, a dense nucleoid, and progressively lucent peripheral zones on transmission electron microscopy is probably simplistic and may be in part a preparation artifact. Electron tomography with three-dimensional reconstruction of platelets is notable for a significant percentage of tubular α-granules that generally lack VWF 4. More recent work using transmission electron microscopy and freeze substitution dehydration of resting platelets shows that α-granules are ovoid with a generally homogeneous matrix and that tubes form from α-granules upon activation 5. Thus, whether or not there exists significant structural heterogeneity among α-granules remains to be completely resolved. α-Granule exocytosis is evaluated primarily by plasma membrane expression of P-selectin (CD62P) by flow cytometry or estimation of the release of PF4, VWF, or other granule cargos. Text was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
Dense granules (also known as δ-granules) are the second most abundant platelet granules, with 3–8 per platelet. They measure about 150 nm in diameter 2. These granules, unique to the platelets, are a subtype of lysosome-related organelles (LROs), a group that also includes melanosomes,
lamellar bodies In cell biology, lamellar bodies (otherwise known as lamellar granules, membrane-coating granules (MCGs), keratinosomes or Odland bodies) are secretory organelles found in type II alveolar cells in the lungs, and in keratinocytes in the skin. The ...
of the type II alveolar cells, and lytic granules of cytotoxic T cells. Dense granules mainly contain bioactive amines (for example, serotonin and histamine), adenine nucleotides, polyphosphates, and pyrophosphates as well as high concentrations of cations, particularly calcium. These granules derive their name from their electron-dense appearance on whole mount electron microscopy, which results from their high cation concentrations . Dense granule exocytosis is typically evaluated by ADP/ATP release by using luciferase-based luminescence techniques, release of preloaded 3Hserotonin, or membrane expression of lysosome-associated membrane protein 2 (LAMP2) or CD63 by flow cytometry. Other platelet granules have been described. Platelets contain about 1–3 lysosomes per platelet and peroxisomes, the platelet-specific function of which remains unclear. Lysosomal exocytosis is typically evaluated by estimation of released lysosomal enzymes such as beta hexosaminidase. An electron-dense granule defined by the presence of Toll-like receptor 9 (TLR9) and protein disulfide isomerase (PDI), termed the T granule, has also been described, although its existence remains controversial. PDI and other platelet-borne thiol isomerases have been reported to be packaged within a non-granular compartment derived from the megakaryocyte endoplasmic reticulum (ER), which may be associated with the dense tubular system.


Insulin granules in beta cells

A specific type of granule found in the
pancreas The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an en ...
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 Beta cells (β-cells) are a type of cell found in pancreatic islets that synthesize and secrete insulin and amylin. Beta cells make up 50–70% of the cells in human islets. In patients with Type 1 diabetes, beta-cell mass and function are dim ...
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.


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. 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.


Germline granules

In 1957, André and Rouiller first coined the term " nuage".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 (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 Germ plasm () is a biological concept developed in the 19th century by the German biologist August Weismann. It states that heritable information is transmitted only by germ cells in the gonads (ovaries and testes), not by somatic cells. The ...
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 The African clawed frog (''Xenopus laevis'', also known as the xenopus, African clawed toad, African claw-toed frog or the ''platanna'') is a species of African aquatic frog of the family Pipidae. Its name is derived from the three short claws ...
, chromatoid bodies in mice, and polar granules in
Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...
''. Molecularly, the ''nuage'' is a tightly interwoven network of differentially localized
RNA-binding protein RNA-binding proteins (often abbreviated as RBPs) are proteins that bind to the double or single stranded RNA in cells and participate in forming ribonucleoprotein complexes. RBPs contain various structural motifs, such as RNA recognition motif ...
s, which in turn localize specific
mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein. mRNA is created during the ...
species for differential storage, asymmetric segregation (as needed for
asymmetric cell division An asymmetric cell division produces two daughter cells with different cellular fates. This is in contrast to symmetric cell divisions which give rise to daughter cells of equivalent fates. Notably, stem cells divide asymmetrically to give rise to ...
), differential splicing and/or translational control. The germline granules appear to be ancestral and universally conserved in the germlines of all
metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
n phyla. Many germline granule components are part of the piRNA pathway and function to repress
transposable elements A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Trans ...
.


Plant cells

Granules are one of the non-living
cell organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' th ...
of plant cell (the others-vacuole and
nucleoplasm The nucleoplasm, also known as karyoplasm, is the type of protoplasm that makes up the cell nucleus, the most prominent organelle of the eukaryotic cell. It is enclosed by the nuclear envelope, also known as the nuclear membrane. The nucleoplasm ...
). It serves as small container of starch in plant cell.


Starch

In
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
, plants use light energy to produce
glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
from
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
. The glucose is stored mainly in the form of starch granules, in plastids such as chloroplasts and especially
amyloplast Amyloplasts are a type of plastid, double-enveloped organelles in plant cells that are involved in various biological pathways. Amyloplasts are specifically a type of leucoplast, a subcategory for colorless, non-pigment-containing plastids. Amylop ...
s. Toward the end of the growing season, starch accumulates in twigs of trees near the buds.
Fruit In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in particu ...
,
seed A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...
s, rhizomes, and
tuber Tubers are a type of enlarged structure used as storage organs for nutrients in some plants. They are used for the plant's perennation (survival of the winter or dry months), to provide energy and nutrients for regrowth during the next growin ...
s store starch to prepare for the next growing season.


See also

*
Chromaffin granule Chromaffin cells, also called pheochromocytes (or phaeochromocytes), are neuroendocrine cells found mostly in the medulla of the adrenal glands in mammals. These cells serve a variety of functions such as serving as a response to stress, monitor ...
*
Kurloff cell Kurloff cells (also known as Foà-Kurloff cells,) were described as mononuclear cells in the peripheral blood and organs of the guinea pig, capybara, paca, agouti and cavie. The Kurloff cell contains a characteristic proteoglycan-containing in ...


References

Cell anatomy {{cell-biology-stub