In cell biology, a granule is a small particle barely visible by light microscopy. The term is most often used to describe a secretory vesicle containing important components of cell physiology. Examples of granules include granulocytes, platelet granules, insulin granules, germane granules, starch granules, and stress granules. It is considered as a cell organelle.
Types
There are mainly two types of granules based on the presence or absence of a membrane: membrane-bound granules, and non-membrane-bound granules. Eosinophilic granules, basophilic granules, and secretory granules are examples of membrane-bound granules, while P-granules and stress granules are examples of non-membrane-bound granules.
In leukocytes
A group of leukocytes, called granulocytes, are white blood cells containing enzyme granules that play a significant role in the immune system. Granulocytes include neutrophils, eosinophils, and basophils which attack bacteria or parasites, and respond to allergens. Each type of granulocyte contains enzymes and chemicals tailored to its function.
Other immune cells, such as natural killer cells, contain granular enzymes, including perforin and proteases which can lead to the lysis of neighboring cells.
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 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 [ 3H] serotonin, or membrane expression of lysosome-associated membrane protein 2 (LAMP2) or CD63 by flow cytometry.
Insulin granules undergo a significant maturation process. First, precursor proinsulin molecules are synthesized in the endoplasmic reticulum and packaged in the golgi network. Insulin granules bud from the trans golgi network and are further sorted via clathrin-coated vesicle transport. After budding, insulin secretory granules are acidified, activating endoproteases PC1/3 and PC2 to convert proinsulin into insulin. The clatherin coating is released and the insulin secretory granules are transported across the cell via actin filaments and microtubules.
In germline cells
In 1957, André and Rouiller first coined the term "nuage". (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 mice, 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.
In plants (starch)
Starch is an insoluble carbohydrate used for energy storage in plant cells. There are two forms of starch, transitionary starch and storage starch. Transitionary starch is synthesised via photosynthesis and found in photosynthetic plant tissue cells, such as the leaves. Storage starch is reserved for longer periods of time and is found in non-photosynthetic tissue cells such as the roots or stem. Storage starch is utilized during germination or regrowth, or when energy demands exceed net energy production from photosynthesis.
thumb|Starch granules in potato cells.
Starch is stored in granule form. Starch granules are composed of a crystalline structure of amylopectin and amylose. Amylopectin forms the structure of the starch granule, with branching and non branching A-chains, B-chains, and C-chains. Amylose fills in the gaps of the amylopectin structure. Under a microscope, starch granules look like concentric layers, referred to as “growth rings”. Starch granules also contain granule-bound starch synthase and amylopectin synthesizing enzymes. Notably, starch granules vary in size and morphology across plant tissues and species.
thumb|Assembly and disassembly of [[stress granules. ]]
Stress granule assembly is dependent upon the conditions of the cell. In yeast, stress granules form under conditions of high heat. Stress granules are of significance for their roles in mRNA localization, cell signaling pathways, and antiviral processes. Once disassembled, the RNA inside stress granules can go back to translation or be removed as cellular waste. Stress granules may provide protection for mRNA from interactions with the cytosol. Moreover, mutations that affect the formation or degradation of stress granules may contribute to neurodegenerative conditions such as ALS and FTLD. However, the effects of stress granules on cell physiology are still under study.