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

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". (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.

Granule cell

The name granule cell has been used by anatomists for a number of different types of neuron whose only common feature is that they all have very small cell bodies. Granule cells are found within the granular layer of the cerebellum, the dentate gyrus of the hippocampus, the superficial layer of the dorsal cochlear nucleus, the olfactory bulb, and the cerebral cortex.

Cerebellar granule cells account for the majority of neurons in the human brain. These granule cells receive excitatory input from mossy fibers originating from pontine nuclei. Cerebellar granule cells send parallel fibers up through the Purkinje layer into the molecular layer where they branch out and spread through Purkinje cell dendritic arbors. These parallel fibers form thousands of excitatory granule-cell-Purkinje-cell synapses onto the intermediate and distal dendrites of Purkinje cells using glutamate as a neurotransmitter.

Layer 4 granule cells of the cerebral cortex receive inputs from the thalamus and send projections to supragranular layers 2-3, but also to infragranular layers of the cerebral cortex.

Cerebellum granule cell

Granule cells of the cerebellum are among the smallest neurons in the brain. (The term granule cell is used for several unrelated types of small neurons in various parts of the brain.) Cerebellar granule cells are also easily the most numerous neurons in the brain: in humans, estimates of their total number average around 50 billion, which means that they constitute about 3/4 of the brain's neurons.

Development

In normal development, endogenous Sonic hedgehog signaling stimulates rapid proliferation of cerebellar granule neuron progenitors (CGNPs) in the external granule layer (EGL). Cerebellum development occurs during late embryogenesis and the early postnatal period, with CGNP proliferation in the EGL peaking during early development (P7, postnatal day 7, in the mouse). As CGNPs terminally differentiate into cerebellum granule cells (also called cerebellar granule neurons, CGNs), they migrate to the internal granule layer (IGL), forming the mature cerebellum (by P20, post-natal day 20 in the mouse). Mutations that abnormally activate Sonic hedgehog signaling predispose to cancer of the cerebellum (medulloblasoma) in humans with Gorlin Syndrome and in genetically engineered mouse models.