খসড়া:গ্লিয়া

গ্লিয়া
গ্লিয়াল কোষ: এপেনডাইমাল কোষ (হালকা গোলাপী), অ্যাস্ট্রোসাইট (সবুজ), মাইকারোগ্লিয়াল কোষ (গাঢ় লাল), ও অলিগোডেনড্রোসাইট (হালকা নীল)
বিস্তারিত
পূর্বভ্রূণ	নিউরোএক্টোডার্ম (ম্যাক্রোগ্লিয়ার জন্য), ও রক্তকোষ উৎপাদনকারী স্টেম কোষ (মাইক্রোগ্লিয়ার জন্য)
তন্ত্র	স্নায়ুতন্ত্র
মাইক্রো শারীরস্থান পরিভাষা

গ্লিয়া হলো কেন্দ্রীয় স্নায়ুতন্ত্রের ( মস্তিষ্ক এবং মেরুদণ্ড ) এবং পেরিফেরাল স্নায়ুতন্ত্রের নন - নিউরন অ্যাল কোষ। একে গ্লিয়াল সেল (একবচন গ্লিওসাইট ) বা নিউরোগ্লিয়াও বলা হয়।এরা হোমিওস্ট্যাসিস বজায় রাখে , পেরিফেরাল নার্ভাস সিস্টেমে মাইলিন তৈরি করে এবং নিউরনের জন্য সমর্থন ও সুরক্ষা প্রদান করে ।  কেন্দ্রীয় স্নায়ুতন্ত্রে, গ্লিয়াল কোষের মধ্যে রয়েছে অলিগোডেনড্রোসাইট, অ্যাস্ট্রোসাইট, এপেনডাইমাল কোষ এবং মাইক্রোগ্লিয়াল কোষ এবং পেরিফেরাল স্নায়ুতন্ত্রের গ্লিয়াল কোষগুলির মধ্যে রয়েছে স্বোয়ান কোষ এবং স্যাটেলাইট গ্লিয়াল কোষ ।

এদের চারটি প্রধান কাজ রয়েছে:

• নিউরনকে ঘিরে রাখা এবং তাদের জায়গায় রাখা;
• নিউরোনগুলিতে পুষ্টি এবং অক্সিজেন সরবরাহ করা;
• একটি নিউরন থেকে অন্য নিউরন নিরোধক করা;
• প্যাথোজেন ধ্বংস এবং মৃত নিউরন অপসারণ. তারা নিউরোট্রান্সমিশন এবং সিনাপটিক সংযোগে ভূমিকা পালন করে,  এবং শ্বাস- প্রশ্বাসের মতো শারীরবৃত্তীয় প্রক্রিয়াগুলিতেও । যদিও গ্লিয়া ১০:১ অনুপাতের দ্বারা নিউরনের সংখ্যাকে ছাড়িয়ে গেছে বলে মনে করা হয়েছিল, সাম্প্রতিক গবেষণাগুলি নতুন পদ্ধতি ব্যবহার করে এবং ঐতিহাসিক পরিমাণগত প্রমাণের পুনর্মূল্যায়নে বিভিন্ন মস্তিষ্কের টিস্যুগুলির মধ্যে উল্লেখযোগ্য পার্থক্য সহ সামগ্রিক অনুপাত ১:১ এর কম হওয়ার পরামর্শ দেয়।

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ধরন

Neuroglia of the brain shown by Golgi's method

Astrocytes can be identified in culture because, unlike other mature glia, they express glial fibrillary acidic protein (GFAP)

Glial cells in a rat brain stained with an antibody against GFAP

Different types of neuroglia

Macroglia

Derived from ectodermal tissue.

Location	Name	Description
CNS	Astrocytes	The most abundant type of macroglial cell in the CNS, astrocytes (also called astroglia) have numerous projections that link neurons to their blood supply while forming the blood-brain barrier. They regulate the external chemical environment of neurons by removing excess potassium ions, and recycling neurotransmitters released during synaptic transmission. Astrocytes may regulate vasoconstriction and vasodilation by producing substances such as arachidonic acid, whose metabolites are vasoactive. Astrocytes signal each other using ATP. The gap junctions (also known as electrical synapses) between astrocytes allow the messenger molecule IP3 to diffuse from one astrocyte to another. IP3 activates calcium channels on cellular organelles, releasing calcium into the cytoplasm. This calcium may stimulate the production of more IP3 and cause release of ATP through channels in the membrane made of pannexins. The net effect is a calcium wave that propagates from cell to cell. Extracellular release of ATP, and consequent activation of purinergic receptors on other astrocytes, may also mediate calcium waves in some cases. In general, there are two types of astrocytes, protoplasmic and fibrous, similar in function but distinct in morphology and distribution. Protoplasmic astrocytes have short, thick, highly branched processes and are typically found in gray matter. Fibrous astrocytes have long, thin, less branched processes and are more commonly found in white matter. It has recently been shown that astrocyte activity is linked to blood flow in the brain, and that this is what is actually being measured in fMRI. They also have been involved in neuronal circuits playing an inhibitory role after sensing changes in extracellular calcium.
CNS	Oligodendrocytes	Oligodendrocytes are cells that coat axons in the central nervous system (CNS) with their cell membrane, forming a specialized membrane differentiation called myelin, producing the myelin sheath. The myelin sheath provides insulation to the axon that allows electrical signals to propagate more efficiently.
CNS	Ependymal cells	Ependymal cells, also named ependymocytes, line the spinal cord and the ventricular system of the brain. These cells are involved in the creation and secretion of cerebrospinal fluid (CSF) and beat their cilia to help circulate the CSF and make up the blood-CSF barrier. They are also thought to act as neural stem cells.
CNS	Radial glia	Radial glia cells arise from neuroepithelial cells after the onset of neurogenesis. Their differentiation abilities are more restricted than those of neuroepithelial cells. In the developing nervous system, radial glia function both as neuronal progenitors and as a scaffold upon which newborn neurons migrate. In the mature brain, the cerebellum and retina retain characteristic radial glial cells. In the cerebellum, these are Bergmann glia, which regulate synaptic plasticity. In the retina, the radial Müller cell is the glial cell that spans the thickness of the retina and, in addition to astroglial cells, participates in a bidirectional communication with neurons.
PNS	Schwann cells	Similar in function to oligodendrocytes, Schwann cells provide myelination to axons in the peripheral nervous system (PNS). They also have phagocytotic activity and clear cellular debris that allows for regrowth of PNS neurons.
PNS	Satellite cells	Satellite glial cells are small cells that surround neurons in sensory, sympathetic, and parasympathetic ganglia. These cells help regulate the external chemical environment. Like astrocytes, they are interconnected by gap junctions and respond to ATP by elevating the intracellular concentration of calcium ions. They are highly sensitive to injury and inflammation and appear to contribute to pathological states, such as chronic pain.
PNS	Enteric glial cells	Are found in the intrinsic ganglia of the digestive system. Glia cells are thought to have many roles in the enteric system, some related to homeostasis and muscular digestive processes.

Microglia

মূল নিবন্ধ: Microglia

Microglia are specialized macrophages capable of phagocytosis that protect neurons of the central nervous system. They are derived from the earliest wave of mononuclear cells that originate in yolk sac blood islands early in development, and colonize the brain shortly after the neural precursors begin to differentiate.

These cells are found in all regions of the brain and spinal cord. Microglial cells are small relative to macroglial cells, with changing shapes and oblong nuclei. They are mobile within the brain and multiply when the brain is damaged. In the healthy central nervous system, microglia processes constantly sample all aspects of their environment (neurons, macroglia and blood vessels). In a healthy brain, microglia direct the immune response to brain damage and play an important role in the inflammation that accompanies the damage. Many diseases and disorders are associated with deficient microglia, such as Alzheimer's disease, Parkinson's disease, and ALS.

Other

Pituicytes from the posterior pituitary are glial cells with characteristics in common to astrocytes. Tanycytes in the median eminence of the hypothalamus are a type of ependymal cell that descend from radial glia and line the base of the third ventricle. Drosophila melanogaster, the fruit fly, contains numerous glial types that are functionally similar to mammalian glia but are nonetheless classified differently.

Total number

In general, neuroglial cells are smaller than neurons. There are approximately 85 billion glia cells in the human brain, about the same number as neurons. Glial cells make up about half the total volume of the brain and spinal cord. The glia to neuron-ratio varies from one part of the brain to another. The glia to neuron-ratio in the cerebral cortex is 3.72 (60.84 billion glia (72%); 16.34 billion neurons), while that of the cerebellum is only 0.23 (16.04 billion glia; 69.03 billion neurons). The ratio in the cerebral cortex gray matter is 1.48, with 3.76 for the gray and white matter combined. The ratio of the basal ganglia, diencephalon and brainstem combined is 11.35.

The total number of glia cells in the human brain is distributed into the different types with oligodendrocytes being the most frequent (45–75%), followed by astrocytes (19–40%) and microglia (about 10% or less).

Development

23-week fetal brain culture astrocyte

মূল নিবন্ধ: Gliogenesis

Most glia are derived from ectodermal tissue of the developing embryo, in particular the neural tube and crest. The exception is microglia, which are derived from hemopoietic stem cells. In the adult, microglia are largely a self-renewing population and are distinct from macrophages and monocytes, which infiltrate an injured and diseased CNS.

In the central nervous system, glia develop from the ventricular zone of the neural tube. These glia include the oligodendrocytes, ependymal cells, and astrocytes. In the peripheral nervous system, glia derive from the neural crest. These PNS glia include Schwann cells in nerves and satellite glial cells in ganglia.

Capacity to divide

Glia retains the ability to undergo cell divisions in adulthood, whereas most neurons cannot. The view is based on the general inability of the mature nervous system to replace neurons after an injury, such as a stroke or trauma, where very often there is a substantial proliferation of glia, or gliosis, near or at the site of damage. However, detailed studies have found no evidence that 'mature' glia, such as astrocytes or oligodendrocytes, retain mitotic capacity. Only the resident oligodendrocyte precursor cells seem to keep this ability once the nervous system matures.

Glial cells are known to be capable of mitosis. By contrast, scientific understanding of whether neurons are permanently post-mitotic, or capable of mitosis, is still developing. In the past, glia had been considered to lack certain features of neurons. For example, glial cells were not believed to have chemical synapses or to release transmitters. They were considered to be the passive bystanders of neural transmission. However, recent studies have shown this to not be entirely true.

History

Although glial cells and neurons were probably first observed at the same time in the early 19th century, unlike neurons whose morphological and physiological properties were directly observable for the first investigators of the nervous system, glial cells had been considered to be merely “glue” that held neurons together until the mid-20th century.

Glia were first described in 1856 by the pathologist Rudolf Virchow in a comment to his 1846 publication on connective tissue. A more detailed description of glial cells was provided in the 1858 book 'Cellular Pathology' by the same author.

When markers for different types of cells were analyzed, Albert Einstein's brain was discovered to contain significantly more glia than normal brains in the left angular gyrus, an area thought to be responsible for mathematical processing and language. However, out of the total of 28 statistical comparisons between Einstein's brain and the control brains, finding one statistically significant result is not surprising and the claim that Einstein's brain is different, is not scientific (c.f. Multiple comparisons problem).

Not only does the ratio of glia to neurons increase through evolution, but so does the size of the glia. Astroglial cells in human brains have a volume 27 times greater than in mouse brains.

These important scientific findings may begin to shift the neuron-specific perspective into a more holistic view of the brain which encompasses the glial cells as well. For the majority of the twentieth century, scientists had disregarded glial cells as mere physical scaffolds for neurons. Recent publications have proposed that the number of glial cells in the brain is correlated with the intelligence of a species.