Astrocytes, or astroglia, are the star shaped glial cells that reside in the brain and spinal cord. They are the most numerous cells in the human brain, performing many tasks. Astrocyte tasks include support of the cells that comprise the blood-brain barrier as well as maintaining the extracellular ion balance, supplying nutrients to nerve tissue and aiding in post-traumatic repair and scarring processes. In addition, astrocytes perform signaling tasks similar to neurons utilizing calcium ions and transmitter molecules.
Role of Astrocytes in the Central Nervous System
Astrocytes, or astrocytic glial cells, collectively form astroglia, which are star-shaped cells surrounding neurons in the brain and spinal cord. Astrocytes outnumber neurons 50:1 and are very active in the central nervous system, unlike previous ideology of astrocytes being “filler” cells. Non-neuronal cells in the nervous system are glial cells with astrocytes constituting a subtype of this category. They serve to maintain, support and repair the nervous tissue that they serve and are responsible for the plasticity of the nervous system. Astrocytes also modulate synaptic transmission by making and releasing glutamate, the principle excitatory neurotransmitter, and their many process often envelope neural synapses. Research suggests that astrocytes communicate with neurons through the release of transmitters, known as gliotransmitters, through a calcium ion dependent mechanism.
Astrocytes control the neuronal circuits by regulating the formation and maintenance of synapses. Synaptic signaling rely on molecules secreted by astrocytes, which is typically cholesterol and thrombin.
Astrocytes respond to central nervous system (CNS) injury or disease via the process called reactive astrogliosis. This pathology has become a characteristic of nervous system structural lesions and is a major area of current research. Astrocytes have been identified as involved in the pathologies of Huntington’s disease, Alzheimer’s disease, concussions, stroke and CNS infections. Current research indicates that reactive astrogliosis plays a major role in many CNS disorders.
There are many functions of glial cells, including building the micro-architecture of brain parenchyma, creating the brain environment, maintaining homeostasis of the brain, storing energy, controlling the development of synaptogenesis, synaptic maintenance, neural cells and brain defense.
Fibrous, Protoplasmic and Radial Astrocytes
There are three forms of astrocytes in the CNS consisting of fibrous, protoplasmic and radial astrocytes.
- Fibrous astroglia are generally found in white matter of the brain, have few organelles and long unbranched processes.
- Protoplasmic glia have a huge quantity of organelles relative to fibrous cells, with short and highly branched cellular processes. They are generally found in gray matter.
- Radial astroglia exist at the intersection of gray matter and the pia mater, which is the innermost layer of the membranes surrounding the brain and spinal cord. Radial glia are also found in the vertebrate eye (form the Mueller Cells of the retina) and as Bergmann glia (epithelial cells in the cerebellum). These cells are bipolar, with elongated processes and ovoid body. Radial glial cells are the initial cells that develop from its neural progenitors and form the main scaffold structures that help in neuronal migration. In the retina, radial astroglia cells transform into Muller cells and are 23% of the total retina cell volume.
Many primary astrocytes express GFAP, the intermediate filament glial fibrillary acidic protein, a characteristic trait.
Astrocyte Research Articles and References
- Characterization of astrocyte cells in response to stimuli: This study developed a protocol to prepare rat astrocytes in culture and then measured their response to chemical stimuli. Among the stimuli tested were hydrogen peroxide exposure, resveratrol (and its ability to protect from oxidation), and endotoxin exposure. PlosOne Journal
- Use of astrocytes as a substrate for other neural cells: Researchers found that survival of CNS cells is improved when they are cultured atop a confluent layer of astrocytes. They have developed a protocol that selects for type I astrocytes in a confluent layer, and successfully plated dorsal root ganglion/dorsal horn co-cultures on top of them. CSHP Protocols