Astrocyte

Recommended video: Cerebral cortex histology [16:38]

The layers and cell types of the cerebral cortex, as seen under the microscope.

Astrocytes are the largest and most abundant type of supporting (glial) cells in the central nervous system (CNS). Together with oligodenrocytes they make up a group of glial cells called macroglia, which originate within the neural plate. In terms of structure and function, astrocytes are the most complex and diverse among the glial cells of the CNS.

There are two main morphological types of astrocytes: protoplasmic and fibrous astrocytes. Protoplasmic astrocytes have a starlike appearance due to their many radiating processes and are primarily found in the grey matter. Fibrous astrocytes, on the other hand, have relatively few but longer cytoplasmic processes that align with the nerve axons and are mainly found in white matter.

Astrocytes are distinguished from other glial cells based on their size, intracellular organization and cytoplasmic projections. Histologically, astrocytes are recognized by immunohistochemical staining for the protein glial fibrillary acidic protein (GFAP), which makes up bundles of intermediate filaments within astrocytes and serves as a histological marker.

The cytoplasmic processes of astrocytes have extensive terminal branching. This allows one single astrocyte to wrap around several nerve cell bodies and make connections with over a million synapses, highlighting the complexity of the structure and function of astrocytes.

Astrocytes form a large network of glial cells that communicate with each other via gap junctions, ensuring coordinated execution of their activities across various regions of the CNS. Astrocytes perform several key functions which include:

• Contributing to the formation and maintenance of the integrity of the blood-brain barrier. Astrocytes cover the outer surface of brain capillary basement membranes with their perivascular feet (pericapillary end-foot), which are the terminal expansions of cytoplasmic processes. Thus, they help ensure that the passage of substances from the blood stream into the brain tissue is tightly regulated.
• Formation of the glia limitans (glial limiting membrane), which is a largely impermeable barrier or between the CNS surface and the pia mater (innermost meningeal layer), formed by the foot processes of astrocytes.
• Providing metabolic support to neurons by modulating the chemical composition of interstitial fluid within the brain through the regulation of the exchange of various ions and molecules between blood and tissue fluid.
• Provision of structural support and plasticity to the nervous tissue in the CNS by their cytoskeletal network.
• Replacement of damaged nerve cells by the process of astrocytosis, which involves the proliferation of astrocytes to occupy space left behind by dead neurons.
• Neuronal development of the fetal brain through the release of chemicals that establish and manage connections between neurons.
• Controlling the diameter of vessels they are in contact with by the release of vasoactive substances that result in dilation or constriction.