The neuroglia or glial cells are defined as supportive cells of the central nervous and peripheral nervous system that primarily support, protect and nourish the neurons. Neuroglial cells are generally smaller than neurons and are more in number by five to ten times. These glial cells comprise about half the total volume of the brain and spinal cord.
What is neuroglia ?
Neuroglia is name given to the group of non-excitable neurological cells that support and protect neurons. Neuroglia maintain homeostasis and form myelin. The neuroglia cells exchange glucose, amino acids and ions with extracellular space fluids. It also influence the neuronal activity of the neurons. the term neuroglia, glial cells or simply glia are same
Four types of neuroglia
There are four types of neuroglial cells in the central and nervous system:
The peripheral nervous system neuroglia comprised of 6. Schwann cells and 7. satellite cells.
Types of Neuroglia:
To answer the question what are types of neuroglia we will discuss individual neuroglia cell and its function with clinical significance.
Astrocytes are neuroglia with small cell bodies with branching processes that extend in all directions. Astrocytes project foot processes that envelop the basement membrane of capillaries, neurons, and synapses.It form the external and internal glial-limiting membranes of the CNS. They play a role in the metabolism of certain neurotransmitters in CNS. There are two types of astrocytes:
- Fibrous astrocytes: Fibrous astrocytes are found mainly in the white matter. Each astrocyte process is long, slender, smooth, and not much branched which pass between the nerve fibers. The cell bodies and processes contain many filaments in their cytoplasm.
- Protoplasmic astrocytes: These are found mainly in the gray matter, where their processes pass between the nerve cell bodies. The processes of protoplasmic are shorter, thicker, and more branched than those of the fibrous astrocyte. The cytoplasm of these cells contains fewer filaments than that of the fibrous astrocyte.
Astrocytes processes end in expansions on blood vessels through perivascular feet, where they form an almost complete covering on the external surface of capillaries. Large numbers of astrocytic processes form the outer and inner glial limiting membranes. Astrocytic processes are also found around the initial segment of most axons and at the nodes of Ranvier.
Function of Astrocytes
These Neuroglia function as
- Astrocytes have branching processes which form a supporting framework for the nerve cells and nerve fibers and their processes are functionally coupled at gap junctions.
- During embryonic life, astrocytes serve as scaffolding for the migration of immature neurons.
- It serves as electrical insulators by covering the synaptic contacts between neurons.
- Astrocytes may even form barriers for the spread of neurotransmitter substances released at synapses.
- Astrocytes appear to be able to take up excess K+ ions from the extracellular space so that they may have an important function during repetitive firing of a neuron.
- Astrocytes store glycogen within their cytoplasm. The stored glycogen can be broken down into glucose and even lactate, both of which are released to surrounding neurons in response to norepinephrine.
- Astrocytes may serve as phagocytes by taking up degenerating synaptic axon terminals.
- Astrocytes can serve as a conduit for the passage of waste material or raw materials from blood capillaries to the neurons through their perivascular feet.
- Astrocytes have a role in the structure of the blood-brain barrier where astrocyte processes terminate as expanded feet on basement membrane of blood vessels.
Oligodendrocytes are the myelin-forming neuroglia cells of the CNS. One oligodendrocyte can myelinate as many as 30 axons. Oligodendrocytes have small cell bodies with delicate processes. It has no filaments in their cytoplasm. Oligodendrocytes are frequently found in rows along myelinated nerve fibers and surround nerve cell bodies. The processes of a single oligodendrocyte join the myelin sheaths of several nerve fibers but one process joins the myelin between two adjacent nodes of Ranvier.
Function of oligodendrocytes:
This neuroglia functions following
- Oligodendrocytes form myelin sheath of nerve fibers in the central nervous system.
- Myelin sheath formed by oligodendrocytes provides the axons with an insulating coat and greatly increases the speed of nerve conduction along these axons.
- Oligodendrocytes can form several internodal segments of myelin on the same or different axons.
- Myelination of nerves by this type of neuroglia begins at about 16th week of intrauterine life and continues postnatally until practically all the major nerve fibers are myelinated by the time the child starts walking.
These neuroglia (microglia) cells are phagocytic cells which are embryologically different from other neuroglial cells. Microglial cells are derived from macrophages outside the nervous system. They are found scattered throughout the central nervous system and the smallest of the neuroglial cells. It gives off numerous spinelike projections from cell bodies. Microglia closely resembles connective tissue macrophages. They migrate into the nervous system during fetal life. When there is damaged nervous tissue resulting from trauma and ischemic injury and in the presence of diseases including Alzheimer disease, Parkinson disease, multiple sclerosis, and AIDS the number of microglial cells increases. Many of these new cells are monocytes.
Function of microglia
MICROGLIA arise from monocytes and function as the scavenger cells (phagocytes) of the CNS. Microglial cells are inactive in the normal brain and spinal cord therefore sometimes called resting microglial cells. During inflammatory disease of the central nervous system, microglia cells become the immune effector cells. This type of microglia cells migrate to the site of the lesion where they proliferate and become antigen presenting cells, which together with the invading T lymphocytes attack invading organisms. They are also actively phagocytic and their cytoplasm becomes filled with lipids and cell remnants. Monocytes join microglial cells from neighboring blood vessels.
EPENDYMAL Cells are ciliated cells that line the central canal and ventricles of the brain. They also line the luminal surface of the choroid plexus. Ependymal cells are type of neuroglia which lines the cavities of the brain and the central canal of the spinal cord. Ependyma cells are cuboidal or columnar in shape and possess microvilli and cilia. The cilia of these cells are motile which contribute to the flow of the cerebrospinal fluid.
Ependymal cells may be divided into three groups:
- Ependymocytes: Ependymocytes line the ventricles of the brain and the central canal of the spinal cord and are in contact with the cerebrospinal fluid.
- Tanycytes: Tancytes line the floor of the third ventricle overlying the median eminence of the hypothalamus.
- Choroidal epithelial cells: these cells cover the surfaces of the choroid plexuses. They have tight junctions which prevent the leakage of cerebrospinal fluid into the underlying tissues.
Function of ependymal cells:
- Ependymocytes help in the circulation of the cerebrospinal fluid inside cavities of the brain and the central canal of the spinal cord by the movements of the cilia.
- Tanycytes function is to transport chemical substances from the cerebrospinal fluid to the hypophyseal portal system. So it may play a part in the control of the hormone production by the anterior lobe of the pituitary.
- Choroidal epithelial cells function is production and secretions of cerebrospinal fluid from the choroid plexuses.
Schwann cells Neuroglia of peripheral nervous system
Schwann cells also called neurolemma cells are neuroglia of the peripheral nervous system that form the myelin sheath around the nerve cell. Other neuroglia or glial to support function of the neurons include satellite cells, olfactory ensheathing cells, and enteric cells. There are two types of cells myelinating Schwann cells and non-myelinating Schwann cells.
Function of Schwann cells
This peripheral neuroglia function as
- Formation of myelin sheath for insulation to decrease membrane capacitance in the axon. It increases conduction velocity through a process called saltatory conduction in which action potential jumps from node to node.
- Non-myelinating Schwann cells are involved in the maintenance of the axon of the neuron.
- Schwann cells are also involved in the conduction of nerve impulse, nerve development and regeneration, modulation of synaptic activity and trophic support.
Development of neuroglia
Embryologically neuroglia develops from ectodermal tissue, in particular, the neural tube and crest. The microglial cells derived from hemopoietic stem cells. The microglia self-renew and multiply when there are injury and disease to the central nervous system. In the central nervous system, the neuroglia develops from the ventricular zone of the neural tube. In the peripheral nervous system, the neuroglia develops from the neural crest.
Clinical significance of neuroglia:
The neuroglial cells in the peripheral nervous system help in regeneration of lost neuronal function but it does not result in the central nervous system from neuroglial cells. Regeneration occurs when there is mild trauma. Studies showed some role in Alzheimer’s disease.
Reactions of Neuroglia to Injury:
following are reactions of neuroglia to the injury
- Reaction of Astrocytes to injury:
When neuroglial cells injury occurs by trauma or vascular occlusion, there is hyperplasia and hypertrophy of the astrocytes. When there is ischemiato the brain there is liquefactive necrosis of the brain. dye to hperplasia and hypertrophy astrocytes become fibrous irrespective of their antecedent morphology. The proliferation of the astrocytes is referred to as astrocytosis or gliosis.
- Reaction of oligodendrocytes to injury:
Oligodendrocytes respond to injury by expanding vacuolation of their cytoplasm, the nuclei also tend to become pyknotic. Severe damage to oligodendrocytes would result in demyelination of the neuron.
- Reaction of microglia to injury:
Microglial cells retract their processes and migrate to the site of the lesion in respond to degeneration and inflammation. Here, they proliferate and are actively phagocytic, and their cytoplasm becomes filled with lipids and cell remnants. Microglial cells are active in a number of diseases including multiple sclerosis, dementia in AIDS, Parkinson disease, and Alzheimer disease.
Tumors of neuroglia:
Tumors of neuroglia are referred to as gliomas which account for half of the intracranial tumors. Tumors of astrocytes are those most common and include astrocytomas and glioblastomas. Apart from the ependymomas, tumors of the neuroglia are highly invasive. This explains the difficulty in achieving complete surgical removal and the great possibility of recurrence after surgery. Tumors of neuroglia are infiltrative. Schwannoma is a tumor of the tissue that covers nerve sheath. It arises from Schwann cells and often benign (not cancerous).