Classification and Origin
The systematic classification of glial elements is based on their anatomical location (CNS or PNS) and embryological origin. The vast majority of glial cells belong to macroglia, which are of neural origin and develop from embryonic glioblasts.
The exception is microglia. Microglia are formed from promonocytes, meaning they have a monocytic (mesenchymal) origin, which largely dictates their macrophage function within nervous tissue.
CNS Macroglia: Astrocytes
Astrocytes (astrocytes) are star-shaped cells that originate from embryonic radial glia (which initially serve as guides for migrating neuroblasts during development).
There are two main morphological types of astrocytes:
- Protoplasmic astrocytes: characterized by thick, short, highly branched processes. They are predominantly localized in the gray matter of the brain.
- Fibrous astrocytes: characterized by thin, long, poorly branching processes. They predominate in the white matter.
The functions of astroglia are mediated through their cytoplasmic processes. They form a three-dimensional supportive framework, participate in neurotransmitter metabolism, and secrete neurotrophic growth factors. The terminal expansions of these processes form glial limiting membranes (glia limitans) — a key structural element of the blood-brain barrier (BBB), which regulates the transport of substances from capillaries to neurons.
Ependymal Glia
Ependymocytes line the cerebral ventricles and the central canal of the spinal cord, forming a tissue known as ependyma. Most commonly, this is a simple columnar epithelium; however, in certain regions (such as the cerebral aqueduct and the III and IV ventricles), it can be stratified.
Ependyma differs from true epithelia by the absence of a distinct basement membrane along most of its extent and the lack of cytoplasmic keratin filaments.
Structural Features and Functions:
- The apical surface of the cells (facing the ventricular lumen) bears microvilli and motile cilia (kinocilia). Their coordinated beating prevents cerebrospinal fluid (CSF) stasis.
- The ependyma of the choroid plexuses is joined by tight junctions, forming the blood-CSF barrier. These specialized cells perform unidirectional secretion of CSF from the blood.
- A specialized subtype of cells, tanycytes, possess long basal processes. They provide structural anchorage and mediate substance transport between the hypothalamus and the pituitary gland (abundant in the floor of the III ventricle).
Oligodendroglia and Peripheral Glia
Oligodendrocytes are small glial cells with a limited number of short, sparsely branched processes (oligo meaning few). They perform barrier, trophic, and electrical insulation functions.
Based on their location and specific roles, they are divided into two types:
- Satellite cells (capsular oligodendrocytes or mantle glial cells): surround neuron cell bodies in the gray matter of the CNS and in PNS sensory ganglia. In ganglia, they form a single-layered capsule ("mantle") around the perikaryon, facilitating contact with the neuron, while being covered externally by connective tissue. In the CNS, they closely appose neurons in a mosaic pattern, interspersed with astrocytic processes.
- Myelinating oligodendrocytes: located in the CNS white matter and peripheral nerves. They wrap around axons and are responsible for myelination. These include, for example, neurolemmocytes (Schwann cells) in the PNS.
Microglia (Glial Macrophages)
Microglial cells are small cells with elongated nuclei that function as the resident macrophages of the nervous tissue. Depending on their functional state, they are classified into three morphological forms:
- Ameboid: motile cells found in the developing brain. They actively phagocytose cellular debris and apoptotic elements before transforming into the resting form.
- Resting (branched/ramified): cells of the mature, fully formed brain with highly branched processes and low baseline phagocytic activity.
- Reactive: activated forms derived from resting microglia following brain injury or ischemia, which reacquire high phagocytic capacity and mobility.
Clinical Significance: Due to their mobility and monocyte lineage, microglial cells can serve as viral reservoirs and vehicles for neurotropic viruses (such as HIV) into the CNS during infection (e.g., AIDS dementia complex).