Ependymal cells are specialized neuroglia, or support cells, found within the central nervous system. They contribute to the overall function and environment of these complex organs. These cells are fundamental to maintaining the delicate balance and processes that keep the central nervous system healthy and functioning.
Where Ependymal Cells Are Found and How They Are Structured
Ependymal cells form a thin lining within the brain’s ventricular system and the central canal of the spinal cord. These cavities are filled with cerebrospinal fluid (CSF). Ependymal cells have a columnar or cuboidal shape, similar to epithelial cells, and originate from neuroepithelial cells during development.
The surface of these cells facing the CSF is equipped with numerous hair-like projections called cilia and smaller structures known as microvilli. The cilia beat in a coordinated manner, while microvilli increase the surface area for absorption and secretion. Ependymal cells are connected by specialized junctions, which create a nearly continuous sheet over the ventricular surfaces, allowing for some diffusion of CSF into the brain tissue. However, in specific areas like the choroid plexus, modified ependymal cells form tighter junctions that strictly regulate substance passage.
Managing Cerebrospinal Fluid
Ependymal cells are involved in the production, circulation, and regulation of cerebrospinal fluid (CSF). Within the ventricles, modified ependymal cells, along with capillaries, form the choroid plexus, the primary site for CSF production. This process involves the selective uptake of water and specific molecules from the blood, which are then secreted into the ventricles as CSF.
Once produced, CSF circulates throughout the central nervous system to deliver nutrients and remove waste. The cilia on ependymal cells play a role in this circulation; their rhythmic beating moves CSF through the ventricular system and the central canal of the spinal cord. This coordinated movement ensures that growth factors and other chemical messengers are distributed throughout the CNS. Ependymal cells also contribute to the blood-CSF barrier, particularly the specialized cells in the choroid plexus, which have tight junctions that control the passage of substances between the blood and the CSF. The microvilli on the ependymal cell surface also assist in absorbing CSF.
Other Key Roles of Ependymal Cells
Beyond their direct involvement in cerebrospinal fluid management, ependymal cells contribute to other important functions within the central nervous system. A subset of ependymal cells acts as a neural stem cell niche. These cells retain the ability to proliferate and differentiate into new neurons and other glial cells throughout adulthood, contributing to neurogenesis. The cerebrospinal fluid itself, in direct contact with these cells, can influence their proliferation and differentiation by providing growth factors and cues.
Specialized ependymal cells, known as tanycytes, are found in specific areas. Unlike other ependymal cells, tanycytes lack cilia but possess long basal processes that extend into the brain tissue, connecting with neurons and blood vessels. This unique position allows tanycytes to act as an interface between the cerebrospinal fluid, blood, and brain tissue, playing a role in neuroendocrine regulation and sensing the metabolic environment of the CSF. They can transport nutrients and hormones and modulate the activity of nearby neurons.
Ependymal Cells and Brain Health
The proper functioning of ependymal cells is linked to overall brain health. Disruptions in their activity can have consequences for the central nervous system. For instance, impaired ciliary movement can lead to an abnormal accumulation of cerebrospinal fluid, a condition known as hydrocephalus. This occurs because the coordinated beating of cilia maintains the directed flow of CSF.
Alterations in the integrity of the ependymal barrier, which regulates substance exchange between CSF and brain tissue, can contribute to neurological disorders. When this barrier is compromised, it may allow the entry of harmful substances or disrupt the balance of the central nervous system environment. Ependymal cells are also involved in the brain’s response to injury or inflammation, changing their morphology and function during disease or injury. They contribute to repair mechanisms and immune regulation, maintaining CNS health.