Ependymal cells are a type of ciliated epithelial tissue which lines the interior surface of the ventricals. Though the term ependyma is sometimes thrown around loosely, it does not include the entirety of the tissue lining the ventricles as some would have you believe. Ependymal cells compose the surface of the choroid plexus, the tissue which secretes cerebral spinal fluid. Given that this tissue is ciliated, it has a strong importance during early development. The ability to provide movement to newly arising cells is a crucial early function, while the function in fully developed organisms is more complicated. This tissue may fill notable roles including neuroprotection and neurosecretory functions. Many functions have been proposed for the ependyma, and while they may be credible there is limited evidence for the majority of them. We will describe several of those which are best documented here. The remainder of this article will describe the development, locations, status, and potential functions of the ependymal cells during various stages of the organism’s life cycle. Information from this post comes from a series of literature reviews conducted by myself and others. Due to the nature of the field, rats are the most studied organism and thus most information refers to lower vertebrates though there are obviously strong correlations across mammals.
SEM of ependymal cells from two different locations in the brain. b. the dense cilia of the caudate nucleus ependyma, c. the far fewer cilia of the more broad corpus callosum ependyma.
Ependyma are very interesting cells. Given their ciliated structure, they could have a huge array of functions in the adult organism. Despite this, the brain of an adult human is almost devoid of them. Given this loss, some scientists were lead to believe that they are actually vestigial. Now though, evidence has amassed which shows that they may have some very exciting neuroprotective and neuroregenerative functions, due in part to their ability to de-differentiate. They’re able to become pluripotent, differentiate asymmetrically, and they possess a structure capable of providing mobility to other cells and substances in the brain (cilia). Given all of these abilities, the question of the function of this cell type becomes a little bit muddy. I like muddy. This is clearly a cell type worth investigating.
This section is meant to emphasize the development of ependymal cells themselves, however these cells actually serve crucial functions in the development of other regions as well and so this will also be explained here. There are two popularly proposed origins for ependymal cell differentiation, either a terminal state of differentiation from ventral zone cells, or an intermediate developmental stage of radial glial cells. The evidence is strong with respect to a radial glial origin, given that ependyma can de-differentiate at certain stages and return to radial glia. In the immature neural tube ependyma are non ciliated a modification which occurs later in development. The exact time during which ependyma differentiate varies across mammals, though 22-28 weeks seems to be a fairly common number in the literature. In the adult human, the density of ependyma throughout the ventricles decreases drastically, becoming almost absent in some areas.
As I mentioned earlier, the functions of ependyma are broad and varied. Evidence for their potential functions is everywhere in the literature. Ependyma are known to die off during pathological conditions, indicating that they function in neuroprotection. Ependyma are known to de-differentiate into pluripotent cells, indicating that they are neuroregenerative. Their ciliated ends which face into the ventricles are known to sweep CSF in the same direction as the bulk fluid flow. This indicates that they may function to create a concentration gradient needed to allow neuroblast movement throughout the CSF. This is just a FEW of the functions proposed for these cells, each of which seems to be more intriguing than the next.
One function not yet mentioned one performed through matrix proteins which can be expressed on their basal surfaces. These proteins provide guidance to developing axons, which on its own has serious implications for things like regeneration, among others. An additional function is the ability to revert to pluripotency, which is a known function of some ependyma as evidenced by the presence of CD133. Further, evidence from studies conducted on rats shows that increased exercise promotes the ability of spinal ependyma to become capable of proliferation and thus able to contribute to the healing of spinal cord injury. Maybe a reason to hit the gym a little bit more? Either way, neuroregeneration is clearly a function of ependyma given any of these functions on their own. Taken together, they are a hugely capable cell group.
The mention of the stem cells brings on an additional function of ependyma which is neurosecretion. In addition to the ability to return to a quiescent state, they are also capable of secreting several growth factors of their own, potentially modulating existing stem cell populations in the brain. Further, the motion of ependymal cilia are coordinated and beat in the same direction as the bulk flow of the CSF. This coordinated beating of cilia may produce a concentration gradient which could guide neuroblasts to their intended destinations. Taken together, these two functions enable ependyma to both initiate production of stem cells and guide their movement.
Neurosecretion is another function of this cell type. Ependymal cells release a broad array of growth factors, including vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF2) enabling them to influence the growth and development of several other cell types.
To conclude, the potential roles of ependymal cells are hugely important for the nervous system. These functions include neuroregeneration through de-differentiation into stem cells, neurosecretion through the production of growth factors, and neuroprotection. All of these functions combined with the question of their potential vestigiality due to a gross absence in the adult human make these cells a hugely intriguing research prospect and an exciting cell type to learn about.
Thanks for reading.
1. Squire, L. R. (2003). Fundamental neuroscience. San Diego: Academic Press