Are the foramen Magendie (or the median aperture) and the two foramina Luschka (or the lateral apertures) genuine defects of the ependymal epithelium? So the ventricular lumen truly communicates with the subarachnoid space? If so, how is the defect maintained? Also, what do the boundaries of the defects even look like?
The fact that there can be non-pathological defects in an epithelium is jarring for me, because I have always been taught that an epithelium consists of an uninterrupted layer of cells juxtaposed against each other, limited by a basement membrane on one side and encasing a lumen or facing the exterior on the other side. So really there are no 'holes' in an epithelium, and even structures that might look like a hole, such as ducts for a gland, are in fact just invaginations of the epithelium.
I've been able to apply this concept to understanding other anatomical structures with no difficulty. If the foramina of the 4th ventricle are really epithelial 'holes' then this is a major anomaly. In fact, I cannot think of a single example other than this one in which a normally developed, intact epithelium (and a basement membrane) has a 'hole' in it.
Also if this is the case, how is the defect maintained? As far as I know, epithelia, when penetrated by trauma, promptly close the gap and re-epithelialize. I believe a persistent epithelial defect must have some kind of supporting mechanism that forces tolerance to such defects.
Lastly, I know almost nothing about cell polarity, but I've learned that that the surface of epithelial cells which faces the lumen is apically polarized, the surface facing neighbor cells is laterally, and the surface in contact with the basement membrane is basally polarized. Would cells at the boundary of the defects have their apical and basal surfaces directly in continuation with each other without an intervening lateral surface, having no neighbor cells to contact?
I've been searching the literature for quite a while, but nobody seems to be really bothered like me by the implications of holes being present in a normal epithelium. Perhaps I'm taking it too seriously and all that 'epithelia are uninterrupted layers of cells' talk is just a rule of thumb, a crude generalization meant to serve only pedagogical purposes. But these foramina of Magendie and Luschka are the only counterexamples to this maxim I could come up with so far. I believe that a rule this widely applicable must have some physiological basis, and therefore that deviations from it demand explanation.