Comparative Anatomy
Because the glia limitans serves such an important structural and physiological function in human beings, it is unsurprising that evolutionary precursors of the glial limiting membrane can be found in many other animals.
Insects have an open circulatory system, so there are no blood vessels found within their ganglia. However, they do have a sheath of perineurial glial cells that envelops the nervous system and exhibit the same tight occluding junctions that are induced by the glia limitans in humans. These cells act as a barrier and are responsible for establishing permeability gradients.
In certain molluscs, a glial-interstitial fluid barrier is observed without the presence of tight junctions. Cephalopod molluscs, in particular, have cerebral ganglia that have microcirculation, often seen in the composition of higher organisms. Often, the glial cells will form a seamless sheath completely around the blood space. The barrier consists of zonular intercellular junctions, rather than tight junctions, with clefts formed by extracellular fibrils. In addition to protection from the blood, these barriers are thought to exhibit local control of the microenvironment around specific neuron groups, a function required for complex nervous systems.
Monkeys and other primates have been found to have a glial limiting membrane extremely similar to humans. Studies on these animals have revealed that the thickness of the glia limitans not only varies greatly among different species, but also within different regions of the central nervous system of the same organism. Further observations of young and old monkeys have proven that the younger subjects have thinner membranes with fewer layers of astrocytic processes while the older monkeys possess much thicker membranes.
Read more about this topic: Glia Limitans
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