Talbert DG
Introduction: Although it is commonly stated that the brain “floats” in the cerebrospinal fluid (CSF), the brain is actually suspended in the CSF-fluid-filled subarachnoid space by trabeculae. Subarachnoid trabaculae are sheets or columns of collagen-reinforced material that stretch between the arachnoid and pia membranes. They can be seen with light microscopes but they are too thin to be seen by ultrasound.
Study: A literature study of the physiology of the subarachnoid space was undertaken. There was a period of interest in trabecular structure in the 1970s, involving electron microscopy. Transmission electron microscopy enabled cell types, and collagen fibre layout, to be determined. The development of scanning electron microscopy techniques allowed the viewing of three dimensional form. Early in mammalian embryo development, a layer of ground substance (gel filled-mesenchyme) advances from the future cervical region into the join between the ectoderm and the neuroepithelium of the telencephalon. It acts as a pia-arachnoid space holder. Randomly spaced fluid-filled “holes” then appear in the gel. These enlarge into randomly spaced and sized, fluid filled, cavities. As the cavities enlarge the remaining mesenchyme elements between them get forced to congregate in the remaining tissue. It appears that when cavities meet, the mesenchyme material lining the two cavities resists further advance leaving, thin walls of mesenchyme which are the origin of trabeculae. The random nature of the original “holes” remains characteristic of trabecular structure thereafter.
Conclusions: The mature subarachnoid space is filled with an ultrasonically invisible “cobweb” of collagen reinforced sheets and cords linking the Arachnoid and Pia Maters. Trabeculae have no coherent structure, they are the result of random removal of tissue, not the generation of new structures.
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