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Electron microscopic studies on the lamprey meninges
Abstract:
Lamprey meninges were revealed to consist of four layers. The I (outermost) layer was made up of densely packed collagen fibrils and microfibrils with fibrocytes interposed between them. It contained a vascular network (epidural vascular plexus). The II layer was a cellular layer, consisting of two parts: the outer, membranous and the inner, spongy part. The membranous part was composed of two sheets of type 1 cells which were held together by desmosomes, gap junctions and tight junctions. Continuous basal laminae were found on both the outer and the inner surface of the membranous part. The spongy part consisted of type 2 cells, collagen fibrils and microfibrils, which were loosely arranged. Type 2 cells were joined together with each other, or with type 1 cells, by gap junctions. These cells lacked a basal lamina covering. The III layer was a peculiar thick layer, being mostly occupied by fine filamentous matrix substance with scattered round cells, fibroblasts and microfibrils. The IV (innermost) layer was a thick fibrous one, consisting of two parts: the outer, vascular and the inner, loose part. The former consisted of a vascular network (perineural vascular plexus) and thick bundles of microfibrils which frequently contained a dense filamentous core. The latter was composed of loosely arranged collagen fibrils. The inner part of the I layer (inside the epidural vascular plexus) was considered to correspond to the true dura, and the II and IV layer to the arachnoid and pia in the meninges of higher animals, respectively. Characteristically the subarachnoid space was absent in lamprey meninges, and instead a peuliar "gelatinous" layer, III layer, was present between the II and IV layer.
Insights
Lamprey meninges possess four distinct layers, differing from higher vertebrates. A unique gelatinous layer replaces the subarachnoid space, offering insights into vertebrate meningeal evolution.
Area of Science:
- Comparative anatomy
- Neuroscience
- Evolutionary biology
Background:
- The meninges, protective membranes surrounding the central nervous system, exhibit variations across vertebrate species.
- Understanding lamprey meninges provides insights into the ancestral structure of vertebrate meninges.
Purpose of the Study:
- To elucidate the detailed histological structure of lamprey meninges.
- To compare the lamprey meningeal structure with that of higher vertebrates.
Main Methods:
- Histological examination of lamprey meninges.
- Microscopic analysis of cellular and extracellular components.
Main Results:
- Lamprey meninges comprise four distinct layers: Layer I (dura homologue), Layer II (arachnoid homologue), Layer III (unique gelatinous layer), and Layer IV (pia homologue).
- Layer I contains collagen, microfibrils, fibrocytes, and an epidural vascular plexus.
- Layer II is cellular with membranous and spongy parts, featuring type 1 and type 2 cells connected by various junctions.
- Layer III is a thick, gelatinous matrix layer.
- Layer IV is fibrous with vascular and loose parts, containing a perineural vascular plexus.
Conclusions:
- Lamprey meninges exhibit a unique four-layered structure with a distinct gelatinous layer (Layer III) replacing the subarachnoid space.
- The identified layers show homologies to the dura, arachnoid, and pia of higher vertebrates, indicating evolutionary relationships.
- The absence of a subarachnoid space and the presence of the gelatinous layer are characteristic features of lamprey meninges.