Abstract
PurposeAn extensive literature has postulated multiple etiologies for aqueductal stenosis. No publications were found, discussing that evolutionary modifications might explain aqueductal anomalies. This study’s objectives were to review the evolutionary modifications of vertebrates’ tectum structures that might explain human aqueduct anomalies. Undertaking vertebrate comparative study is currently not feasible in view of limitations in obtaining vertebrate material. Thus, vertebrate material collected, injected, dissected, and radiographed in the early 1970s was analyzed, focusing on the aqueduct and components of the midbrain tectum.MethodsPhotographs of brain dissections and radiographs of the cerebral ventricles and arteries of adult shark, frog, iguana, rabbit, cat, dog, and primate specimens, containing a barium-gelatin radiopaque compound, were analyzed focusing on the aqueduct, the optic ventricles, the quadrigeminal plate, and collicular ventricles. The anatomic information provided by the dissections and radiographs is not reproducible by any other radiopaque contrast currently available.ResultsDissected and radiographed cerebral ventricular and arterial systems of the vertebrates demonstrated midbrain tectum changes, including relative size modifications of the mammalian components of the tectum, simultaneously with the enlargement of the occipital lobe. There is a transformation of pre-mammalian optic ventricles to what appear to be collicular ventricles in mammals, as the aqueduct and collicular ventricle form a continuous cavity.ConclusionsThe mammalian tectum undergoes an evolutionary cephalization process consisting of relative size changes of the midbrain tectum structures. This is associated with enlargement of the occipital lobe, as part of overall neocortical expansion. Potentially, aqueductal anomalies could be explained by evolutionary modifications.
Highlights
For over a century, an extensive literature has investigated the anatomy of the cerebral aqueduct, and explored the various potential etiologies of non-tumoral aqueductal stenosis [1, 24, 30]
The comparative anatomic information may have the potential of pointing out that evolutionary modifications might explain the formation of forking, septa, and membranes resulting in aqueductal stenosis in humans
There is the possibility that the enlargement of the periaqueductal tissue in the human changed the increased vertical height to a horizontal one. This investigation emphasized the role of vertebrate anatomic dissections and radiography in helping introduce comparative anatomic changes in expanding the list of potential explanations regarding non-tumoral aqueductal stenosis
Summary
An extensive literature has investigated the anatomy of the cerebral aqueduct, and explored the various potential etiologies of non-tumoral aqueductal stenosis [1, 24, 30]. Evolutionary modifications of the midbrain tectum are not discussed in the extensive aqueductal stenosis literature. The objectives of this investigation were to review evolutionary material that contributed to the understanding of the evolutionary changes of the optic lobes and optic ventricles, the colliculi, the quadrigeminal plate of the midbrain tectum, and aqueduct. The comparative anatomic information may have the potential of pointing out that evolutionary modifications might explain the formation of forking, septa, and membranes resulting in aqueductal stenosis in humans
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