The accessibility of the nervous system of Manduca sexta to horseradish peroxidase and lanthanum during metamorphosis has been studied. In the fifth instar larva, peroxidase penetrates the thick neural lamella but does not reach the perineurium. In the first week after larval-pupal ecdysis, when the intercellular channels between hypertrophied perineurial I cells are the least tortuous, the enzymic tracer bypasses the perineurial I tight and gap junctions and penetrates as far as the perineurial II bracelet layer. During this time, there is active uptake of peroxidase by the perineurial I cells but no reaction product is found in the underlying perineurial II cell bracelet. In the second week after larval-pupal ecdysis, when the perineurium is a flattened interdigitating cell layer, peroxidase does not penetrate as far as the perineurial I and II interspace and there is little or no uptake of the protein by the perineurial I cells. Lanthanum, in contrast, passes the perineurial I tight and gap junctions, which are presumed to be macular, and travels within the entire perineurial I and II interspace throughout metamorphosis. As it travels within this interspace, it gains access to the perineurial II bracelet connexions, which differ in their ability to restrict lanthanum. Lanthanum is able to penetrate the entire overlapping connexion with its predominant gap junctions before being stopped, but penetrates the more complex interdigitating connexion with its extensive tight, gap and septate junctions for only a short distance. The relevance of the perineurial II bracelet layer and its intercellular connexions to the maintenance of a blood-brain barrier in this developing nervous system is discussed.

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