Oligodendrocyte-specific protein (OSP)/claudin-11, which is the third most abundant CNS myelin protein, is localized to a variant of tight junctions in myelin sheaths. The distribution of this protein was investigated immunohistochemically in 7 human fetal brains (from 24 to 32 weeks of gestation. At 24 weeks of gestation, OSP/claudin-11-immunoreactive (IR) cell bodies are observed in the ganglionic eminence, internal capsule, medial medullary lamina and subthalamic nucleus. Between 30 and 32 weeks of gestation, cellular and fibrous immunolabeling is found in the above-mentioned brain areas. In addition, IR cell bodies are seen in the dorsal thalamus, globus pallidus and striatum. The distribution pattern of OSP/claudin-11-IR structures reflects which regions of the forebrain first display myelin sheaths. In addition, OSP/claudin-11 is expressed in the ganglionic eminence and adjacent cell clusters, indicating a role of this protein in the proliferation and migration of oligodendrocytes. Moreover, it is obvious that OSP/claudin-11 is first restricted to the somata of oligodendrocytes and later accumulates within their processes. This accumulation is in accordance with data in the literature demonstrating that OSP/claudin-11 is a specific protein of myelinic tight junctions. The latter may form a barrier isolating the extracellular compartment between myelin sheaths.

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