= Blood-testis barrier
The <blood-testis barrier> is a selective interface within the <seminiferous tubule>, formed principally by junctions between neighboring <Sertoli cells>. Its <tight junctions> restrict the extracellular route toward developing <germ cells>, helping maintain a specialized environment for <spermatogenesis>. Despite its name, the key junctional seal is between supporting epithelial <cells>, rather than simply between <blood> vessel walls and the rest of the <testis>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section27.1: Sertoli-cell organization and blood-testis barrier]{ref}
A 1999 experiment tested a molecular component. Mice lacking the <membrane protein> <Claudin-11> were male-sterile, and <freeze-fracture electron microscopy> showed absence of the normal <tight junction> strands between their <Sertoli cells>. The conjunction of a defined genetic alteration, an altered junctional structure and impaired reproduction provides stronger evidence than identifying a dark line in one section.<CNS myelin and Sertoli cell tight junction strands are absent in Osp claudin-11 null mice by A. Gow and others>[Abstract: null mice, freeze-fracture strands and reproductive deficit]{ref}
Selective separation does not mean complete isolation. Supporting <cells> still exchange substances through controlled cellular routes. A useful tracer experiment would compare whether a probe can pass between <Sertoli cells> under matched conditions; its distribution would test access for that probe, not prove that every <solute> crosses identically.
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