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Evidence That Tubulobulbar Complexes in the Seminiferous Epithelium Are Involved with Internalization of Adhesion Junctions¹

Department of Anatomy and Cell Biology,³ Faculty of Medicine, The University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z3 Department of Molecular Biology and Biochemistry,⁴ Osaka University Medical School, Osaka 565-0871, Japan

Tubulobulbar complexes may be part of the mechanism by which intercellular adhesion junctions are internalized by Sertoli cells during sperm release. These complexes develop in regions where Sertoli cells are attached to adjacent cells by intercellular adhesion junctions termed ectoplasmic specializations. At sites where Sertoli cells are attached to spermatid heads, tubulobulbar complexes consist of fingerlike processes of the spermatid plasma membrane, corresponding invaginations of the Sertoli cell plasma membrane, and a surrounding cuff of modified Sertoli cell cytoplasm. At the terminal ends of the complexes occur clusters of vesicles. Here we show that tubulobulbar complexes develop in regions previously occupied by ectoplasmic specializations and that the structures share similar molecular components. In addition, the adhesion molecules nectin 2 and nectin 3, found in the Sertoli cell and spermatid plasma membranes, respectively, are concentrated at the distal ends of tubulobulbar complexes. We also demonstrate that double membrane bounded vesicles are associated with the ends of tubulobulbar complexes and nectin 3 is present on spermatids, but is absent from spermatozoa released from the epithelium. These results are consistent with the conclusion that Sertoli cell and spermatid membrane adhesion domains are internalized together by tubulobulbar complexes. PKC, a kinase associated with endocytosis of adhesion domains in other systems, is concentrated at tubulobulbar complexes, and antibodies to endosomal and lysosomal (LAMP1, SGP1) markers label the cluster of vesicles associated with the ends of tubulobulbar complexes. Our results are consistent with the conclusion that tubulobulbar complexes are involved with the disassembly of ectoplasmic specializations and with the internalization of intercellular membrane adhesion domains during sperm release.

¹ Supported by CIHR MOP 62728 awarded to A.W.V. and by a CIHR doctoral research award awarded to J.A.G.

² Correspondence: A. Wayne Vogl, 313-2177 Wesbrook Mall, University of British Columbia, Department of Anatomy and Cell Biology, Vancouver, BC, Canada V6T1Z3. FAX: 604 822 2316; vogl{at}interchange.ubc.ca

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