An Alternative Model of Tubulobulbar Complex Internalization During Junction Remodeling in the Seminiferous Epithelium of the Rat Testis

Tubulobulbar complexes (TBCs) are elongate subcellular machines responsible for internalizing intercellular junctions during sperm release. Each complex consists of a double-membrane tubular core terminating in a clathrin-coated pit. The core is surrounded by a network of actin filaments, and a distinct swelling or bulb, which lacks an association with actin, develops in the distal third of the structure. The bulb eventually buds from the complex and enters endocytic compartments of the Sertoli cell. The relationship of the actin cuff to the formation and budding of the bulb is not known. To gain insight into this relationship, we perturbed the actin networks of TBCs with cytochalasin D. When isolated testes were perfused with a physiological buffer containing cytochalasin D, apical TBCs at stage VII of spermatogenesis were associated with lower levels of actin compared to controls. At the ultrastructural level, the actin networks in cytochalasin D-treated testes appeared patchy, and ectopic bulbs and swollen tubular regions occurred. When normal untreated samples at early stage VII were analyzed, large elongate bulbs and short tubular sections were observed. Together, these results suggest a new model for TBC vesiculation in which the actin network begins to disassemble and the tubular region begins to swell into a bulb. As actin disassembly continues, the coated pit and most of the tubular region are incorporated into the enlarging bulb. The remaining short neck of the bulb near the base of the complex undergoes scission, and the bulb is internalized.