A histological study of the protracted dismantling of the spent (Sertoli-only) shark spermatocyst post-spermiation: Insight from species with or without testis-associated lymphomyeloid tissue

Sertoli cells of sharks are non-permanent components of the spermatocyst that they share exclusively with only one germ cell stage. After spermiation, all Sertoli cells, and thus the whole spent cyst, are disposed of in an area adjacent to the spermatozoal spermatocysts, that is, the resorption zone (RZ). Differences in the histology and magnitude of the RZ of the mature blue shark and Greenland shark correlate with differences in how spent cysts are dismantled. In the blue shark's RZ, the spent cyst's Sertoli nuclei were synchronously and stepwise fragmented into pyknotic bodies that were eventually resorbed in a whorl in the RZ interstitium. Conversely, cyst dismantling in the Greenland shark, that also lacked a spatially definitive RZ, revealed redundancy. One mode entailed the sloughing of the bulky Sertoli nuclei through an indistinct cyst–ductule transition area into its attached collecting ductule. A second mode entailed the asynchronous, progressive fragmentation of the bulky Sertoli nuclei into membrane-enclosed pyknotic bodies. Both these modes solely entailed an internally coordinated demise of the spent cyst and whose basal lamina remained intact almost right to the end. Whatever the underlying mechanisms of these differences, these findings nonetheless reveal species-specificity in the clearing up of the elasmobranch testicular parenchyma after the completion of a round of spermiogenesis. One consideration is the blue shark's expansive immune cell augmented RZ, that adjoins the animal's bone marrow equivalent tissue. The notable finding of a second conspicuous Sertoli cell type in the Greenland shark's spent cysts is also discussed.