The epigonal organ and mature pole of the testis in the recreationally fished blue shark (Prionace glauca): histochemico‐functional correlates

Abstract

The exact role of the immune system in normal spermatogenesis is poorly understood. The attachment, however, of the lymphomyeloid epigonal organ specifically to the testis's mature pole in many shark species is a curious finding. Unlike the histology of the lymphomyeloid tissues of many other elasmobranchs, the epigonal organ leukocytes of wild-caught blue shark (Prionace glauca), besides exhibiting extensive nuclear heterogeneity, contain some of the largest known granules ever seen in vertebrate white blood cells. It was previously shown that the blue shark epigonal organ remains unremarkable and functionally unchanged despite cestode parasites embedded into its surface, suggesting that it might have other functions in addition to microbial defense. We show here that Prionace epigonal leukocytes shed their granule-laden cytoplasm into the cyst resorption zone (RZ) of the testis, i.e. the region separating the spermatogenic tissue from the epigonal organ, as they begin to migrate into the RZ. Using the immunoreactivity of the conserved transcription factor (proliferating cell nuclear antigen) as marker, it is shown that the granule-lacking leukocytes exclusively infiltrated spermatozoal cysts leftover after the wave of wide-spread multinuclear cell death in summer-breeding males in a seasonally dependent manner. By contrast, Prionace caught 2 months later showed fully recovered testes containing numerous completely intact spermatozoal cysts. Conversely, degenerating immature spermatids were gradually phagocytized by their accompanying Sertoli cells, and leukocytes did not infiltrate such cysts. The autoimmune response described here resembles in every aspect the testicular autoimmune response induced experimentally in a teleost fish. These observations suggest functional adaptation of shark leukocytes in response to specific changes in the testicular microenvironment.

A Comparative Study of Intratesticular Ductules in the Spermatogenically Active Testes of Shortfin Mako and Thresher Sharks

“…Based on the presence of degraded remains of spermatids/sperm among the cell debris, it is plausible therefore that degeneration, once initiated in these bodies, proceeds at a pace that outstrips the degradation of spermatids they might harbor. These developments are peculiar, as they are in stark contrast to the involvement of the infiltrating, epigonal‐derived leukocytes in the removal of degenerated spermatid‐containing cysts as reported in the blue shark (McClusky and Sulikowski, ). To what extent the rectangular and other bizarre shaped ductules, which could have deep blind pockets, and their non‐apoptosis‐related histological changes can be linked to the phenomena of circular degenerative bodies, if at all, is unclear at this stage and await further elucidation.…”

“…Tissue sections of thresher shark testes and attached epigonal organs were probed with an antibody to an evolutionary conserved protein that is synthesized in early G1 phase of the cell cycle, namely PCNA. This approach also had utility for the immunodetection of epigonal organ‐derived leukocytes as the full shape of the leukocyte nucleus was fortuitously revealed by the brown color of the DAB chromogen (McClusky and Sulikowski, ). As shown in Figure , a group of closely lying degenerative bodies with distinctly cellular morphologies were, except for a few scattered intensely PCNA immunoreactive nuclei on the outer periphery, decidedly PCNA‐negative.…”

“…To immunodetect cycling cells in tissues that are presumably proliferatively active, tissue sections were probed with an antibody to an evolutionary conserved protein (proliferation cell nuclear antigen, PCNA) that is synthesized in early G1 phase of the cell cycle (Georgescu et al, ). Procedures followed were as previously described for the study of elasmobranch tumors (Borucinska et al, ), the testis and epigonal organ (McClusky and Sulikowski, ) of the blue shark. Owing to the lack of available molecular and other markers of elasmobranch lymphomyeloid cells, the strict nuclear staining patterns of PCNA immunohistochemistry fortuitously informed about the shape of the leukocyte nucleus, and thus also the possible identity of the labeled cell, as previously reported (McClusky and Sulikowski, ).…”

“…Procedures followed were as previously described for the study of elasmobranch tumors (Borucinska et al, ), the testis and epigonal organ (McClusky and Sulikowski, ) of the blue shark. Owing to the lack of available molecular and other markers of elasmobranch lymphomyeloid cells, the strict nuclear staining patterns of PCNA immunohistochemistry fortuitously informed about the shape of the leukocyte nucleus, and thus also the possible identity of the labeled cell, as previously reported (McClusky and Sulikowski, ).…”

A first observation of spermatogenesis in mature male scalloped hammerheads (Sphyrna lewini) from Zinkwazi, KwaZulu-Natal, South Africa

“…Spermatogenesis is the process of spermatozoa development from germ cells to mature spermatozoa in the seminiferous tubules within the testes. Shark testes consist of seminiferous tubules which form part of a larger lobular structure with germ cells in different stages of development (Pratt 1988 ; Grier 1992 ; Parsons and Grier 1992 ; Girard et al 2000 ; McClusky and Sulikowski 2014 ; Do Rêgo et al 2016 ). Germ cell development takes place in the seminiferous tubules, with initial development from the basal membrane on the periphery of the seminiferous tubule.…”