The gonadotropins (GtHs), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are heterodimers composed of a common subunit (GP) and a unique subunit (FSH or LH); they are synthesized in and secreted from gonadotrophs (FSH and LH cells) in the pituitary. Little is known about the roles of FSH and LH during spermatogenesis in perciform fishes. In this study, we examined immunoreactive changes in FSH and LH cells, and changes in the gene expression of the three gonadotropin subunits in the pituitary of male chub mackerel Scomber japonicus during testicular development. FSH-immunoreactive (ir) and LH-ir cell area were measured immuno-histochemically based on the FSH and LH cell-occupying area in the proximal pars distalis. The FSH-ir cell area increased significantly during spermiation, while FSH mRNA levels, already high at the beginning of spermatogenesis, increased further, peaking during spermiation. In contrast, LH-ir cell area and LH mRNA levels, which were low at the beginning of spermatogenesis, increased significantly during late spermatogenesis, peaking during spermiation. For both FSH and LH, GtH-ir cell area and GtH mRNA levels decreased until gonadal resting. GP mRNA levels showed similar changes to LH mRNA levels. These results suggest that in the chub mackerel, FSH may play an important role in the early and late phases of spermatogenesis, and that LH may play a role during late spermatogenesis and spermiation. Moreover, our results demonstrate that changes in GtH-ir cell area were accompanied by similar changes in the expression of the FSH and LH genes, both of which increased during testicular development.
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