International Journal of Aquaculture, 2024, Vol.14, No.3, 154-164 http://www.aquapublisher.com/index.php/ija 157 (Anguilla japonica). The study found that injecting salmon pituitary homogenate (SPH) and 17α-methyltestosterone (MT) significantly promoted synchronous follicle development. Experimental results showed that the ovarian growth index (GSI) and follicle-stimulating hormone receptor (fshr) mRNA expression levels in the SPH + MT group were higher than those in the SPH-only group (Figure 1). Additionally, it was found that using pituitary extract and maturation-inducing hormone to induce follicle maturation in European eels resulted in successful vitellogenesis and enhanced embryonic development capabilities (Kottmann et al., 2021). Figure 1 Histological comparison of ovarian development among the control and three weekly SPH- and SPH + MT- injected groups (Adopted from Lee and Lou, 2019) Image caption: The figure shows that primary follicle growth in the control group (A-D) was slow (A, GSI = 0.42%; B, GSI = 0.46%; C, GSI = 0.5%; D, GSI = 0.75%), while the group injected with salmon pituitary homogenate (SPH) (E-G) showed greater follicle development (E, GSI = 0.63%; F, GSI = 1.02%; G, GSI = 1.07%). The group injected with both SPH and 17α-methyltestosterone (MT) (H-K) (H, GSI = 0.93%; I, GSI = 1.34%; J, GSI = 1.38%; K, GSI = 1.41%) exhibited the most significant follicle development, with a notable increase in the number of primary follicles. The results indicate that the combined action of androgens and SPH promotes synchronized ovarian development (Adapted from Lee and Lou, 2019) Another study explored the impact of cold seawater treatment on the early sexual development stages of European eel. Rozenfeld et al. (2019) found that cold seawater (10°C) treatment significantly promoted the activity of the male reproductive gland axis (BPG axis) in European eel. Specifically, the cold seawater treatment increased the proliferation and differentiation of reproductive cells, especially at the type A spermatogonia stage, and elevated plasma testosterone and 11-ketotestosterone levels (Figure 2). Gene expression analysis showed significant differences in gene expression profiles between the treatment group and the control group, particularly in aspects such as circadian rhythm, histone modification, and meiotic nuclear division. The results suggest that cold seawater treatment may influence temperature perception through epigenetic mechanisms, thereby promoting the early sexual development stages of eel. Furthermore, the cryopreservation of eel sperm has been optimized, with recent protocols using methanol as a cryoprotectant showing improved motility and fertilization rates, which is crucial for large-scale breeding programs (Herranz-Jusdado et al., 2019).
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