Animal Molecular Breeding 2024, Vol.14, No.3, 217-227 http://animalscipublisher.com/index.php/amb 218 produced in vitro and transferred to recipient queens, demonstrating the feasibility of these techniques. More recent advancements have focused on optimizing cryoprotectant exposure techniques to improve the survival and developmental competence of vitrified oocytes and embryos (Tharasanit et al., 2011). 2.2 Comparison with other reproductive technologies Embryo transfer is one of several assisted reproductive technologies (ART) used in feline breeding programs. Compared to other methods such as artificial insemination (AI) and somatic cell nuclear transfer (SCNT), ET offers unique advantages. For instance, ET allows for the transfer of embryos produced from genetically valuable or endangered individuals, thereby enhancing genetic diversity and conservation efforts (Zambelli and Cunto, 2022). In contrast, AI primarily focuses on the direct insemination of sperm into the female reproductive tract, which may not be as effective in species with complex reproductive physiology. SCNT, while promising, has shown limited success in felines, with challenges in achieving full-term development of cloned embryos (Swanson, 2012). ET, therefore, remains a critical tool in both domestic and wild feline breeding programs, offering a higher success rate and broader applicability. 2.3 Challenges specific to feline reproductive physiology Feline reproductive physiology presents several challenges that complicate the application of embryo transfer techniques. One significant issue is the induction of ovulation, which in felines is typically triggered by mating. Hormonal treatments using eCG and hCG have been developed to induce ovulation in donor and recipient queens, but the timing and dosage must be carefully managed to ensure successful embryo recovery and implantation. Additionally, the maintenance of pregnancy in recipient queens, especially during the non-breeding season, requires the administration of sustained action progesterone to prevent spontaneous abortion (Tsutsui, 2000). Another challenge is the high sensitivity of feline oocytes and embryos to cryoinjury during vitrification, necessitating the development of optimized cryoprotectant exposure techniques to improve survival rates. These physiological complexities underscore the need for continued research and refinement of ET protocols to enhance their efficacy and reliability in feline breeding programs (Pelican et al., 2006). By addressing these challenges and leveraging advancements in reproductive technologies, researchers aim to optimize embryo transfer techniques, thereby improving the outcomes of feline breeding programs and contributing to the conservation of endangered species. 3 Embryo Collection Techniques 3.1 Hormonal stimulation protocols Hormonal stimulation is a critical step in optimizing oocyte retrieval in feline breeding programs. The use of follicle-stimulating hormone (FSH) has been shown to significantly enhance follicular development and oocyte yield (Table 1). For instance, studies on Eld's deer have demonstrated that FSH stimulation increases the main follicular diameter, although it does not necessarily increase the number of oocytes retrieved or their quality (Thongphakdee et al., 2017). In domestic cats, repeated FSH treatments have been effective in stimulating follicular development, with a mean recovery of 24.1 mature oocytes per laparoscopic retrieval. Additionally, minimal ovarian stimulation protocols combined with elective single embryo transfer have been shown to yield acceptable live birth rates in human IVF programs, suggesting potential applicability in feline breeding (Chinarov et al., 2021). 3.2 Oocyte retrieval methods Oocyte retrieval methods vary, but laparoscopic ovum pick-up (LOPU) and transvaginal ovum pick-up (TVOPU) are commonly used. LOPU has been extensively used in cats, with over 1 603 laparoscopic oocyte retrievals performed, resulting in the recovery of more than 38 000 mature oocytes (Pope, 2014). In cows, different OPU protocols have been tested, showing that dominant follicle removal followed by FSH treatment and subsequent OPU can significantly increase the number of follicles aspirated and oocytes retrieved (Konstantinidou et al., 2023). These methods can be adapted for use in feline species to optimize oocyte yield and quality.
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