The never ending search of an ideal culture system for cat oocytes and embryos

An ideal culture system for cat oocytes and embryos has yet to be achieved. In vitro simulation of complex in vivo models, as follicles and oviducts, is extremely challenging. Chemical and physical factors are involved, and several studies have been performed to approach as close as possible to the physiological microenvironments, which interact with oocytes and embryos promoting their competence. Chemical factors Follicular fluid contains a variety of hormonal and regulatory factors and a wide range of supplementation were tested for cat oocyte in vitro maturation (IVM). Our first studies date back more than 20 years ago, but the research is still ongoing today. For example, the influence of two different FSH additives on the IVM rate and gene expression of feline cumulus–oocytes complexes (COCs) has recently been investigated [1]. Oviductal fluid generated from secretory cells, is regulated by hormonal status during estrous cycle and it is well known its environmental and nutritional support for early embryonic development. The need of an optimized culture medium for feline embryos, prompted the investigation of metabolic requirements and the results showed that sequential culture systems based on the metabolism of cat embryos along development are beneficial [2]. Physical factors From a physical point of view, the oocyte, surrounded by layers of somatic granulosa cells, is located in the ovarian follicle. This three-dimensional (3D) structure cannot be maintained in bi-dimensional culture drops. For this purpose, we used 3D innovative culture system, as alginate microcapsules, for cat oocyte IVM and the results were encouraging [3]. Microcapsules were also beneficial for cat embryo culture [4] and, if enriched with granulosa or oviductal cells, can be considered as an alternative to the co-culture on cell monolayer. Changes of physical microenvironment might provide more adequate chemical conditions for oocyte and embryos. For instance, the physiological spatial arrangement of COCs in 3D systems may improve the secretion of oocyte regulatory factors involved in the acquisition of developmental competence. Other sophisticated systems based on customized 3D printing combined with microfluidic technology, might be the evolution toward the creation of artificial follicles and oviducts (organs-on-a-chip) that could drive toward the end of the search. [1] Zahmel J, Mundt H, Jewgenow K, et al. Analysis of gene expression in granulosa cells post-maturation to evaluate oocyte culture systems in the domestic cat. Reprod Domest Anim 2017;52 (Suppl. 2): 65-70. [2] Herrick JR, Bond JB, Magarey GM, et al. Toward a feline-optimized culture medium: impact of ions, carbohydrates, essential amino acids, vitamins, and serum on development and metabolism of in vitro fertilization-derived feline embryos relative to embryos grown in vivo. Biol Reprod 2007;76: 858–870. [3] Morselli MG, Canziani S, Vigo D, Luvoni GC. A three-dimensional alginate system for in vitro culture of cumulus-denuded feline oocytes. Reprod Domest Anim, 2017;52: 83-88. [4] Morselli MG, Luvoni GC, Comizzoli P. The nuclear and developmental competence of cumulus-oocytes complexes is enhanced by 3-dimensional co-culture with conspecific denuded oocytes during in vitro maturation in the domestic cat model. Reprod Domest Anim 2017; 52 (Suppl. 2): 82-87.