Effects of maturational age of recipient oocytes and activation conditions on the development of porcine fetal fibroblast nuclear transfer embryos

Recent advances in critical nodes of embryo engineering technology

The normal development and maturation of oocytes and sperm, the formation of fertilized ova, the implantation of early embryos, and the growth and development of foetuses are the biological basis of mammalian reproduction. Therefore, research on oocytes has always occupied a very important position in the life sciences and reproductive medicine fields. Various embryo engineering technologies for oocytes, early embryo formation and subsequent developmental stages and different target sites, such as gene editing, intracytoplasmic sperm injection (ICSI), preimplantation genetic diagnosis (PGD), and somatic cell nuclear transfer (SCNT) technologies, have all been established and widely used in industrialization. However, as research continues to deepen and target species become more advanced, embryo engineering technology has also been developing in a more complex and sophisticated direction. At the same time, the success rate also shows a declining trend, resulting in an extension of the research and development cycle and rising costs. By studying the existing embryo engineering technology process, we discovered three critical nodes that have the greatest impact on the development of oocytes and early embryos, namely, oocyte micromanipulation, oocyte electrical activation/reconstructed embryo electrofusion, and the in vitro culture of early embryos. This article mainly demonstrates the efforts made by researchers in the relevant technologies of these three critical nodes from an engineering perspective, analyses the shortcomings of the current technology, and proposes a plan and prospects for the development of embryo engineering technology in the future.

Effects of recipient oocyte age and interval from fusion to activation on development of buffalo (Bubalus bubalis) nuclear transfer embryos derived from fetal fibroblasts

The objective was to investigate the effect of recipient oocyte age and the interval from activation to fusion on developmental competence of buffalo nuclear transfer (NT) embryos. Buffalo oocytes matured in vitro for 22 h were enucleated by micromanipulation under the spindle view system, and a fetal fibroblast (pretreated with 0.1 μg/mL aphidicolin for 24 h, followed by culture for 48 h in 0.5% fetal bovine serum) was introduced into the enucleated oocyte, followed by electrofusion. Both oocytes and NT embryos were activated by exposure to 5 μM ionomycin for 5 min, followed by culture in 2 mM 6-dimethyl-aminopurine for 3 h. When oocytes matured in vitro for 28, 29, 30, 31, or 32 h were activated, more oocytes matured in vitro for 30 h developed into blastocysts in comparison with oocytes matured in vitro for 32 h (31.3 vs 19.9%, P < 0.05). When electrofusion was induced 27 h after the onset of oocyte maturation, the cleavage rate (78.0%) was higher than that of electrofusion induced at 28 h (67.2%, P < 0.05), and the blastocyst yield (18.1%) was higher (P < 0.05) than that of electrofusion induced at 25 or 26 h (7.4 and 8.5%, respectively). A higher proportion of NT embryos activated at 3 h after electrofusion developed to the blastocyst stage (18.6%) in comparison with NT embryos activated at 1 h (6.0%), 2 h (8.3%), or 4 h (10.6%) after fusion (P < 0.05). No recipient was pregnant 60 d after transfer of blastocysts developed from NT embryos activated at 1 h (0/8), 2 h (0/10), or 4 h (0/9) after fusion. However, 3 of 16 recipients were pregnant following transfer of blastocysts developed from the NT embryos activated at 3 h after fusion, and two of these recipients maintained pregnancy to term. We concluded that the developmental potential of buffalo NT embryos was related to recipient oocyte age and the interval from fusion to activation.

Effects of different oocyte activation procedures on development and gene expression of porcine pre-implantation embryos

Among the factors that affect the efficiency of somatic cell nuclear transfer (SCNT) in pigs, the activation protocol is the most variable among the current SCNT procedures. The aim of this study is focused on defining an efficient activation treatment of porcine oocytes. In Experiment 1, we studied the effects of nine different oocyte activation procedures (including chemical- and electrical-based treatments) on parthenogenetic embryo development. In Experiment 2, we studied the effect of the more efficient activation procedures on the gene expression profile of Oct4 and Igf2r in parthenogenetic blastocysts. In conclusion, ionomycin as a first calcium stimulus is not able to activate porcine oocytes efficiently in comparison with electric procedures. Electrical treatments with 6-DMAP significantly increased the level of Oct4 expression, whereas the single and double pulse treatments alone maintained the same profile as the IVF group.

Oocyte activation procedures and influence of serum on porcine oocyte maturation and subsequent parthenogenetic and nuclear transfer embryo development

The viability of SCNT embryos is poor, with an extremely low cloned piglet production rate. In the present work, we studied the effect of three activation protocols based on ionomycin treatment (5 μM ionomycin for 5 min and incubated in 2 mM 6-DMAP for 3.5 h) or electric stimuli (two square wave electrical DC pulses of 1.2 kV/cm for 30 μs) combined or not with 6-DMAP on parthenogenetic embryo development. Oocytes activated by ionomycin plus 6-DMAP showed lower cleavage (47.2 vs. 78.5–81.5; p < 0.05) and blastocyst rates (11.3 vs. 29.2–32.1; p < 0.05) than those activated by electrical and electrical plus 6-DMAP treatments. Also, we studied the effect of addition of serum to maturation medium (0% vs. 10%) on nuclear maturation and further parthenogenetic and SCNT embryo development. We observed in the parthenogenetic embryos that cleavage rates in the serum-free group were significantly higher than in the serum-supplemented group (81.8 vs. 69.6% respectively; p < 0.05), although these differences were not detected in blastocyst rates or blastocyst nuclei numbers. Regarding SCNT embryos, no significant differences were observed in cleavage or blastocyst rates between different experimental groups of SCNT embryos. In conclusion, electrical pulse followed or not by 6-DMAP was found to be an efficient procedure to artificially activate MII porcine oocytes. Moreover, the addition of serum to oocyte maturation media did not seem to improve parthenogenetic or SCNT porcine embryo development.