Park, J. E., Oh, H. J., Hong, S. G., Jang, G., Kim, M. K., & Lee, B. C. (2010). Effects of Activin A on the In Vitro Development and mRNA Expression of Bovine Embryos Cultured in Chemically‐Defined Two‐Step Culture Medium. Reproduction in domestic animals, 45(4), 585-593.
doi: 10.1111/j.1439-0531.2008.01306.x

The purpose of this study was to evaluate the effects of activin A on the developmental competence of in vitro fertilized (IVF) bovine embryos derived from a two-step defined culture system (C1/C2 medium) during the early or later stages of embryo development. The authors suggest that activin A, during the later stage of in vitro bovine embryo development, can enhance in vitro development of embryos by increasing hatching rates and affecting expression levels of genes related to hatching and implantation in defined culture medium.

The formula of C1/C2 medium is listed in a former article.--> Lim, K. T., Jang, G., Ko, K. H., Lee, W. W., Park, H. J., Kim, J. J., ... & Kang, S. K. (2007). Improved in vitro bovine embryo development and increased efficiency in producing viable calves using defined media. Theriogenology, 67(2), 293-302. See table 1 for the composition of the two-step culture medium for culturing bovine embryo's.