Have you looked down your dissecting microscope and felt that the level of cumulus expansion of IVM-derived cumulus-oocyte complexes (COCs) is not as you have seen in other articles?  Maybe you have seen pictures of in vivo matured COCs (such as in human IVF) and they look much more expanded.  Do not be too concerned!

Many factors impact the degree of cumulus expansion during cattle IVM; although the composition of the matrix is mostly hyaluronic acid, the cumulus matrix composition is subtly different between in vivo and in vitro matured COCs, but this difference means a lot.  For in vivo matured COCs within the follicle post-LH surge, the extent of cumulus expansion does relate to oocyte competence. However, IVM medium composition, especially the levels of glucose and glutamine (the substrates for hyaluronic acid production), impact the degree of cumulus expansion, as does the type and concentration of hormone preparation used that initiates expansion.

These are either FSH, EGF, or the EGF-like peptides, with or without oestradiol, or combinations.  The level of serum will also affect the level of cumulus expansion, so will the density of COCs per volume of medium.   All of these factors will interact with each other, causing differences in the degree of expansion.

The ‘take-home’ message is that cumulus expansion itself during cattle IVM likely indicates helpful ‘signalling pathways’ for oocyte competence are turned on, but it shouldn’t be used as a direct measure of oocyte competence.  Each IVM medium system will likely cause differences in the extent of cumulus expansion, but it is not comparable to in vivo maturation, where the degree of maturation does reflect oocyte competence. 

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