Effect of cycloheximide on nuclear maturation of horse oocytes and its relation to initial cumulus morphology

The period of protein synthesis necessary for meiotic maturation in horse oocytes initially having compact or expanded cumulus cells was studied. Oocytes incubated in the presence of cycloheximide after 0, 4, 8, 12 or 16 h maturation in vitro (total incubation time 24 h) were matured for 24 h, or were incubated with cycloheximide for 24 h and then matured for 24 h. Incubation with cycloheximide from 0 h was effective in suppressing maturation (no significant increase in maturing oocytes compared with controls fixed directly after removal from the follicle) in both expanded and compact groups and was completely reversible, as there was no difference in the proportion of oocytes reaching metaphase II between controls and treatment groups of either cumulus type. Addition of cycloheximide after 4 h maturation resulted in no significant difference in distribution of oocytes compared with addition at 0 h in either cumulus group. A significant decrease in the proportion of germinal vesicle stage oocytes, and an increase in oocytes in metaphase I occurred in oocytes with expanded cumulus cells in the 8 h treatment and in oocytes with compact cumulus cells in the 12 h treatment, compared with oocytes treated after 0 h. A significant increase in the proportion of oocytes at metaphase II occurred in the 12 h treatment for expanded cumulus-oocyte complexes and in the 16 h treatment for compact cumulus-oocyte complexes. These data show that nuclear maturation of horse oocytes can be reversibly suppressed by incubation with cycloheximide from the onset of culture. Oocytes with different initial cumulus type differed in the time required for protein synthesis essential for maturation: expanded cumulus-oocyte complexes required less time to prepare for germinal vesicle breakdown, maturation to metaphase I, and maturation to metaphase II, than did compact cumulus-oocyte complexes.